What clamp() actually does#
clamp(min, preferred, max) picks the middle value clamped between the other two — it grows or shrinks with the preferred value (usually viewport-relative) but never goes below the min or above the max. The tricky part isn't the clamp() syntax itself, it's the preferred value in the middle: a vw-based expression whose coefficient has to be calculated from the actual slope between your min and max sizes across your chosen viewport range, and that's the part this tool works out.
Why the middle term looks like "1rem + 2.5vw", not a plain vw#
A pure vw value scales to zero at a zero-width viewport, which is never actually what you want — you want a size that scales but never collapses. The formula adds a fixed offset (the point where the line defined by your two size/viewport pairs would cross a zero-width viewport) to a vw term, so the two together reproduce a straight line that passes exactly through your min size at your min viewport and your max size at your max viewport.
rem versus px for the output#
rem output respects a user's browser font-size setting — someone who has bumped their default text size up gets a proportionally larger clamp() range too, which is the accessible default. px output is exact and predictable but ignores that setting entirely; it makes sense for things like a fixed-width icon or spacing value that genuinely should not respond to text-size preferences.