Online ruler › How to Read a Ruler: Inches, cm and mm
Last updated: October 25, 2026 · By the TrueRuler team
To read a ruler, find the zero mark, then count the lines from zero to the edge of the object. On a metric scale the longest lines are whole centimeters and the shortest are millimeters; on an inch scale the longest lines are whole inches and the shortest are usually sixteenths.
To measure 1/2 inch, find the medium line halfway between 0 and 1 on the inch scale. It sits at 0.5 in, which is 12.7 mm. Eight sixteenths make 1/2 inch and four sixteenths make 1/4 inch.
| Fraction | Decimal inches | Millimeters |
|---|---|---|
| 1/16 in | 0.0625 | 1.59 |
| 1/8 in | 0.125 | 3.18 |
| 1/4 in | 0.25 | 6.35 |
| 3/8 in | 0.375 | 9.53 |
| 1/2 in | 0.5 | 12.70 |
| 5/8 in | 0.625 | 15.88 |
| 3/4 in | 0.75 | 19.05 |
| 7/8 in | 0.875 | 22.23 |
2 inches equal 50.8 mm, or 5.08 cm. One inch is exactly 25.4 mm, so multiply inches by 25.4 for millimeters and by 2.54 for centimeters.
| Inches | Centimeters | Millimeters |
|---|---|---|
| 1 in | 2.54 | 25.4 |
| 2 in | 5.08 | 50.8 |
| 3 in | 7.62 | 76.2 |
| 4 in | 10.16 | 101.6 |
| 6 in | 15.24 | 152.4 |
| 8 in | 20.32 | 203.2 |
| 12 in | 30.48 | 304.8 |
Practice with the online ruler: switch between cm, mm and inch, then use the Show an exact size box to display 1/2 in, 3 cm or 5 mm.