Inches to CM Converter

How to convert inches to centimeters

Because the inch is defined as exactly 25.4 millimeters, converting inches to centimeters is a single multiplication by 2.54. Your answer is only as rough as your measurement: 12 in is exactly 30.48 cm, with no rounding at all. The cm to inches converter divides by the same 2.54 for the opposite direction, and the length converter shows the result in millimeters, meters and feet as well.

The step people get wrong is the fraction. A tape reading of 3 5/8 in has to become a decimal first: 5 ÷ 8 = 0.625, so the length is 3.625 in, and 3.625 × 2.54 = 9.2075 cm. Multiplying only the 3 and forgetting the 5/8 is a classic error, as is treating “3.5/8” as 3.5. Type the decimal into the converter above, or use the fraction table on the cm to inches page to look up sixteenths.

How many decimals should you keep? Multiplying by 2.54 adds at most two decimal places, so a measurement to the nearest 1/16 in (0.0625 in) becomes a centimeter value to about 0.16 cm. Quoting more digits than that suggests a precision the tape never had. For woodworking and sewing, rounding to the nearest millimeter (one decimal in cm) is plenty.

Step by step

1. Write the inches as a decimal (3 5/8 in becomes 3.625 in).
2. Multiply by 2.54.
3. Read the result in centimeters, or multiply by 10 more for millimeters.

See it on a scale

Why is one inch exactly 2.54 cm?

It is exact by agreement, not by measurement: since 1959 the inch has been defined as 25.4 mm, one thirty-sixth of the international yard of 0.9144 m.

For most of the 20th century there was no single inch. The US defined its yard from the meter in the 1893 Mendenhall Order as 3600/3937 m, which made the US inch about 2.540005 cm. Britain kept a physical standard yard, and its inch was very slightly shorter.

Engineers and toolmakers needed parts to fit across borders, so in 1959 six countries agreed on an international yard of exactly 0.9144 m. Dividing by 36 gives an inch of exactly 0.0254 m. The older US definition survived only as the US survey foot used in land surveying, about 2 parts per million longer. NIST and NOAA retired the survey foot at the end of 2022, so today every inch you meet is 2.54 cm.

What this means in practice

Any inch dimension, from a 1/4 in bolt to a 72 in sofa, converts with the same factor. You never need a country-specific or industry-specific inch, and a result such as 1/4 in = 0.635 cm is exact, not an approximation.

How do you estimate inches to cm without a calculator?

The easiest shortcut is multiply by 2.5: double the inches, then add half the original. For 20 in that gives 40 + 10 = 50 cm. Because 2.5 is 1.6% below 2.54, the estimate runs slightly low; the exact answer is 50.8 cm. Add about 1 cm for every 60 cm if you want to close the gap.

• Feet as a checkpoint: 1 ft = 30.48 cm, so 5 ft ≈ 152 cm and 6 ft ≈ 183 cm.
• Millimeters directly: multiply inches by 25.4. A 1/2 in socket is 12.7 mm and a 3/4 in drill bit is 19.05 mm.
• Height: convert feet to inches first (× 12), add the inches, then multiply by 2.54. 5′ 9″ is 69 in, or 175.26 cm.

Other everyday conversions follow the same pattern: see the pounds to kilograms converter for body weight, or the full length converter for yards and miles.