How do CSS triangles work?

There're plenty of different CSS shapes over at CSS Tricks - Shapes of CSS and I'm particularly puzzled with a triangle:

CSS Triangle

#triangle-up {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
}
<div id="triangle-up"></div>

How and why does it work?

csscss3geometrypolygoncss-shapes

Answers

answered 7 years ago alex #1

The borders use an angled edge where they intersect (45° angle with equal width borders, but changing the border widths can skew the angle).

Border example

jsFiddle.

By hiding certain borders, you can get the triangle effect (as you can see above by making the different portions different colours). transparent is often used as an edge colour to achieve the triangle shape.

answered 7 years ago sdleihssirhc #2

CSS Triangles: A Tragedy in Five Acts

As alex said, borders of equal width butt up against each other at 45 degree angles:

borders meet at 45 degree angles, content in middle

When you have no top border, it looks like this:

no top border

Then you give it a width of 0...

no width

...and a height of 0...

no height either

...and finally, you make the two side borders transparent:

transparent side borders

That results in a triangle.

answered 7 years ago Mouna Cheikhna #3

Start with a basic square and borders. Each border will be given a different color so we can tell them apart:

.triangle {
    border-color: yellow blue red green;
    border-style: solid;
    border-width: 200px 200px 200px 200px;
    height: 0px;
    width: 0px;
}

which gives you this:

square with four borders

But there's no need for the top border, so set its width to 0px. Now our border-bottom of 200px will make our triangle 200px tall.

.triangle {
    border-color: yellow blue red green;
    border-style: solid;
    border-width: 0px 200px 200px 200px;
    height: 0px;
    width: 0px;
}

and we will get this:

bottom half of square with four borders

Then to hide the two side triangles, set the border-color to transparent. Since the top-border has been effectively deleted, we can set the border-top-color to transparent as well.

.triangle {
    border-color: transparent transparent red transparent;
    border-style: solid;
    border-width: 0px 200px 200px 200px;
    height: 0px;
    width: 0px;
}

finally we get this:

triangular bottom border

answered 7 years ago HerrSerker #4

Here is an animation in JSFiddle I created for demonstration.

Also see snippet below.

This is an Animated GIF made from a Screencast

Animated Gif of Triangle

transforms = [
         {'border-left-width'   :'30', 'margin-left': '70'},
         {'border-bottom-width' :'80'},
         {'border-right-width'  :'30'},
         {'border-top-width'    :'0', 'margin-top': '70'},
         {'width'               :'0'},
         {'height'              :'0', 'margin-top': '120'},
         {'borderLeftColor'     :'transparent'},
         {'borderRightColor'    :'transparent'}
];


$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
		  for ( var i=0; i < transforms.length; i++ ) {
        $(this)
         .animate(transforms[i], duration)
		  }
    }).end()
}(jQuery))
.border {
    margin: 20px 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>


Random version

/**
 * Randomize array element order in-place.
 * Using Durstenfeld shuffle algorithm.
 */
function shuffleArray(array) {
    for (var i = array.length - 1; i > 0; i--) {
        var j = Math.floor(Math.random() * (i + 1));
        var temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
    return array;
}

transforms = [
         {'border-left-width'   :'30', 'margin-left': '70'},
         {'border-bottom-width' :'80'},
         {'border-right-width'  :'30'},
         {'border-top-width'    :'0', 'margin-top': '70'},
         {'width'               :'0'},
         {'height'              :'0'},
         {'borderLeftColor'     :'transparent'},
         {'borderRightColor'    :'transparent'}
];
transforms = shuffleArray(transforms)



$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
		  for ( var i=0; i < transforms.length; i++ ) {
        $(this)
         .animate(transforms[i], duration)
		  }
    }).end()
}(jQuery))
.border {
    margin: 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>


All at once version

$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
        $(this)
         .animate({'border-top-width': 0            ,
         					 'border-left-width': 30          ,
         					 'border-right-width': 30         ,
         					 'border-bottom-width': 80        ,
         					 'width': 0                       ,
         					 'height': 0                      ,
                   'margin-left': 100,
                   'margin-top': 150,
         					 'borderTopColor': 'transparent',
         					 'borderRightColor': 'transparent',
         					 'borderLeftColor':  'transparent'}, duration)
    }).end()
}(jQuery))
.border {
    margin: 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>

answered 7 years ago PseudoNinja #5

Taking it one step further, using css based on this I added arrows to my back and next buttons (yes I know its not 100% cross-browser, but slick none the less).

