How to Convert Inches to Centimeters

An exact definition, not an approximation

One inch is exactly 2.54 centimeters, or 25.4 millimeters. The value comes from the 1959 agreement between the United States and several Commonwealth countries that fixed the international yard at 0.9144 meters. A yard is 36 inches, and 0.9144 ÷ 36 = 0.0254 meters, which is 2.54 centimeters.

Since the factor is exact, converting inches to centimeters involves no hidden error. If an answer looks slightly off, the cause is rounding or a typing slip, not an uncertain constant. That makes the inch easier to convert than many older units whose metric equivalents were once measured rather than defined.

Inches are still the everyday unit for many products, including television and computer screens and the diameter of vehicle wheel rims. Product listings from abroad therefore often give a size in inches while a local shop or form asks for centimeters, which is exactly where this conversion comes up.

Inches to centimeters

Multiply the number of inches by 2.54. Writing the units out shows the logic: 8.5 in × 2.54 cm/in = 21.59 cm. The inches cancel and centimeters remain.

Common sizes make good reference points. A sheet of US letter paper is 8.5 by 11 inches, which is 21.59 by 27.94 cm. A 32-inch screen, measured along its diagonal, is 81.28 cm. A 65-inch screen is 165.1 cm. Each is a single multiplication, and the last digit is exact, not rounded.

If you want millimeters, multiply by 25.4 instead. A length of 3 in is 76.2 mm, which is the same as 7.62 cm.

Centimeters to inches

Divide centimeters by 2.54, or multiply by about 0.393701. For 30 cm, 30 ÷ 2.54 = 11.811 inches, rounded to three decimals. For 100 cm the answer is 39.370 inches.

Unlike the forward direction, this result usually does not end cleanly. The decimal 11.811 inches is not a ruler mark, so you need one more step to read it on a tape. Multiply the decimal part by 16 to find the nearest sixteenth: 0.811 × 16 = 12.98, so 11.811 inches is almost exactly 11 and 13/16 inches.

Fractions of an inch

Rulers and tape measures in inches are marked in fractions, so convert the fraction to a decimal first. Divide the top number by the bottom, then multiply by 2.54.

• 1/16 in = 0.0625 in = 0.15875 cm = 1.5875 mm
• 1/8 in = 0.125 in = 0.3175 cm = 3.175 mm
• 3/8 in = 0.375 in = 0.9525 cm = 9.525 mm
• 1/2 in = 1.27 cm = 12.7 mm
• 3/4 in = 1.905 cm = 19.05 mm

Mixed numbers follow the same route. For 5 3/8 inches, first write 5.375, then multiply: 5.375 × 2.54 = 13.6525 cm. That is 136.525 mm, or 137 mm to the nearest whole millimeter, but the extra digits are worth keeping if the number feeds into another calculation.

Feet and inches together

Lengths given in feet and inches need one extra step: change everything to inches before converting. Since 1 foot is 12 inches, 3 ft 4 in becomes 36 + 4 = 40 inches. Then 40 × 2.54 = 101.6 cm, which is 1.016 meters.

The shortcut of converting the feet and the inches separately also works, as long as you add the results at the end. Three feet is 91.44 cm and 4 inches is 10.16 cm, and 91.44 + 10.16 = 101.6 cm. Both routes agree, which is a handy way to double-check a result.

Estimating and rounding sensibly

For a rough figure, use 2.5 centimeters per inch. It is about 1.6 percent low, so 4 inches comes out at 10 cm against an exact 10.16 cm. Another easy rule is that 10 centimeters is just under 4 inches, and 1 meter is a little over 39 inches.

How many digits to keep depends on the job. Clothing and furniture dimensions are usually fine at the nearest centimeter. Woodworking or fitting parts often needs the nearest millimeter or better, and rounding too early can add up when several measurements are combined. Keep the full value until the final step, then round once.

Always convert the number you were given, not a rounded copy of it. A shelf listed as 47.5 inches is 120.65 cm, not 120 cm, and a half-centimeter gap can decide whether it fits an alcove. When the answer will be used for a purchase or a cut, round only at the very end and in the safe direction.

Using a converter and when to check the source

A unit converter such as this one lists inch and centimeter in its Length category, shows the full equation, and lets you set the number of decimal places, so you can see 13.6525 cm in full or rounded to two decimals as needed. It also shows the same length in every other unit at once, which is useful for millimeters.

For manufacturing drawings, medical measurements, or anything where a wrong dimension has a cost, take the figure from the original specification rather than from a converted copy, and note which unit the source treated as authoritative.