Update your font-size to REM units

Elad Shechter
2 min readMay 13, 2015

A lot of websites are using pixel units that are difficult to adjust in the increasing variety of different devices.

CSS3 introduces a few new units, including the REM unit, which stands for “root em”.

So, how are we using REM ?

let’s say we have this CSS:


article h2 {font-size:20px;}
article p {font-size:12px;}

First of all we need to decide what size of px will be relative to all font’s. and to make it easy, the best practice I have done is to make root font-size 1px like this:


html {font-size:1px;}

Second of all we need to replace the rest of the font size values from pixel to rem units.


article h2 {font-size:20rem;}
article p {font-size:12rem;}

What REM does, it takes 20REM and Multiply it with the root element:

20 REM * 1 PX = 20PX.

Browser support

IE7 & IE8 still need to use PX values. This will force us to write font size twice, one time in PX and second time with REM.


article h2 {font-size:20px; font-size:20rem;}
article p {font-size:12px; font-size:12rem;}


Why REM units are good for you?

Lets assume we need to enlarge all fonts in our website by 20%, all we have to do is to change the size of the font-size in the root element, like this:

html {font-size:1.2px;}

If you wand to reduce by 20%, you will do it like this:


Really nice and easy!

REM for Responsive Design

When you want to change all font size according to breakpoints in your responsive design it is a lot more easy. See example:

@media (min-width: 320px){
html{ font-size:1.4px; }
@media (min-width: 600px){
html{ font-size:1.2px; }

Now in smaller screens we re-size all fonts by 40% bigger, and in medium screens we re-size it by 20% bigger.

Using less to resolve the needs of writing everything twice — for supporting old browsers

in less or sass you can add functions to save you the time of writing everything twice.

less — font-size function and calling It

.font-size(@font-size) { 
font-size : @font-size * 1px;
font-size : @font-size * 1rem;

article h2 { .font-size(20); }

The compiled CSS will look like this:

article h2{
font-size:20px;/*Support IE7 & IE8*/


If you like this tip, I will be happy to get your LIKE. You are welcome to follow me and endorse my skills.

Elad Shechter

Originally published at coderwall.com by me, on May 28, 2013.