How to convert inches to centimeters – and read a tape measure

The one number to remember

One inch is exactly 2.54 centimeters. This is not an approximation: since 1959 the inch has been defined in terms of the meter by international agreement. Everything else follows from it. To go from inches to centimeters you multiply by 2.54; to go from centimeters to inches you divide by 2.54.

Quick check: 10 inches is 25.4 cm, 20 inches is 50.8 cm, and 100 inches is 254 cm. If your answer to “how many cm is 12 inches” is not 30.48, something went wrong.

Converting inches to cm step by step

Take the length in inches, for example 8.5 inches. Multiply by 2.54. 8.5 × 2.54 = 21.59 cm. For a quick mental estimate, multiply by 2.5 and add a little: 8.5 × 2.5 = 21.25, close to the true 21.59.

Working the other way, 50 cm ÷ 2.54 = 19.685 inches. As a rule of thumb, divide centimeters by 2.5 and subtract about 1.5%.

Reading fractions on a tape measure

In the United States, tape measures divide each inch into halves, quarters, eighths and sixteenths. The longest mark between whole numbers is ½ inch, the next longest ¼ and ¾, then eighths, then sixteenths. A reading of 5 3/8 inches means five whole inches plus three eighths.

To convert a fraction, divide the numerator by the denominator and add the whole number: 5 3/8 = 5.375 inches = 13.6525 cm. Our calculator accepts the fraction directly, so you can type 5 3/8 without doing any arithmetic.

Feet and inches

One foot is 12 inches, or exactly 30.48 cm. A height of 5 ft 8 in is 5 × 12 + 8 = 68 inches, which is 172.72 cm. Going backwards, divide centimeters by 2.54 to get inches, then divide by 12: the whole number is feet and the remainder is inches.

Example: 180 cm ÷ 2.54 = 70.87 inches = 5 ft 10.87 in, about 5′ 11″.

Millimeters and workshop sizes

Fasteners, drill bits and sheet materials are often specified in both systems. 1 inch is 25.4 mm, so 1/4 inch is 6.35 mm and 3/8 inch is 9.525 mm. If you are matching a metric part to an imperial hole, check the fractions chart for the nearest standard size.

Common mistakes to avoid

Mixing up the direction is the most frequent error: inches to cm makes the number bigger, cm to inches makes it smaller. Another is treating 5.8 feet as 5 ft 8 in; 5.8 feet is 5 ft 9.6 in. Decimal feet and feet-and-inches are different notations, which is why this site has separate feet-and-inches converters.