rem unit

A size equal to the root (html) font size, usually 16px.

What is the rem unit?

A rem is the font size of the root <html> element (usually 16px). Unlike em, it doesn't change with nesting, and it respects the user's font settings.

h1 { font-size: 2rem; }      /* 32px by default */
.card { padding: 1.5rem; }   /* 24px */

Level: Novice · More CSS terms at this level

Updated September 27, 2026