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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31 mm = 1.2205 in
Worked example
31 ÷ 25.4 = 1.2205 inches. On a ruler marked in sixteenths, that is about 1 1/4″; in thirty-seconds, 1 7/32″; in sixty-fourths, 1 7/32″. The nearest 1/16 mark (1 1/4″ = 31.75 mm) is off by 0.75 mm.
31 mm is also 3.1 cm.
| Millimeters | Inches | Nearest 1/16 |
|---|---|---|
| 26 mm | 1.0236 | 1″ |
| 27 mm | 1.063 | 1 1/16″ |
| 28 mm | 1.1024 | 1 1/8″ |
| 29 mm | 1.1417 | 1 1/8″ |
| 30 mm | 1.1811 | 1 3/16″ |
| 31 mm | 1.2205 | 1 1/4″ |
| 32 mm | 1.2598 | 1 1/4″ |
| 33 mm | 1.2992 | 1 5/16″ |
| 34 mm | 1.3386 | 1 5/16″ |
| 35 mm | 1.378 | 1 3/8″ |
| 36 mm | 1.4173 | 1 7/16″ |
Frequently asked questions
How many inches is 31 mm?
1.2205 inches, about 1 1/4″.
What fraction of an inch is 31 mm?
About 1 7/32″ to the nearest 1/32.