The answer to how many cm in an inch is exactly 2.54, which is 25.4 mm or 0.0254 m. That figure is a definition, not a measurement. From 1 July 1959 the US National Bureau of Standards counted the yard as exactly 0.9144 meter, and because a yard holds 36 inches, the inch became 0.9144 ÷ 36 = 0.0254 meter.
Converting therefore takes one step. Multiply inches by 2.54 to get centimeters, or divide centimeters by 2.54 to get inches. Below you will find the formulas with worked examples, a computed conversion chart, what the inch was before 1959, fractions of an inch in millimeters, the cost of rounding to 2.5, and where inch sizes still appear on products.
How many cm are in an inch?
One inch (in) is 2.54 cm, 25.4 mm and 0.0254 m. The question of how many mm in an inch has the same answer in a different unit: 25.4. All three values are exact, with nothing rounded. The reverse is different. One centimeter is 1 ÷ 2.54 = 0.393700787... inch, and the decimals never end. An inches to cm result is therefore always exact, while a cm to inches result is a number you round when you write it down.
How to convert inches to cm and cm to inches
The conversion uses three formulas:
- Inches to centimeters: cm = inches × 2.54
- Centimeters to inches: inches = cm ÷ 2.54
- Inches to millimeters: mm = inches × 25.4
A 27-inch measurement is 27 × 2.54 = 68.58 cm. A half-inch value works the same way: 5.5 in × 2.54 = 13.97 cm. Going the other way, 30 cm ÷ 2.54 = 11.811 in. On a ruler that sits close to 11 13/16 in, because 0.811 × 16 = 12.98 and the nearest fraction is 13/16.
For a height written in feet and inches, turn everything into inches first. As a hypothetical example, 5 ft 7 in is 5 × 12 + 7 = 67 in, and 67 × 2.54 = 170.18 cm. In reverse, 170 cm ÷ 2.54 = 66.93 in, which is 5 ft 6.93 in, so 170 cm is about 5 ft 7 in. If you have many values to convert, the length converter shows mm, cm, m, inches, feet, yards and miles together.
Inches to cm conversion chart
The first chart was computed by multiplying the number of inches by 2.54, so every centimeter and millimeter value is exact.
| Inches (in) | Centimeters (cm) | Millimeters (mm) |
|---|---|---|
| 1 | 2.54 | 25.4 |
| 2 | 5.08 | 50.8 |
| 5 | 12.7 | 127 |
| 10 | 25.4 | 254 |
| 12 | 30.48 | 304.8 |
| 20 | 50.8 | 508 |
| 30 | 76.2 | 762 |
| 36 | 91.44 | 914.4 |
| 50 | 127 | 1,270 |
| 100 | 254 | 2,540 |
For the other direction, divide the centimeters by 2.54. Because that division never ends, the values below are rounded.
| Centimeters (cm) | Inches (4 decimals) | Inches (2 decimals) |
|---|---|---|
| 1 | 0.3937 | 0.39 |
| 5 | 1.9685 | 1.97 |
| 10 | 3.9370 | 3.94 |
| 20 | 7.8740 | 7.87 |
| 30 | 11.8110 | 11.81 |
| 50 | 19.6850 | 19.69 |
| 100 | 39.3701 | 39.37 |
| 150 | 59.0551 | 59.06 |
| 170 | 66.9291 | 66.93 |
TV sizes need a different chart, because a screen also has a width and a height. Those are in the TV dimensions chart.
Why an inch is exactly 2.54 cm
Whether an inch is exactly 2.54 cm depends on the year: since 1959 it is, and before that the US and UK inches were slightly different. The short answer is that the inch is no longer defined on its own. The yard is defined from the meter, and the inch from the yard: 1 yard is exactly 0.9144 m, a yard is 36 inches, so 1 inch is exactly 0.0254 m. NIST shows the same chain as 0.9144 m ÷ 36 = 0.0254 m.
The 1959 definition
A National Bureau of Standards and Coast and Geodetic Survey notice dated 30 June 1959 (24 FR 5348) set 1 yard equal to exactly 0.9144 meter from 1 July 1959. It says the same equivalents would be used by the national standards laboratories of Australia, Canada, New Zealand, South Africa and the United Kingdom. The notice also has a separate line for the inch: derived from the new yard, it is exactly 25.4 millimeters.
