clamp() Calculator

Handy little tool for calculating viewport-based clamped values for use in CSS.

Visual Example
  • Root Size: 16px
  • Clamped Size: 16–18px
  • Current Size: 16px
  • Viewport Size: 9001px

How does this work?

This calculator uses rem units which require knowing the font size at the document’s root. In most cases, that refers to the html element and this calculator is currently operating with a root font size of 16px.

We need to figure out the rate of change: how much the size needs to change across a distance across the viewport. In other words, for every pixel of change across the viewport, how many pixels of change should there be in size? We can calculate this by dividing the difference in size by the difference in viewport.

Change = (sizeMax - sizeMin) / (viewportMax - viewportMin)
Change = (18px - 16px) / (1000px - 500px)
Change = 0.004

Next, let’s figure out what our preferred size should be (this is different from our minimum font size). We do this by asking how much change is applied to the size when the viewport is at its maximum size, and subtracting that from our desired size at the maximum viewport size.

A = sizeMax - viewportMax * Change
A = 18px - 1000px * 0.004
A = 14px = 0.875rem

The last piece of the puzzle is the most straightforward: we need to represent the rate of change as a value relative to the viewport, and we do so by multiplying the rate of change with 100vw.

B = 100vw * Change
B = 100vw * 0.004
B = 0.4vw

Let’s put everything together with the clamp() function, otherwise our calculated value will be incorrect above and below our designated viewport bounds.

Result = clamp(sizeMin, A + B, sizeMax)
Result = clamp(1rem, 0.875rem + 0.4vw, 1.125rem)

We can perform a simple check that our calculations are correct by plugging in our minimum and maximum viewport values and seeing if they result in our desired minimum and maximum sizes.

Let’s not forget that viewport-based units (vw, vh, etc.) are kind of like a percentage, which we can factor into our calculations by dividing B by 100 when figuring out the true pixel-value result.

Note: Because of rounding, there may be some loss of accuracy around the 3rd decimal place in these calculations, but the browser will interpret them well.

A + B = 0.875rem + 0.4vw
A + B = 14px + 0.4vw

Minimum Size ≈ 14px + (0.004 * 500px)
Minimum Size ≈ 14px + 2px
Minimum Size ≈ 16px

Maximum Size ≈ 14px + (0.004 * 1000px)
Maximum Size ≈ 14px + 4px
Maximum Size ≈ 18px

Learn more

Wondering why we don’t need calc() around the middle value of the clamp() function? Michelle Barker’s got you covered with this quick tip, and check out clamp() on MDN to learn more.

Shoutout to James Gilyead and Trys Mudford for building Utopia which first introduced me to this amazing technique! Utopia helps you in calculating fluid type, space, and grids for fluid responsive designs, and includes tonnes of great information about Utopian Design.