Understanding CSS Units

CSS Units come in a variety of categories such as font-sizes, character-sizes, viewport dimensions, etc. Broadly there are two categories of absolute and relative units which consist of above mentioned sub categories.

Following are the CSS absolute units −

Sr.No	Unit & Name
1	cm Centimeters (1 cm = 100 mm)
2	in Inches (1 in = 2.54 cm)
3	mm Millimeters
4	pc Picas (1 pc = 12 pt)
5	pt Points (1 pt = 1/72 in)
6	px Pixels (1 px = 0.75 pt)

Let us see an example of CSS absolute units −

Example

<!DOCTYPE html>
<html>
<head>
<title>CSS Absolute Units</title>
<style>
form {
   width:70%;
   margin: 0 auto;
   text-align: center;
}
* {
   padding: 2px;
   margin:5px;
}
input[type="button"] {
   border-radius: 10px;
}
#container {
   display: flex;
}
.child{
   margin: 5px 0;
   height: 40px;
   color: white;
   border-width: 4px 0 4px 4px;
   border-color: black;
   border-style: solid;
}
.child:nth-of-type(1){
   background-color: #FF8A00;
   width:1cm;
}
.child:nth-of-type(2){
   background-color: #F44336;
   width:1in;
}
.child:nth-of-type(3){
   background-color: #C303C3;
   width:1mm;
}
.child:nth-of-type(4){
background-color: #4CAF50;
width:1pc;
}
.child:nth-of-type(5){
   background-color: #03A9F4;
   width:1pt;
}
.child:nth-of-type(6){
   background-color: #FEDC11;
   width:1px;
   border-right-width: 4px;
}
</style>
</head>
<body>
<form>
<fieldset>
<legend>CSS-Absolute-Units</legend>
<div id="container">
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
</div><br>
<div><span class="child">cm</span><span class="child">in</span><span class="child">mm</span><span class="child">pc</span><span class="child">pt</span><span class="child">px</span></div><br>
<input type="number" id="numSelect" value="1" min="1">
<input type="button" onclick="changeWidth()" value="Preview">
<script>
var numSelect = document.getElementById("numSelect");
function changeWidth() {
   var divChild = document.getElementsByClassName('child');
   for(var i=0; i<6; i++){
      divChild[i].style.width = numSelect.value+divChild[i+6].textContent;
}}
</script>
</body>
</html>

Output

Before clicking any button −

Understanding CSS Units

After clicking ‘Preview’ button with number field set −

Understanding CSS Units

Following are the CSS relative units −

Sr.No	Unit & Relative to
1	% Parent element dimensions
2	em Font size of the element
3	ex x-height of the element's font
4	rem Font size of the root element
5	vh 1% of viewport's height
6	vmax 1% of viewport's larger dimension
7	vmin 1% of viewport's smaller dimension
8	vw 1% of viewport's width

Let us see an example of CSS relative units −

Example

<!DOCTYPE html>
<html>
<head>
<title>CSS Relative Units</title>
<style>
form {
   width:70%;
   margin: 0 auto;
   text-align: center;
}
* {
   padding: 2px;
   margin:5px;
}
input[type="button"] {
   border-radius: 10px;
}
#container {
   display: flex;
}
.child{
   margin: 5px 0;
   height: 40px;
   color: white;
   border-width: 4px 0 4px 4px;
   border-color: black;
   border-style: solid;
}
.child:nth-of-type(1){
   background-color: #FF8A00;
   width:1%;
}
.child:nth-of-type(2){
   background-color: #F44336;
   width:1em;
}
.child:nth-of-type(3){
   background-color: #C303C3;
   width:1ex;
}
.child:nth-of-type(4){
   background-color: #4CAF50;
   width:1rem;
}
.child:nth-of-type(5){
   background-color: #03A9F4;
   width:1vh;
}
.child:nth-of-type(6){
   background-color: #FEDC11;
   width:1vw;
   border-right-width: 4px;
}
</style>
</head>
<body>
<form>
<fieldset>
<legend>CSS-Relative-Units</legend>
<div id="container">
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
<div class="child"></div>
</div><br>
<div><span class="child">%</span><span class="child">em</span><span class="child">ex</span><span class="child">rem</span><span class="child">vh</span><span class="child">vw</span></div><br>
<input type="number" id="numSelect" value="1" min="1">
<input type="button" onclick="changeWidth()" value="Preview">
<script>
var numSelect = document.getElementById("numSelect");
function changeWidth() {
   var divChild = document.getElementsByClassName('child');
   for(var i=0; i<6; i++){
      divChild[i].style.width = numSelect.value+divChild[i+6].textContent;
}}
</script>
</body>
</html>

Output

Before clicking any button −

Understanding CSS Units

After clicking ‘Preview’ button with ‘em’ option selected and empty text field −

Understanding CSS Units