Converters guide
Pixels to rem at a 16 px root (and when the root is not 16)
Designers still spec 16, 18, 24 px. CSS type that should scale with the user wants rem. The default root is 16 px in most browsers until someone changes html { font-size }.
Updated 2026-09-19
Divide by the root you actually shipped
rem = pixels ÷ root font-size. At 16 px, 16 px = 1 rem, 24 px = 1.5 rem, 32 px = 2 rem.
If the design system set html to 62.5% (10 px), then 16 px is 1.6 rem. The converter lets you change the base. Guessing 16 when the codebase is 10 is how a heading jumps a size.
em is not rem
em is relative to the parent’s font size. Nested em compounds. rem is relative to the root. Use the em page when a component should scale with its wrapper, not the document.
CSS px is a reference pixel, not a hardware pixel and not a DTP point. Print still wants pt and inches on those pages.
Key takeaways
- Default math is px ÷ 16.
- Read the real html font-size before you convert a whole type scale.
- em and rem are different relatives.