.triangle {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
  margin:20px auto;
}

.triangle-down {
  border-bottom:none;
  border-top: 100px solid red;
}

.triangle-left {
  border-left:none;
  border-right: 100px solid red;
  border-bottom: 50px solid transparent;
  border-top: 50px solid transparent;
}

.triangle-right {
  border-right:none;
  border-left: 100px solid red;
  border-bottom: 50px solid transparent;
  border-top: 50px solid transparent;
}

.triangle-after:after {
  width: 0;
  height: 0;
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  border-bottom: 5px solid red;
  margin:0 5px;
  content:"";
  display:inline-block;
}

.triangle-after-right:after {
  border-right:none;
  border-left: 5px solid blue;
  border-bottom: 5px solid transparent;
  border-top: 5px solid transparent;

}

.triangle-before:before {
  width: 0;
  height: 0;
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  border-bottom: 5px solid blue;
  margin:0 5px;
  content:"";
  display:inline-block;
}

.triangle-before-left:before {
  border-left:none;
  border-right: 5px solid blue;
  border-bottom: 5px solid transparent;
  border-top: 5px solid transparent;

}
<div class="triangle"></div>
<div class="triangle triangle-down"></div>
<div class="triangle triangle-left"></div>
<div class="triangle triangle-right"></div>

<a class="triangle-before triangle-before-left" href="#">Back</a>
<a class="triangle-after triangle-after-right" href="#">Next</a>

answered 6 years ago Daniel Imms #6

Consider the below triangle

.triangle {
    border-bottom:15px solid #000;
    border-left:10px solid transparent;
    border-right:10px solid transparent;
    width:0;
    height:0;
}

This is what we are given:

Small triangle output

Why it came out in this shape? The below diagram explains the dimensions, note that 15px was used for the bottom border and 10px was used for left and right.

Large triangle

It's pretty easy to make a right-angle triangle also by removing the right border.

Right angle triangle

answered 5 years ago lima_fil #7

Different approach. With linear gradient (for IE, only IE 10+). You can use any angle:

.triangle {
    margin: 50px auto;
    width: 100px;
    height: 100px;
/* linear gradient */
    background: -moz-linear-gradient(-45deg,  rgba(255,0,0,0) 0%, rgba(255,0,0,0) 50%, rgba(255,0,0,1) 50%, rgba(255,0,0,1) 100%);
 /* FF3.6+ */
    background: -webkit-gradient(linear, left top, right bottom, color-stop(0%,rgba(255,0,0,0)), color-stop(50%,rgba(255,0,0,0)), color-stop(50%,rgba(255,0,0,1)), color-stop(100%,rgba(255,0,0,1)));
 /* Chrome,Safari4+ */
    background: -webkit-linear-gradient(-45deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* Chrome10+,Safari5.1+ */
    background: -o-linear-gradient(-45deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* Opera 11.10+ */
    background: -ms-linear-gradient(-45deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* IE10+ */
    background: linear-gradient(135deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* W3C */;
}
<div class="triangle"></div>

Here is jsfiddle

answered 5 years ago Nedudi #8

I made a sample in enjoycss

http://enjoycss.com/5p#border

you can play with it and see how the thing changes ;)

enter image description here

answered 4 years ago Rai Ammad Khan #9

Lets say we have the following div:

<div id="triangle" />

Now Edit the CSS step-by-step, so you will get clear idea what is happening around

STEP 1: JSfiddle Link:

 #triangle {
        background: purple;
        width :150px;
        height:150PX;
        border-left: 50px solid black ;
        border-right: 50px solid black;
        border-bottom: 50px solid black;
        border-top: 50px solid black;
    }

This is a simple div. With a very simple CSS. So a layman can understand. Div has dimensions 150 x 150 pixels with the border 50 pixels. The image is attached:

enter image description here

STEP 2: JSfiddle Link:

#triangle {
    background: purple;
    width :150px;
    height:150PX;
    border-left: 50px solid yellow ;
    border-right: 50px solid green;
    border-bottom: 50px solid red;
    border-top: 50px solid blue;
}

