Font Size Converter (px, pt, em, rem, %)
Convert a font size between px, pt, em, rem, %, picas and mm at the root and parent sizes you set, with ready-to-paste CSS declarations for each unit.
The same font size in every CSS and print unit — including em and rem that respect your actual root and parent sizes — with the declaration ready to copy.
Example: 18 px at a 16 px root is 1.125 rem, 1.125 em, 112.5 % and 13.5 pt; the same 18 px inside a 20 px parent is 0.9 em while rem stays 1.125.
Absolute units,
relative units, one pixel.
How px, pt and mm relate, why em and rem need a reference size, and what the converter assumes.
Absolute units
CSS defines the pixel as 1/96 of an inch and the point as 1/72 of an inch, so 1 pt = 1.333 px and 12 pt = 16 px exactly; a pica is 12 pt and a millimetre is 96/25.4 px. These are the CSS reference units, not the physical dots of a screen — a phone with a 3× display renders a CSS pixel as nine device pixels. The conversions here follow CSS Values and Units Level 4 and are exact; print software that uses a different pixel definition (72 px per inch in some older tools) will disagree by a third.
em, rem and %
rem is a multiple of the root element's font size and em a multiple of the parent's; a percentage is em × 100. Both reference sizes are inputs — 16 px is prefilled because that is what browsers ship with, but a user who has raised the browser font size, or a stylesheet that sets html to 62.5 %, changes the rem basis, and any nested element with its own font-size changes the em basis. The warning appears whenever the two references differ so that em and rem are not mixed up.
What it does not do
It converts lengths; it does not judge legibility or accessibility. Sizes below about 9 px are flagged because many browsers apply a minimum font size, but the arithmetic is unchanged. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
Last reviewed 19 September 2026. How results are checked: How we verify.