Fallbacks for CSS `progress()`

The new CSS progress() function calculates where a value sits between a start and end bound, returning a unitless number between 0 and 1.

While we wait for browser support to grow, we can manually calculate progress and use its underlying mechanics.

The math behind progress(value, start, end) is simple:

progress = (current value - start) / (end - start)

This is also known as normalization.

Depending on the types of values you are working with, implementing this in CSS ranges from a straightforward calc() to some more complicated CSS tricks.

All Unitless Numbers

If you are working purely with unitless numbers (like in Preethi Sam’s article), you can implement the formula directly in calc(). To match the default behavior of progress(), we’ll use clamp().

For example, to find where 50 sits between 10 and 100:

--progress: clamp(0, (50 - 10) / (100 - 10), 1);

That’s it, almost as easy as progress().

Working with Units

Most of the time, we want to use units like px, rem, or vw while still outputting a unitless value.

Browsers that support typed arithmetic can already do this. Both these lines should produce the same result:

--progress: clamp(0, (100vw - 20rem)/(100rem - 20rem), 1);
--progress: progress(100vw, 20rem, 100rem);

Sadly, this still isn’t cross-browser (looking at you, Firefox).

Enter the tan(atan2()) trick.

We could wrap every value in this, but we can optimize it further.

The progress formula requires division, and atan2(y, x) already divides its values. Pass the offset (value - start) as the first argument and the span (end - start) as the second.

If all your values use the same unit, you can plug them straight in:

/* Where does 500px sit between 320px and 1200px? */
--progress: tan(atan2(500px - 320px, 1200px - 320px));

Mixing Units

Usually, your units won’t match. In theory, atan2() should handle mixed units, and in some browsers, it does. But for cross-browser support, we need to do a little more work.

Let’s look at fluid typography.

For fluid type, we want the progress value to be based on the viewport width (100vw), relative to lower and upper bounds (320px and 1200px).

Here we’re mixing vw and px, so we need to convert 100vw to pixels for the math to work everywhere.

We can do this by registering the viewport width as a <length> custom property, with 100vw as its initial value:

@property --vw {
  syntax: "<length>";
  inherits: true;
  initial-value: 100vw;
}

:root {
  --progress: clamp(0, tan(atan2(var(--vw) - 320px, 1200px - 320px)), 1);
}

Registering --vw as a <length> makes the browser resolve 100vw to its px equivalent. Now all values passed to atan2() share the same unit, making this work cross-browser.

The Robust Approach

What if you want to use rem for your bounds?

Registering a custom property as <length> resolves it to pixels; you can’t force it to resolve to rem. To avoid mixing units, we’d have to register every value so they all resolve to pixels.

Fortunately, we can optimize this. We don’t need to register the value, start, and end separately, only the two values passed to atan2(): the offset and the span.

Let’s make those values custom properties and register them with @property:

@property --offset {
  syntax: "<length>";
  inherits: true;
  initial-value: 0px;
}

@property --span {
  syntax: "<length>";
  inherits: true;
  initial-value: 0px;
}

:root {
  --offset: calc(100vw - 20rem);
  --span: calc(80rem - 20rem);
  --progress: clamp(0, tan(atan2(var(--offset), var(--span))), 1);
}

And that’s it: the same calculation as progress(), with much better browser support.

If you want to emulate the unclamped version of progress(), remove the clamp().

Which method should you use?

Your approach depends on the units you are using:

All unitless numbers: Calculate the value directly with calc() or clamp().
Using units: Use the tan(atan2()) trick.
One unit: You don’t need to register any custom properties.
Pixels + one non-pixel unit: Register the non-pixel value as a <length>. This is the most common case in fluid typography.
Multiple non-pixel units: Make the offset and span custom properties, register both as <length>, and pass them to atan2().

The progress() function is already in modern browsers, so it won’t be long until you can use it natively without worry. Until then, hopefully this helps you choose the simplest fallback for your situation.