Inches to Centimeters Converter

How to convert inches to centimeters (the formula)

Converting inches to centimeters is a single multiplication: centimeters = inches × 2.54. Going the other way, inches = centimeters ÷ 2.54, which is the same as multiplying by 0.39370079. Because the factor 2.54 is exact, the only rounding in the whole calculation is the one you choose when you write the answer down.

Two quick examples: a 32-inch monitor is 32 × 2.54 = 81.28 cm, and a 3/4-inch copper pipe is 0.75 × 2.54 = 1.905 cm. Notice that you never have to know anything else, no feet, no yards, no fractions of an inch. As long as the number you start with is in inches, one multiplication finishes the job.

The calculator on this page works in both directions at once. Type in the inches box and the centimeters box fills in live; type in the centimeters box and the inches box updates instead. That matters when you are copying a measurement off a ruler and you are not sure which unit the spec sheet used.

Why one inch is exactly 2.54 centimeters

The number 2.54 is not a measured value that was rounded, it is a definition. On 1 July 1959, the national standards laboratories of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed on a single international yard of exactly 0.9144 meters. Since one inch is one thirty-sixth of a yard, dividing 0.9144 by 36 gives exactly 0.0254 meters, which is 2.54 centimeters. The agreement was published by the US National Bureau of Standards (now NIST) as Letter Circular 1033.

Before 1959 the US and British inches differed slightly. The old US inch was defined as exactly 1/39.37 of a meter, which works out to about 25.4000508 millimeters, roughly two millionths of an inch longer than the international inch. That tiny gap mattered in precision machining and instrument calibration, which is why the six countries harmonized the definition.

Today the exact relationship is codified in the US federal measurement guidelines, so any inch-to-cm conversion you do with the factor 2.54 is legally and scientifically correct. See NIST SP 811, "Guide for the Use of the International System of Units (SI)", for the exact factor table.

Inches to cm conversion chart

The table below gives both directions for the values people look up most often. Every figure is exact for the factor 2.54; the reverse column is rounded to four decimal places for readability.

A useful anchor to memorize: 1 inch is a hair under 2.5 cm, so 4 inches is almost exactly 10 cm. From there you can sanity-check any result before you even reach for a calculator.

Worked examples: common inches to cm conversions

Here are four conversions US users run into most, each worked out step by step with the exact factor.

• A 55-inch TV: 55 × 2.54 = 139.7 cm diagonal. That is about 1.4 meters of screen.
• 5 foot 9 in height: first turn feet into inches, 5 × 12 + 9 = 69 inches, then 69 × 2.54 = 175.26 cm. This is the standard height conversion for a passport or medical form.
• A 0.5-inch bolt: 0.5 × 2.54 = 1.27 cm, or 12.7 mm. Hardware often lists both.
• A 12-inch ruler: 12 × 2.54 = 30.48 cm, which is why a metric ruler is usually 30 cm long with a little extra.

Notice the pattern: multiply inches by 2.54 and you are done. The only trap is mixing units, if a measurement is "5 foot 9", convert the feet to inches first so that the entire quantity is in one unit before you multiply.

Where inches-to-cm conversions go wrong

Almost every inches-to-cm error comes from one of three causes, none of which is the arithmetic itself.

• Multiplying instead of dividing (or the reverse). Inches are the bigger unit, so a length in cm is always the larger number. If your answer came out smaller, you divided when you should have multiplied.
• Mixing feet and inches. "6 foot 2" is 74 inches, not 62 inches. Add the feet times twelve to the leftover inches before converting.
• Rounding too early. If you round 2.54 down to 2.5 mid-calculation, a 100-inch measurement drifts by 4 cm. Round only the final answer.

Screen sizes are a frequent source of confusion: a "55-inch TV" is the diagonal in inches, but the marketing spec sheet in a metric country may list 139 cm. Both describe the same panel. When in doubt, convert the number you measured and keep the units labeled on the paper.

Inches to cm for height, screens and DIY projects

Three everyday situations send US users to an inches-to-cm converter.

Height. Medical and immigration forms often ask for height in centimeters. Take your height in feet and inches, convert the whole thing to inches, then multiply by 2.54. Someone 5 foot 9 is 175.26 cm; someone 6 foot 0 is 182.88 cm.

Screens and electronics. Monitors, phones and TVs are sold by diagonal in inches worldwide, but the bezel and stand dimensions on a spec sheet are usually metric. A 27-inch monitor is 68.58 cm diagonally, which tells you whether it fits a desk gap you measured in cm.

Home projects. Imported furniture, picture frames and fasteners are frequently labeled in cm. Before drilling, convert the cm dimension back to inches so your tape measure agrees with the part. The reverse conversion (divide by 2.54) is just as important as the forward one, and this page does both instantly.

For the underlying definitions and the exact factor table, see NIST SP 811 and the SI Brochure from the BIPM.

Fractions of an inch in centimeters

US measurements are full of fractional inches, 1/8, 1/4, 1/2, and metric tools and hardware are labeled in millimeters or centimeters. The conversion is the same multiplication by 2.54, applied to the decimal value of the fraction.

To use the table, turn your fraction into a decimal first: 3/8 inch is 3 ÷ 8 = 0.375 inches, then 0.375 × 2.54 = 0.9525 cm, or 9.525 mm. Sockets, bolts and drill bits sold in metric countries use these millimeter values, so a 1/2-inch bolt is a 12.7 mm bolt. Because 1 inch equals exactly 25.4 mm, every whole-number inch is a round millimeter figure, 2 inches is 50.8 mm and 3 inches is 76.2 mm, which makes cross-checking hardware quick.

Where these conversion factors come from

The relationship between inches and centimeters is fixed by international agreement rather than by local convention. The values used on this page follow the NIST guide to the SI (US Department of Commerce) and the BIPM SI Brochure, the two documents that define the exact values used worldwide.

Because these factors are exact by definition rather than measured, converting a number and converting it back always returns the number you started with, there is no rounding drift to worry about.