REM vs EM vs PX — Which CSS Unit Should You Use in 2025?

Picking the right CSS unit affects accessibility, scalability, and how predictable your layouts feel. Here's a clear comparison of px, em, and rem.

Use the PX ⇄ REM Converter →

PX — absolute and predictable

Pixels are fixed. Great for borders and hairlines, but they ignore user font-size preferences.

EM — relative to the parent

EMs compound with nesting. Powerful for component-local scaling, but easy to lose track of.

REM — relative to the root

REMs stay predictable and respect the user's root font-size. Best default for typography and spacing.

Recommendation

Use rem for type and spacing, em when you want a component to scale with its own font-size, and px for borders.