Cm to Inches Calculator
Convert between centimetres and inches — bidirectional, exact, no rounding tricks. 1 in = 2.54 cm exactly; 1 cm ≈ 0.3937007874 in.
1 cm =
0.39
1 inch = 25.4 mm exactly (1959 International Yard and Pound Agreement, NIST SP 811). So 1 in = 2.54 cm exactly and 1 cm = 1/2.54 = 0.3937007874 in. 1 cm × 0.3937007874 = 0.39370078740157477 in.
How to use this calculator
Type a value, pick "Centimetres → Inches" or "Inches → Centimetres" in the Direction select, and the result updates instantly. Flip the direction to invert the conversion.
How the calculation works
The factor is exact by international agreement. Under the 1959 International Yard and Pound Agreement — restated by NIST SP 811 — 1 inch = 25.4 mm exactly, which means 1 inch = 2.54 cm exactly. Going the other way, 1 cm = 1/2.54 in = 0.3937007874015748… inches. Because 2.54 has no rounding, IEEE 754 double precision preserves the round-trip to about 15 significant figures.
Worked example
A 180 cm ceiling in inches: 180 × 0.3937007874 = 70.8661417323 in, i.e. 70 in + 0.8661 × 8 sixteenths ≈ 70 27⁄32 in. Going back: 70.8661417323 × 2.54 = 180 cm exactly. A 6-inch ruler: 6 × 2.54 = 15.24 cm.
Frequently asked questions
How many inches are in a centimetre?
1 cm = 1/2.54 in ≈ 0.3937007874 inches. The factor is not rounded — it comes from the exact definition 1 in = 2.54 cm.
How many centimetres are in an inch?
Exactly 2.54 cm. This is the international definition set by the 1959 Yard and Pound Agreement between the US, UK, Canada, Australia, New Zealand, and South Africa, and adopted by NIST as SP 811.
How do I convert cm to inches without a calculator?
Divide the centimetre value by 2.54. A quick mental shortcut is "cm × 0.4" — this gives 4% high (e.g. 100 cm × 0.4 = 40 in vs the correct 39.37 in), which is close enough for rough estimates like clothing sizes.
Is the inch–centimetre factor really exact?
Yes. Since 1 July 1959, one international inch has been defined as exactly 25.4 mm (25.4 hundredths of a metre = 2.54 cm) by international treaty. There is no measurement uncertainty — the two units are related by a rational number.
What about the US survey inch?
The US survey inch (1/12 of a US survey foot) was 1.000002 international inches, a difference of 2 parts per million. It was formally retired by NIST on 1 January 2023. For all everyday use — clothing, DIY, engineering drawings, screens — the international inch is standard.
How accurate is this converter?
It uses the exact factor 2.54 in IEEE 754 double precision. The only rounding happens at display; the underlying math is good to roughly 15 significant figures, far beyond what any ruler, tape measure, or micrometer can reach.