Divide by 2.54, and a shortcut for a rough figure
The inch is defined as exactly 2.54 cm, so the factor never changes. For a quick mental estimate, divide the centimetres by 2.5: 30 cm → 12 inches (the exact answer is 11.81), which runs a touch high but is close enough to picture a size.
Quick reference
| Centimetres | Inches (decimal) | Inches (nearest 1/16) |
|---|---|---|
| 2.54 cm | 1.00 in | 1 in |
| 10 cm | 3.94 in | 3 15/16 in |
| 15 cm | 5.91 in | 5 15/16 in |
| 30.48 cm | 12.00 in | 12 in (1 foot) |
| 50 cm | 19.69 in | 19 11/16 in |
| 100 cm | 39.37 in | 39 3/8 in |
| 180 cm | 70.87 in | 70 7/8 in |
Screen sizes work the same way: a 24-inch monitor is about 61 cm and a 15.6-inch laptop is about 40 cm, measured on the diagonal.
Decimal inches vs fractional inches
- Decimal (5.75 in) is the form for calculation, engineering and code. It is the raw division result.
- Fractional (5 3/4 in) is the form on tape measures and sewing patterns. The calculator rounds the decimal part to the nearest sixteenth, so 5.75 reads as 5 12/16, which simplifies to 5 3/4.
- Rounding to 1/16. Multiply the decimal part by 16 and round: 0.69 × 16 ≈ 11, giving 11/16. That is close enough for cutting fabric or timber.
Common questions
How many inches is one centimetre?
One centimetre is 0.3937 inches, and one inch is exactly 2.54 centimetres. To go from centimetres to inches you divide by 2.54.
Why exactly 2.54 and not 2.5?
The 1959 international yard agreement fixed the yard at exactly 0.9144 metres. Since a yard is 36 inches, one inch works out to exactly 2.54 cm, with no rounding.
How do I convert a decimal inch to a fraction?
Take the part after the decimal point, multiply by 16 and round to a whole number, then write it over 16. For 5.69 inches: 0.69 x 16 = 11, so it is about 5 and 11/16 inches. Sixteenths are the common step in sewing, woodworking and construction.
