An exact factor of 2.54
Since 1959 the inch has been defined as exactly 2.54 cm, so this conversion has no rounding error at all. For a mental estimate, multiply by 2.5 and nudge up 1–2%: 10 inches → 25 + a touch = 25.4 cm.
Common lengths
| Inches | cm | Context |
|---|---|---|
| 1" | 2.54 | Base unit |
| 6" | 15.24 | Large phone screen |
| 12" | 30.48 | 1 foot |
| 36" | 91.44 | 1 yard |
| 60" | 152.4 | 5 feet |
| 72" | 182.9 | 6 feet |
Screens are measured on the diagonal: a 55" TV is 139.7 cm corner to corner, not across.
Fractions of an inch
Inch fractions are binary, not decimal: 3/8" is 0.375", never 0.38". Read them straight off a tape measure by counting sixteenths from the last whole inch.
| Fraction | Decimal | cm |
|---|---|---|
| 1/16" | 0.0625" | 0.159 |
| 1/8" | 0.125" | 0.318 |
| 1/4" | 0.25" | 0.635 |
| 3/8" | 0.375" | 0.953 |
| 1/2" | 0.5" | 1.270 |
| 5/8" | 0.625" | 1.588 |
| 3/4" | 0.75" | 1.905 |
| 7/8" | 0.875" | 2.223 |
Common questions
How many centimetres are in an inch?
One inch is 2.54 centimetres exactly. This is not a rounded figure — since 1959 it has been the international definition of the inch.
What is 5/8 inch in cm?
5/8 inch is 1.588 cm. Inch fractions are halves of halves (1/2, 1/4, 1/8, 1/16), so 5/8 is 0.625 inch, then times 2.54.
How do I convert inches to cm quickly?
Multiply by 2.54. For a rough figure multiply by 2.5 and add a little: 6 inches is about 15 cm, exactly 15.24. To reverse it, divide centimetres by 2.54.
