Where do these numbers come from?
- NISTSP 811 exact unit definitions (1 in = 25.4 mm, 1 lb = 453.59237 g)current edition
- NISTHandbook 44, weights and measurescurrent edition
- King Arthur BakingIngredient Weight Chart (kitchen tools)retrieved Oct 23, 2026
Checked Oct 23, 2026. 1 inch is exactly 25.4 mm (NIST SP 811), so 100 mm ÷ 25.4 = 3.937 in.
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21 mm = 0.8268 in
Worked example
21 ÷ 25.4 = 0.8268 inches. On a ruler marked in sixteenths, that is about 13/16″; in thirty-seconds, 13/16″; in sixty-fourths, 53/64″. The nearest 1/16 mark (13/16″ = 20.64 mm) is off by 0.36 mm.
21 mm is also 2.1 cm.
| Millimeters | Inches | Nearest 1/16 |
|---|---|---|
| 16 mm | 0.6299 | 5/8″ |
| 17 mm | 0.6693 | 11/16″ |
| 18 mm | 0.7087 | 11/16″ |
| 19 mm | 0.748 | 3/4″ |
| 20 mm | 0.7874 | 13/16″ |
| 21 mm | 0.8268 | 13/16″ |
| 22 mm | 0.8661 | 7/8″ |
| 23 mm | 0.9055 | 7/8″ |
| 24 mm | 0.9449 | 15/16″ |
| 25 mm | 0.9843 | 1″ |
| 26 mm | 1.0236 | 1″ |
Frequently asked questions
How many inches is 21 mm?
0.8268 inches, about 13/16″.
What fraction of an inch is 21 mm?
About 13/16″ to the nearest 1/32.