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The essential imperial-to-metric length conversion
Inches appear everywhere in US product specs, screen sizes, plumbing, and clothing. Converting to centimeters bridges the gap for the metric world. The conversion is exact: 1 inch = 2.54 cm by international definition since 1959.
The formula
cm = in × 2.54
- in — the length in inches.
- cm — the equivalent length in centimeters.
- 2.54 — the exact factor (1 in = 2.54 cm, 1959 international agreement).
To go the other way: in = cm ÷ 2.54.
Worked example
Using the default of 5 inches:
- Multiply by 2.54: 5 × 2.54 = 12.7 cm.
So 5″ = 12.7 cm exactly.
Conversion table
| Inches | Centimeters |
|---|---|
| 1 | 2.54 |
| 2 | 5.08 |
| 3 | 7.62 |
| 4 | 10.16 |
| 5 | 12.70 |
| 6 | 15.24 |
| 8 | 20.32 |
| 10 | 25.40 |
| 12 | 30.48 |
| 18 | 45.72 |
| 24 | 60.96 |
| 36 | 91.44 |
History & standards
The inch has roots in Roman measurement (the “uncia”, one-twelfth of a foot) and in the English tradition of measuring by the width of a thumb. The centimeter is 1/100 of the meter, the SI base unit defined in 1799 and redefined in 1983 by the speed of light. The exact relationship between the two — 1 in = 2.54 cm — was established by the 1959 International Yard and Pound Agreement. NIST retired the US survey inch (which differed at the ppm level) in 2023.
Common applications
- Screen diagonals. Monitors, TVs and phones are rated in inches in the US; EU specifications list cm.
- Clothing. US shirt collar and sleeve sizes are in inches; European sizing uses cm.
- Pipe & fitting sizes. US plumbing fittings use nominal inch sizes; metric replacements are listed in mm/cm.
- Woodworking. US lumber and hardware dimensions appear in inches; metric plans require cm.
- Medical. Wound and lesion sizes are documented in cm in most clinical settings, but US practitioners may measure in inches.
Reverse conversion
To convert cm back to inches, divide by 2.54: in = cm ÷ 2.54. Use the cm to inches calculator.
Limitations & gotchas
- Decimal inches. Enter fractional inches as decimals (e.g. 5.5 for 5½″). The result will be exact.
- Unit confusion. Ensure your measurement is in inches, not millimeters; 1″ = 25.4 mm, so mixing the two inflates results by 25×.
- US survey inch (deprecated 2023). The international inch is now the sole legal US definition.
Sources & references
- NIST SP 811, Guide for the Use of the International System of Units (SI).
- BIPM, The International System of Units (SI), 9th edition.
- 1959 International Yard and Pound Agreement.
- Federal Register Vol. 88, No. 60 (2023) — deprecation of the US survey foot/inch.
FAQs
Multiply the number of inches by 2.54: cm = in × 2.54. For example, 5 in × 2.54 = 12.7 cm. The factor 2.54 is exact, fixed by the 1959 International Yard and Pound Agreement.
Exactly 2.54 cm per inch. This is a legally defined exact value, not an approximation.
10 × 2.54 = 25.4 cm exactly.
12 × 2.54 = 30.48 cm exactly — this is also the definition of one foot in centimeters.
36 × 2.54 = 91.44 cm, which is also exactly 1 yard (0.9144 m).
Yes. The 1959 International Yard and Pound Agreement defined 1 inch = 25.4 mm = 2.54 cm as an exact value. Any displayed rounding in the calculator result is only for readability.