21 px in rem — 1.3125 rem, 15.75 pt
21 px is 1.3125 rem and 15.75 pt — 0.21875 inches, 5.556 mm, and 131.25% of the root font size.
- rem (root 16 px)
- 1.3125rem
- pt (points)
- 15.75pt
- pc (picas)
- 1.3125pc
- percent of root
- 131.25%
- inches
- 0.21875in
- millimetres
- 5.556mm
- centimetres
- 0.5556cm
Why rem divides by 16
Because browsers default the root font size to 16 px, and rem is a multiple of it. When a reader enlarges text in their browser settings, only the parts written in rem grow — anything pinned in px stays put.
Nearby sizes
On screen, pt and inches are still pixels
CSS fixes one inch at 96 px and one point at 1/72 inch. So 16 px is exactly 12 pt — but hold a ruler to the screen and that "inch" will not measure an inch.
Sizes you meet most
Body text at 16, small text at 14, spacing at 8, 24 and 32 — the values that keep coming back.
How to read this
- rem = px ÷ 16, because browsers default the root font size to 16 px.
- pt = px × 0.75, since CSS fixes an inch at 96 px and a point at 1/72 inch.
- One pc is 12 pt, and one inch is both 96 px and 72 pt.
- Use rem for text and spacing; keep px for hairlines such as borders.
Frequently asked questions
Q. How many rem is 21 px?
1.3125 rem — that is 21 divided by 16.
Q. How many points is 21 px?
15.75 pt. A point is 1/72 inch and an inch is 96 px, so you multiply by 0.75.
Q. What length is that?
0.21875 inches, or 5.556 mm — though a ruler on the screen will not agree.
Q. And as a percentage?
131.25% of the root font size.