Now I just changed the border-color of all 4 sides. The image is attached.

enter image description here

STEP:3 JSfiddle Link:

#triangle {
    background: purple;
    width :0;
    height:0;
    border-left: 50px solid yellow ;
    border-right: 50px solid green;
    border-bottom: 50px solid red;
    border-top: 50px solid blue;
}

Now I just changed the height & width of div from 150 pixels to zero. The image is attached

enter image description here

STEP 4: JSfiddle:

#triangle {
    background: purple;
    width :0px;
    height:0px;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 50px solid red;
    border-top: 50px solid transparent;
}

Now I have made all the borders transparent apart from the bottom border. The image is attached below.

enter image description here

STEP 5: JSfiddle Link:

#triangle {
    background: white;
    width :0px;
    height:0px;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 50px solid red;
    border-top: 50px solid transparent;
}

Now I just changed the background color to white. The image is attached.

enter image description here

Hence we got the triangle we needed.

answered 4 years ago web-tiki #10

Different approach:

CSS3 triangles with transform rotate

Triangular shape is pretty easy to make using this technique. For people who prefer to see an animation explaining how this technique works here it is :

gif animation : how to make a triangle with transform rotate

Otherwise, here is detailed explanation in 4 acts (this is not a tragedy) of how to make an isosceles right-angled triangle with one element.

  • Note 1 : for non isosceles triangles and fancy stuff, you can see step 4.
  • Note 2 : in the following snippets, the vendor prefixes aren't included. they are included in the codepen demos.
  • Note 3 : the HTML for the following explanation is always : <div class="tr"></div>

STEP 1 : Make a div

Easy, just make sure that width = 1.41 x height. You may use any techinque (see here) including the use of percentages and padding-bottom to maintain the aspect ratio and make a responsive triangle. In the following image, the div has a golden yellow border.

In that div, insert a pseudo element and give it 100% width and height of parent. The pseudo element has a blue background in the following image.

Making a CSS triangle with transform roate step 1

At this point, we have this CSS :

.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
}

.tr: before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #0079C6;
}

STEP 2 : Let's rotate

First, most important : define a transform origin. The default origin is in the center of the pseudo element and we need it at the bottom left. By adding this CSS to the pseudo element :

transform-origin:0 100%; or transform-origin: left bottom;

Now we can rotate the pseudo element 45 degrees clockwise with transform : rotate(45deg);

Creating a triangle with CSS3 step 2

At this point, we have this CSS :

.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
}

.tr:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #0079C6;
    transform-origin: 0 100%;        
    transform: rotate(45deg);
}

STEP 3 : hide it

To hide the unwanted parts of the pseudo element (everything that overflows the div with the yellow border) you just need to set overflow:hidden; on the container. after removing the yellow border, you get... a TRIANGLE! :

DEMO

CSS triangle

CSS :

.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
    overflow: hidden;
}

.tr:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: #0079C6;
    transform-origin: 0 100%;
    transform: rotate(45deg);
}

STEP 4 : go further...

As shown in the demo, you can customize the triangles :

  1. Make them thinner or flatter by playing with skewX().
  2. Make them point left, right or any other direction by playing with the transform orign and rotation direction.
  3. Make some reflexion with 3D transform property.
  4. Give the triangle borders
  5. Put an image inside the triangle
  6. Much more... Unleash the powers of CSS3!

Why use this technique?

  1. Triangle can easily be responsive.
  2. You can make a triangle with border.
  3. You can maintain the boundaries of the triangle. This means that you can trigger the hover state or click event only when the cursor is inside the triangle. This can become very handy in some situations like this one where each triangle can't overlay it's neighbours so each triangle has it's own hover state.
  4. You can make some fancy effects like reflections.
  5. It will help you understand 2d and 3d transform properties.

Why not use this technique?

  1. The main drawback is the browser compatibility, the 2d transform properties are supported by IE9+ and therefore you can't use this technique if you plan on supporting IE8. See CanIuse for more info. For some fancy effects using 3d transforms like the reflection browser support is IE10+ (see canIuse for more info).
  2. You don't need anything responsive and a plain triangle is fine for you then you should go for the border technique explained here : better browser compatibility and easier to understand thanks to the amaizing posts here.

answered 4 years ago user133408 #11

And now something completely different...