NIST adds that English-speaking nations agreed on the new values at a 1958 conference and that the factors were announced in the Federal Register in 1959. So 1 July 1959 is the effective date in the United States. The United Kingdom writes the same yard into law today: the Weights and Measures Act 1985 says the yard is exactly 0.9144 meter. According to the English Wikipedia article, the UK adopted the value through a 1963 act that took effect on 1 January 1964. The same notice fixed the pound, which the post on how many kg in a pound covers.
What the inch was before 1959
In the United States, an 1866 law defined the meter as 39.37 inches. An International Hydrographic Review article dates that law to 28 July 1866. In 1893 the yard was written as 3600/3937 meter. From this, the old US inch works out to 1000 ÷ 39.37 = 25.4000508 mm, which is longer than 25.4 mm by 0.0000508 mm, or 2 parts in a million.
In the United Kingdom the legal ratio was 1 meter = 39.370113 inches. The same article says the ratio came from an 1898 comparison. That gives an old British inch of 1000 ÷ 39.370113 = 25.399978 mm, shorter than 25.4 mm by 0.87 parts in a million. Neither inch value appears in the notice itself, so we computed them from the ratios above. The English Wikipedia article gives the same two values, 25.4000508 mm and 25.399977 mm. The notice does say the new yard is smaller than the 1893 yard by 2 parts in a million and that the change would have no effect at all on ordinary trade and commerce, only on very precise measurements.
The gap is small but grows with length. Over one mile, which is 63,360 inches, the old US inch adds 63,360 × 0.0000508 = 3.2 mm and the old British inch removes 63,360 × 0.0000221 = 1.4 mm.
Why 25.4 mm was chosen
The notice gives no reason for 25.4 mm. It only says the value was approved by the American Standards Association in 1933 for inch-millimeter conversion, adopted by the National Advisory Committee for Aeronautics in 1952 and by many standards bodies in other countries. Industry was already using 25.4 mm long before 1959. The English Wikipedia article pushes that history back further: in 1912 gauge-block maker Carl Edvard Johansson built his inch-sized blocks on exactly 25.4 mm, and the British Standards Institution adopted that inch in 1930.
The arithmetic makes the choice easy to see. A value of 25.4 mm was within 2 parts in a million of the old US inch and within 0.87 parts in a million of the old British inch. Any other centimeter value with two decimals sits much further away, since 2.53 and 2.55 each differ from 2.54 by about 0.4%. That reading is our own calculation, not a reason the official text gives.
Feet, yards and meters
NIST Handbook 44 lists 12 inches = 1 foot (ft) and 3 feet = 1 yard (yd). Every figure follows from the 2.54 factor. One foot is 12 × 2.54 = 30.48 cm, or 0.3048 m, and one yard is 36 × 2.54 = 91.44 cm, or 0.9144 m. One mile is 63,360 inches, which equals 1,609.344 m. One meter is 1 ÷ 0.0254 = 39.3700787... inches.
So the answer to how many inches in a foot is exactly 12, a yard holds 36, and a meter holds about 39.37. A 12-inch ruler is 30.48 cm long, which makes it 4.8 mm longer than a 30 cm ruler. The same figures hold in the UK since 1964.
Fractions of an inch in millimeters
The inch side of a ruler or tape measure is usually divided into halves, quarters, eighths and sixteenths. To turn a fraction into millimeters, multiply it by 25.4. The answer to what is 1/2 inch in mm is 12.7 mm. The table lists the common fractions, all computed this way and all exact.
| Fraction (in) | Decimal (in) | Millimeters (mm) |
|---|---|---|
| 1/16 | 0.0625 | 1.5875 |
| 1/8 | 0.125 | 3.175 |
| 3/16 | 0.1875 | 4.7625 |
| 1/4 | 0.25 | 6.35 |
| 3/8 | 0.375 | 9.525 |
| 1/2 | 0.5 | 12.7 |
| 5/8 | 0.625 | 15.875 |
| 3/4 | 0.75 | 19.05 |
| 7/8 | 0.875 | 22.225 |
| 1 | 1 | 25.4 |
For a mixed number, convert the fraction to a decimal first. 5 3/8 in is 5 + 0.375 = 5.375 in, and 5.375 × 25.4 = 136.525 mm, which is 13.6525 cm.
Rounding: 2.54 or 2.5?
