Viewport-relative, scalable text

When creating fluid designs, controlling the line length is a challenge. You don't want your lines to expand indefinitely, making them unreadable, even if the layout can expand indefinitely. This article details how I solved the problem on

max-width is old-school

The traditional approach is to set a max-width on your content element. This works great, but can create way too much whitespace on large viewports, leaving your content floating in the middle of a sea of whitespace.

When I redesigned to be fluid and responsive and otherwise make it buzzword compliant, I opted for a somewhat different approach.

I used viewport-relative, scalable text sizing.

Try resizing your browser window. Apart from your run-of-the-mill responsiveness at smaller displays, the text starts growing larger as your viewport increases above a certain threshold.

vw: Viewport width

The work horse behind this trick is vw - a CSS unit that is relative to the viewport. This means that 1vw is equal to 1% of the viewports width (and 1vh is equal to 1% of the height).

Where we’re used to setting the width of an element to a specific amount of pixels (using 50px), we can nowadays use 50vw to set the width to 50% of the viewports width.

This unit naturally works for font-sizes as well. So if your viewport is 1000 pixels wide and you set font-size: 1em, you have effectively set the font-size to 10px (1% of 1000 pixels). If the viewport is then changed to only be 500 pixels wide, the font size will change to 5px.

Using this one, weird trick…

Setting our font-size relative to the viewport width has the effect of ensuring or content always has the same size - relative to the viewport.

Line lengths stay the same, word wrapping appears exactly in the same place - all regardless of the width of the browser.

This isn’t just for text

Scaling your font size with your viewport is intriguing for sure, but the potential is even greater. It also proves vastly more reliable than previous techniques for making your layout fluid.

Percentage units go a long way for making fluid layouts. The problem is they are measured relatively to their parent elements. Which means that when you are 4 levels deep in parent elements it gets really tricky figuring out exactly what those 2% are going to be.

And good luck trying to size out borders, margins and paddings using percentages. It is doable, but it quickly ends up being unwieldy.

Viewport-relative units give us the benefits from both fixed units like px and em and relative units like %, practically without any of the drawbacks.


What I have found to be the most effective way of controlling your scalable design is to have one central, scalable font-size setting.

That base setting is the one you make scalable using the vw unit. Every other font-sizes is made relative to that base, using another favourite of mine; the good old em unit.

This ensures that we get to control the overall size of our design in just one place. If we need to make exceptions and changes using media queries, this also provides a single location to do it.


Now, scaling your designs with the width of your users viewport does come with some drawbacks.

  1. You don’t actually know what size your site is going to be, so you’ll want to make sure to scale everything; images in particular.
  2. If your visitors have wide displays with wide browsers your font sizes can grow to levels suitable for billboards. These are outliers, though, and it should be fairly easy to remedy with a media query.
  3. Browser support isn’t 100% yet. It’s currently clocking in at only 77%. Luckily the fallback is reasonably easy and you could simple use px or em for those browsers without support for vw.

If you want to read more about this, CSS-tricks has a fine introduction to viewport-sized typography.