Instead of using css tricks don't forget about solutions as simple as html entities:

&#9650;

Result:

See: What are the HTML entities for up and down triangles?

answered 4 years ago Doml The-Bread #12

here is another fiddle:

.container:after {
    position: absolute;
    right: 0;
    content: "";
    margin-right:-50px;
    margin-bottom: -8px;
    border-width: 25px;
    border-style: solid;
    border-color: transparent transparent transparent #000;
    width: 0;
    height: 0;
    z-index: 10;
    -webkit-transition: visibility 50ms ease-in-out,opacity 50ms ease-in-out;
    transition: visibility 50ms ease-in-out,opacity 50ms ease-in-out;
    bottom: 21px;
}
.container {
    float: left;
    margin-top: 100px;
    position: relative;
    width: 150px;
    height: 80px;
    background-color: #000;
}

.containerRed {
    float: left;
    margin-top: 100px;
    position: relative;
    width: 100px;
    height: 80px;
    background-color: red;
}

https://jsfiddle.net/qdhvdb17/

answered 3 years ago vsync #13

SASS (SCSS) triangle mixin

I wrote this to make it easier (and DRY) to automatically generate a CSS triangle:

// Triangle helper mixin (by Yair Even-Or)
// @param {Direction} $direction - either `top`, `right`, `bottom` or `left`
// @param {Color} $color [currentcolor] - Triangle color
// @param {Length} $size [1em] - Triangle size
@mixin triangle($direction, $color: currentcolor, $size: 1em) {
  $size: $size/2;
  $transparent: rgba($color, 0);
  $opposite: (top:bottom, right:left, left:right, bottom:top);

  content: '';
  display: inline-block;
  width: 0;
  height: 0;
  border: $size solid $transparent;
  border-#{map-get($opposite, $direction)}-color: $color;
  margin-#{$direction}: -$size;
}

use-case example:

span {
  @include triangle(bottom, red, 10px);
}

Playground page


Important note:
if the triangle seems pixelated in some browsers, try one of the methods described here.

answered 3 years ago Tom Sarduy #14

This is an old question, but I think will worth it to share how to create an arrow using this triangle technique.

Step 1:

Lets create 2 triangles, for the second one we will use the :after pseudo class and position it just below the other:

2 triangles

.arrow{
    width: 0;
    height: 0;
    border-radius: 50px;
    display: inline-block;
    position: relative;
}

    .arrow:after{
        content: "";
        width: 0;
        height: 0;
        position: absolute;
    }


.arrow-up{
     border-left: 50px solid transparent;
     border-right: 50px solid transparent;
     border-bottom: 50px solid #333;
}
    .arrow-up:after{
         top: 5px;
         border-left: 50px solid transparent;
         border-right: 50px solid transparent;
         border-bottom: 50px solid #ccc;
         right: -50px;
    }
<div class="arrow arrow-up"> </div>

Step 2

Now we just have to set the predominant border color of the second triangle to the same color of the background:

enter image description here

.arrow{
    width: 0;
    height: 0;
    border-radius: 50px;
    display: inline-block;
    position: relative;
}

    .arrow:after{
        content: "";
        width: 0;
        height: 0;
        position: absolute;
    }


.arrow-up{
     border-left: 50px solid transparent;
     border-right: 50px solid transparent;
     border-bottom: 50px solid #333;
}
    .arrow-up:after{
         top: 5px;
         border-left: 50px solid transparent;
         border-right: 50px solid transparent;
         border-bottom: 50px solid #fff;
         right: -50px;
    }
<div class="arrow arrow-up"> </div>

Fiddle with all the arrows:
http://jsfiddle.net/tomsarduy/r0zksgeu/

answered 3 years ago HelloWorldNoMore #15

Others have already explained this well. Let me give you an animation which will explain this quickly: http://codepen.io/chriscoyier/pen/lotjh

Here is some code for you to play with and learn the concepts.

