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Katie Langerman
Katie Langerman

Posted on • Originally published at on

Converting Fluid Typography LESS mixin to CSS clamp()

I've been waiting for clamp() browser support for awhile. Finally we're at a point where it feels safe to implement with a fallback for production sites. I spent a day converting an existing fluid type scale to utilize clamp() and documented the process along the way! My goal with this conversion was to keep the compiled sizing as close to the original LESS mixin as possible. A few benefits I already see for using clamp():

  • The syntax is immensely easier to read and write
  • The clamp() definition could live in a CSS custom property, allowing for themes to utilize one type scale file without losing control over individual heading sizes
  • You no longer need to rely on a complicated LESS mixin

Skip ahead to my codepen for the results

out with the old

Below is the original LESS mixin I've been using for years. I've seen this several times on codepen and in various articles, so I'm not sure who to credit. I use rems and assume a base font size of 16px.

    .fluid-typography-rem(@min-screen: 320px, @max-screen: 1600px, @base-font: 16px, @min-font, @max-font, @fallbackFontSize) {
        // min font size
        font-size: @min-font;

        @media screen and (min-width: unit((@min-screen / @base-font), ~'rem')) {
            font-size: ~'@{fallbackFontSize}';
            font-size: calc(
                @min-font ~' + ' unit(@max-font - @min-font) ~' * ((100vw - '
                    unit((@min-screen / @base-font), ~'rem') ~') /' unit(
                        (@max-screen / @base-font) - (@min-screen / @base-font)
                    ) ~')'

        @media screen and (min-width: unit((@max-screen / @base-font), ~'rem')) {
            // max font size
            font-size: @max-font;

    /* the output looks like this for h1 */
    /* minimum size of 25.6px */
    h1 {
        font-size: 1.6rem;

    /* starting at 320px screen width */
    @media screen and (min-width: 20rem) {
        h1 {
            font-size: 1.75rem; // fallback of 28px
            font-size: calc(1.6rem + 0.4 * ((100vw - 20rem) / 70));

    /* anything beyond 1440px */
    @media screen and (min-width: 90rem) {
        h1 {
            font-size: 2rem; // max of 32px

I've always felt that this logic is really hard to follow. Essentially we are setting a min font size, a max font size, and asking the browser to scale up and down fluidly between the two based on the viewport width in the calc function... which is also calculated based on the screen size range and base font size we have specified. I think clamp() is doing the same thing, though I haven't been able to get an exact match between the mixin and clamp. I'm okay with that.

converting to clamp()

Since I already have a fluid type scale, I can reuse those values within my clamp definitions. I still need to support ie11 and other browsers that have yet to support clamp() so I'll be using @supports with a fallback. I found this article really helpful in understanding clamp() and using it with @supports, but I'm going to elaborate on each part of clamp for my own sake (it took me a bit to fully grasp).

clamp() breakdown

Read the spec here

clamp(min, val, max)

min the smallest unit you ever want

max the biggest unit you ever want

val the ideal unit

The middle value is referred to as a preferred value. I found this part a little confusing to figure out. If the computed val is less than min : min value will be used. If the computed val is greater than max : max value will be used.

By setting the preferred value in vw units, we can leverage the "fluid" aspect of relative units. And by using clamp() we can enforce a min and max font-size to regain control over the fluidity whenever necessary (eg: not allowing the font-size to go below or above a certain point.) Having a good understanding of how vw works helps here! As a little overview:

  • vw = 1% of the viewport's width
  • 3vw = 30% of the viewport's width
  • vw and vh are relative units, not absolute, so they scale with the width or height of the viewport (perfect for fluid typography)


Say this is our clamp definition: font-size: clamp(1.6rem, 4vw, 2rem);

If the viewport window is 768px, font-size: 4vw computes to 30.7167px

768 * 0.4 = 307.2 or 30.72px

If that calculated value of 30.72px was higher than the max value we defined in the clamp, the font-size would instead take the max value.

In plain english, clamp says: "Here's my min font-size, here's my max font-size, and here's my PREFERRED font-size. Please use my preferred font-size whenever possible, but don't let it exceed my max font-size or go below my min font-size."

the comparison

From what I can tell, there's a slight difference between the fluid typography mixin logic and clamp(). You can see the difference in the codepen below where I'm printing out each computed font size side by side. Now that I understand how to utilize the val part of clamp() it's quite easy to control the type scale, but trying to match it with logic in a mixin I barely understand was a struggle. Since I have further improvements I'd like to do to this particular type scale, I'm happy with the results being close enough for now!

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