How is px related to physical units like pt and in?

CSS fixes 1 inch to 96px, so 1pt = 96/72 ≈ 1.333px, 1pc = 16px, 1cm ≈ 37.795px and 1mm ≈ 3.7795px. These are reference pixels, not necessarily the device's physical pixels.

Why do rem and em need a font size?

rem is relative to the root element's font size and em is relative to the parent element's font size. With the default 16px root, 1rem = 16px; change the root or parent fields to match your CSS.

Is anything uploaded?

No. All conversions run entirely in your browser — nothing you enter is sent to a server.

CSS Unit Converter

Convert between px, rem, em, pt, pc and physical CSS units.

CSS unit converter

Convert a length between every CSS unit at once: px, rem, em, pt, pc, in, cm, mm and Q. The absolute units are anchored to the CSS rule that 1 inch equals 96px, while rem and em use the root and parent font sizes you set (both default to 16px).

Enter a value, pick its unit, and read the equivalent in every other unit instantly. Everything runs in your browser; nothing is uploaded.