HTML:

<html>
  <body>
    <div id="border-demo">
    </div>
  </body>
</html>

CSS:

/*border-width is border thickness*/
#border-demo {
    background: gray;
    border-color: yellow blue red green;/*top right bottom left*/
    border-style: solid;
    border-width: 25px 25px 25px 25px;/*top right bottom left*/
    height: 50px;
    width: 50px;
}

Play with this and see what happens. Set height and width to zero. Then remove top border and make left and right transparent, or just look at the code below to make a css triangle:

#border-demo {
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 100px solid blue;
}

answered 3 years ago Stewartside #16

CSS clip-path

This is something I feel this question has missed; clip-path

clip-path in a nutshell

Clipping, with the clip-path property, is akin to cutting a shape (like a circle or a pentagon) from a rectangular piece of paper. The property belongs to the “CSS Masking Module Level 1” specification. The spec states, “CSS masking provides two means for partially or fully hiding portions of visual elements: masking and clipping”.


clip-path will use the element itself rather than its borders to cut the shape you specify in its parameters. It uses a super simple percentage based co-ordinate system which makes editing it very easy and means you can pick it up and create weird and wonderful shapes in a matter of minutes.


Triangle Shape Example

div {
  -webkit-clip-path: polygon(50% 0%, 0% 100%, 100% 100%);
  clip-path: polygon(50% 0%, 0% 100%, 100% 100%);
  background: red;
  width: 100px;
  height: 100px;
}
<div></div>


Downside

It does have a major downside at the moment, one being it's major lack of support, only really being covered within -webkit- browsers and having no support on IE and only very partial in FireFox.


Resources

Here are some useful resources and material to help better understand clip-path and also start creating your own.

answered 1 year ago Alireza #17

OK, this triangle will get created because of the way that borders of the elements work together in HTML and CSS...

As we usually use 1 or 2px borders, we never notice that borders make a 45° angles to each others with the same width and if the width changes, the angle degree get changed as well, run the CSS code I created below:

.triangle {
  width: 100px;
  height: 100px;
  border-left: 50px solid black;
  border-right: 50px solid black;
  border-bottom: 100px solid red;
}
<div class="triangle">
</div>

Then in the next step, we don't have any width or height, something like this:

.triangle {
  width: 0;
  height: 0;
  border-left: 50px solid black;
  border-right: 50px solid black;
  border-bottom: 100px solid red;
}
<div class="triangle">
</div>

And now we make the left and right borders invisible to make our desirable triangle as below:

.triangle {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
}
<div class="triangle"></div>

If you not willing to run the snippet to see the steps, I've created an image sequence to have a look at all steps in one image:

enter image description here

answered 6 months ago Temani Afif #18

Almost all the answers focus on the triangle built using border so I am going to elaborate the linear-gradient method (as started in the answer of @lima_fil).

Using a degree value like 45° will force us to respect a specific ratio of size in order to obtain the triangle we want and this won't be responsive:

.tri {
  width:100px;
  height:100px;
  background:linear-gradient(45deg, transparent 50%,red 0);
  
  /*To illustrate*/
  border:1px solid;
}
Good one
<div class="tri"></div>
bad one
<div class="tri" style="width:150px"></div>
bad one
<div class="tri" style="height:30px"></div>

Instead of doing this we should consider predefined values of direction like to bottom, to top, etc. In this case we can obtain any kind of triangle shape while keeping it responsive.

1) Rectangle triangle

To obtain such triangle we need one linear-gradient and a diagonal direction like to bottom right, to top left, to bottom left, etc

.tri-1,.tri-2 {
  display:inline-block;
  width:100px;
  height:100px;
  background:linear-gradient(to bottom left, transparent 50%,red 0);
  border:1px solid;
  animation:change 2s linear infinite alternate;
}
.tri-2 {
  background:linear-gradient(to top right, transparent 50%,red 0);
  border:none;
}

@keyframes change {
  from {
    width:100px;
    height:100px;
  }
  to {
    height:50px;
    width:180px;
  }
}
<div class="tri-1"></div>
<div class="tri-2"></div>

2) isosceles triangle

For this one we will need 2 linear-gradient like above and each one will take half the width (or the height). It's like we create a mirror image of the first triangle.