Using 2.5 is easy to do in your head, but it makes every result 1.57% too small. The table below converts 100 inches and 100 cm with different factors. The difference is measured against the result from the exact factor.
| Direction | Factor used | Result | Difference (%) |
|---|---|---|---|
| 100 in → cm | 2.5 | 250 cm | −1.57 |
| 100 in → cm | 2.54 (exact) | 254 cm | 0 |
| 100 cm → in | 0.4 (same as ÷ 2.5) | 40 in | +1.60 |
| 100 cm → in | 0.39 | 39 in | −0.94 |
| 100 cm → in | 0.394 | 39.40 in | +0.08 |
| 100 cm → in | 0.3937 | 39.37 in | −0.0002 |
| 100 cm → in | ÷ 2.54 (exact) | 39.3701 in | 0 |
The percentage stays the same while the absolute error grows with size. A 55-inch diagonal comes out at 137.5 cm with 2.5 and 139.7 cm with 2.54, and that 2.2 cm gap can decide whether furniture or a wall space works. Four rules help keep the error small:
- Do the calculation with 2.54 (or 25.4 in millimeters) and round only the final result.
- Use 2.5 for a quick mental estimate, but do not report the answer as exact.
- When going from centimeters to inches, use 0.3937 or divide by 2.54. A 0.4 factor inflates the result by 1.6%.
- The tolerance of the job decides how many digits to write. Only 2.54 is exact, so any digits you drop are your own rounding.
Where inch sizes show up
Many countries that use the metric system still size screens, tires and pipes in inches. In the three examples below, the inch number does not always mean a real measured size.
Screens and phones
A "55-inch" TV is a screen diagonal, and the TV dimensions chart gives width and height for each size. Phones follow the same rule. Apple lists the iPhone 16 display as 6.1-inch (diagonal). In centimeters, 6.1 inches is 6.1 × 2.54 = 15.494 cm. Apple adds that the display has rounded corners. Measured as a standard rectangle, the diagonal is 6.12 inches (15.54 cm), and the actual viewable area is less. The same page gives the phone's height as 5.81 inches (14.76 cm), so a "6.1-inch" phone is not 6.1 inches tall.
Tires and bike wheels
On a car tire, millimeters and inches sit side by side on one label. Michelin explains that in a size code such as 205/55 R17, the 205 is the section width in millimeters and the number after the R is the wheel diameter in inches. For a 205/55 R16 tire, that wheel diameter is 16 × 25.4 = 406.4 mm.
Bicycles break that link. Sheldon Brown's tire sizing page lists a "26 inch" wheel as 559 mm in ISO terms, which is 22.01 inches. A "29 inch" is 622 mm (24.49 in), and the "27 inch" of older road bikes is 630 mm (24.80 in). The page says traditional inch names were based on a tire's outside diameter and lost contact with reality. When you buy a bike tire, compare the ISO size in millimeters, not the name.
Pipes
The number in "1/2 inch pipe" is nominal. According to Wikipedia's Nominal Pipe Size article, the outside diameter of NPS 1/2 pipe is 0.840 inch, and 0.840 × 25.4 = 21.34 mm. Do not confuse that with the 12.7 mm of a true half inch. For fittings and valves, check the real outside diameter in the product's data sheet.
The inch symbol and how to write inches
The standard symbol for the inch is "in", and the NIST conversion table uses it too. In everyday writing the inch is marked with a double prime (″), usually typed as a straight double quote, while the foot gets a single prime (′). The English Wikipedia article gives the ISO 31-1 annex as the source for "in". So 5′7″ means 5 ft 7 in, which is the 170.18 cm computed above. The inch symbol and the foot symbol are different marks, and mixing them up changes a measurement by a factor of 12.
The US survey foot was retired in 2022
The 1959 notice left one exception for the foot. Geodetic survey data in the United States would stay on the 1893 definition, where 1 foot equals 1200/3937 meter. That foot is called the US survey foot and equals 0.3048006... meter. Handbook 44 says 1 international foot is exactly 0.999998 survey foot.
The difference is only 2 parts per million, but it accumulates with distance. The 2019 NOAA and NIST notice puts it at about 0.01 foot (0.12 inch) over a one-mile survey. The two agencies ended the double definition. Handbook 44 gives 31 December 2022 as the deprecation date and says that from 1 January 2023 the survey foot should be used only for historic and legacy applications. NIST's announcement says that at midnight on 1 January 2023 the international foot, exactly 0.3048 meter, superseded it. This exception concerned the foot and the survey units built on it. Since 1959 the inch itself has had one definition: 25.4 mm.