Guide: EM vs REM vs PX. Which should you use?

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The delta will always be uniform irrespective of the measurement used, but the scale factor providing the maths are done correctly will adjust as needed for say a widescreen monitor vs a standard monitor, or rectangle pixels vs square pixels. Ems are becoming increasingly popular in web documents due to scalability and their mobile-device-friendly nature.

The markup of the bigger header element might be something like this: header! Generally Always ensure height is not set on text blocks. Just by changing the font size of the container div, we can see that the paragraph font size updated accordingly.

Confused About REM and EM?

The question is, which should you use. It makes complete sense if we substituted point-size with font-size though. We have to look at the parent element in order to compute the 's font-size. Instead, you can either choose to use a percentage value, or leave out the font-size declaration entirely. Note: font-size will be set to 100% if you left it out entirely. This is where many people start to get confused with em values. What should the margin-bottom value be for both the and elements. Assume font-size of is set to 100%. Since the font-size in is now set to 2em. What throws people off is that 1em can take on different values in different parts of the code. It is a unit of typography equal to the root font-size. This means 1rem is always equal to the font-size defined in. Consider the same code above, written in rems instead. What are the computed margin-bottom values now. Some developers avoid rem entirely, claiming that using rem make their components less modular. Others use rem for everything, font size rem the simplicity that rem provides. Oddly, I fell into the trap of strictly only rem or em at different points in my development career. I loved how em helped me make modular components, but I loathed the complexity it brought to my code. I also loved font size rem rem made calculations simple, but I hated the hacks I used to make my components modular. Turns out, rem and em have their strengths and weaknesses. They should be used differently, depending on the circumstances. The markup of the bigger header element might be something like this: header. Not enough breathing space between edge and text on this large header If you insist on using only rems, the only way to fix this problem is to redeclare the padding on the large header:. What would happen if we have more headers of different sizes, or if the headers have to change in size. You can already see how coding the entire site in rem can cause duplication and super complex code. This is where the first rule was born. All we have to do is change rem to em. We have to consider how this header interacts with other elements on your page. If you insist on using only em, the only way to fix this problem is to redeclare the padding-left and padding-right properties on the large header:. Yet, the padding-left and padding-right of. Like in the above case, we can simplify the code if you are open to using a combination of rem and em in your code. Specifically, rem for left and right paddings and em for top and bottom paddings:. This was everything we covered in the earlier sections. Moving on, this component can be found in differents of a website. Header components on a grid. The markup would look like this: And the style for this variant is:. As with the header element, you can identify which properties to size in em by seeing if they interact with the rest of the page. If your component behaves in this manner when resized, you need to size everything in ems. Imagine if you had a grid like font size rem. The vertical and horizontal spaces between each grid item needs to remain the same across all devices for of good aesthetics. Equal margins on a 1 + 2 grid. In order words, the computed width of the gutter is 32px. The challenge in this grid is to separate small component A and small component B with a margin of 32px. We can try setting a margin-top of component Font size rem to be 2em for start. The margin between small component A and small component B is larger than the gutter width at a viewport above 800px. Space between the two smaller components isn't equal to the gutters when the screen is wider than 800px Boo : This happens because the font-size of the component is 1. Since the font-size is 24px, the computed value of 2em becomes 48px, which is different from the 32px we were looking for. See the Pen by Zell Liew on. Note: You need to use Flexbox to build this grid. Chris Coyier a year ago. Anyway, are we good so far. Sometimes you just need to tune font size rem small section of your component instead of resizing everything at once. For example, you might want to change only the header font-size at a larger viewport. Font size rem how only the header font-size changes as we resize the browser now. This way, it becomes easy to ensure that your typography remains consistent across all components. Wrapping Up So, should you use rem or em. Both rem and em has their strengths and weaknesses, and they can be used together to help you make simple, modular components. Did this article help you out. If it did, I hope you consider. You might help someone else out.

Always allow scroll down, never to the side. Imagine if you had a grid like this. But then each module on the page has a font-size set in rem. Normally when we look at font sizes, many people often think of pixels px as a unit of size. Additionally, end-users can use hot keys to scale the text in all browsers. While the problems associated with px in Internet Explorer have dissipated, % and em remained the best choices — especially for responsive designs. If it did, I hope you consider. By expressing widths, margins, and padding in rem units, it becomes possible to create an interface that grows or shrinks in tune with the root font size. If you insist on using only em, the only way to fix this problem is to redeclare the padding-left and padding-right properties on the large header:. It is a unit of typography equal to the root font-size. Turns out, rem and em have their strengths and weaknesses. So what I will do here is give a simplified description of the differences between em, rem, %, and px, but I will try not to get too technical to descibe it.