.tri {
  display:inline-block;
  width:100px;
  height:100px;
  background-image:
  linear-gradient(to bottom right, transparent 50%,red 0),
  linear-gradient(to bottom left, transparent 50%,red 0);
  background-size:50.9% 100%; /* I use a value slightly bigger than 50% to avoid having a small gap between both gradient*/
  background-position:left,right;
  background-repeat:no-repeat;
  
  animation:change 2s linear infinite alternate;
}


@keyframes change {
  from {
    width:100px;
    height:100px;
  }
  to {
    height:50px;
    width:180px;
  }
}
<div class="tri"></div>

3) equilateral triangle

This one is a bit tricky to handle as we need to keep a relation between the height and width of the gradient. We will have the same triangle as above but we will make the calculation more complex in order to transform the isosceles triangle to an equilateral one.

To make it easy, we will consider that the width of our div is known and the height is big enough to be able to draw our triangle (height >= width).

enter image description here

We have our two gradient g1 and g2, the blue line is the width of the div w and each gradient will have 50% of it (w/2) and each side of the triangle sould be equal to w. The green line is the height of both gradient hg and we can easily obtain the formula below:

(w/2)² + hg² = w² ---> hg = (sqrt(3)/2) * w ---> hg = 0.866 * w

So we need to rely on cacl() in order to do our calculation and to obtain the needed result:

.tri {
  --w:100px;
  width:var(--w);
  height:100px;
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 50%,red 0),
  linear-gradient(to bottom left, transparent 50%,red 0);
  background-size:calc(var(--w)/2 + 0.5%)  calc(0.866 * var(--w));
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
  
}
<div class="tri"></div>
<div class="tri" style="--w:80px"></div>
<div class="tri" style="--w:50px"></div>

4) Random triangle

To obtain a random triangle, it's easy as we simply need to remove the condition of 50% of each one BUT we should keep two condition (both should have the same height and the sum of both width should be 100%).

.tri-1 {
  width:100px;
  height:100px;
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 50%,red 0),
  linear-gradient(to bottom left, transparent 50%,red 0);
  background-size:20% 60%,80% 60%;
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
  
 
}
<div class="tri-1"></div>

But what if we want to define a value for each side? We simply need to do calculation again!

enter image description here

Let's define hg1 and hg2 as the height of our gradient (both are equal to the red line) then wg1 and wg2 as the width of our gradient (wg1 + wg2 = a). I will not going to detail the calculation but at then end we will have:

wg2 = (a²+c²-b²)/(2a)
wg1 = a - wg2
hg1 = hg2 = sqrt(b² - wg1²) = sqrt(c² - wg2²)

Now we have reached the limit of CSS as even with calc() we won't be able to implement this so we simply need to gather the final result manually and use them as fixed size:

.tri {
  --wg1: 20px; 
  --wg2: 60px;
  --hg:30px; 
  width:calc(var(--wg1) + var(--wg2));
  height:100px;
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 50%,red 0),
  linear-gradient(to bottom left, transparent 50%,red 0);

  background-size:var(--wg1) var(--hg),var(--wg2) var(--hg);
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
  
}
<div class="tri" ></div>

<div class="tri" style="--wg1:80px;--wg2:60px;--hg:100px;" ></div>

Bonus

We should not forget that we can also apply rotation and/or skew and we have more option to obtain more triangle:

.tri {
  --wg1: 20px; 
  --wg2: 60px;
  --hg:30px; 
  width:calc(var(--wg1) + var(--wg2));
  height:100px;
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 50%,red 0),
  linear-gradient(to bottom left, transparent 50%,red 0);

  background-size:var(--wg1) var(--hg),var(--wg2) var(--hg);
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
  
}
<div class="tri" ></div>

<div class="tri" style="transform:skewY(25deg)"></div>

<div class="tri" style="--wg1:80px;--wg2:60px;--hg:100px;" ></div>


<div class="tri" style="--wg1:80px;--wg2:60px;--hg:100px;transform:rotate(20deg)" ></div>

And of course we should keep in mind the SVG solution which can be more suitable in some situation:

svg {
 width:100px;
 height:100px;
}

polygon {
  fill:red;
}
<svg viewBox="0 0 100 100"><polygon points="0,100 0,0 100,100" /></svg>
<svg viewBox="0 0 100 100"><polygon points="0,100 50,0 100,100" /></svg>
<svg viewBox="0 0 100 100"><polygon points="0,100 50,23 100,100" /></svg>
<svg viewBox="0 0 100 100"><polygon points="20,60 50,43 80,100" /></svg>

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