Convert in to cm inches ↔ centimetres Free Online Secure
Type a value in inches and read it in centimetres at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert in to cm (inches ↔ centimetres)
| in | cm |
|---|---|
| 1 | 2.54 |
| 2 | 5.08 |
| 3 | 7.62 |
| 5 | 12.7 |
| 10 | 25.4 |
| 20 | 50.8 |
| 50 | 127 |
| 100 | 254 |
Where these units come from
The metre was first defined in 1793 as a ten-millionth of the distance from the equator to the North Pole — a definition nobody could check without surveying a quarter of the planet. It has been redefined four times since, each time to something steadier: a platinum bar, then a prototype kept in Paris, then a wavelength of krypton light, and since 1983 the distance light travels in vacuum in 1/299,792,458 of a second. The inch stopped being a separate thing in 1959, when six countries agreed to define it as exactly 25.4 mm. That agreement is why every conversion on this page between a metric and an imperial length is exact rather than measured: both sides now descend from the same definition.
Where you meet them
Millimetres are the working unit of engineering drawings and of anything made in a workshop; centimetres belong to tailoring, furniture and school rulers; metres to rooms, fabric and running tracks. Feet and inches survive in aviation altitudes worldwide, in American and British construction, and in the diagonal of every screen you have ever bought. Nautical miles are not a quaint survival — one is a minute of latitude, which is why ships and aircraft still navigate in them.
Questions people ask
Why is 1 inch exactly 25.4 mm?
Because in 1959 the standards bodies of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed to make it so. Before that the American inch and the British inch differed in the seventh decimal place, which was enough to matter when parts made on one side of the Atlantic had to fit parts made on the other. The agreed value is a definition, not a measurement, so it will not be revised.
Where is the centimetre used in everyday life?
For anything a person measures against their own body: height, clothing sizes, the length of a sleeve, the width of a shelf, the dimensions of a parcel. A centimetre is roughly the width of a fingernail, which makes it easy to picture. It is large enough to avoid long strings of digits and small enough that you rarely need a decimal.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
From American size charts to a metric tape: how to convert in to cm
A package arrives from an online shop in the United States and the description says "32 x 18 x 12 in". A recipe calls for a "9 inch" cake tin, a carpenter's plan wants a board "1 by 6", and a laptop review boasts a "16-inch" display. Each is a perfectly clear instruction to someone who grew up with inches, and a small puzzle to someone who works in centimetres. This page turns inches into centimetres at once. Type a length in inches and the page returns the exact figure, based on the definition of the inch rather than a rule of thumb.
The conversion has one pleasant feature: it is exact. One inch is 2.54 centimetres, no more and no less, so multiplying by 2.54 never introduces a rounding error. That makes it easy to convert in to cm by hand if you do not mind a little arithmetic, and easy to trust when the machine does it for you. The sections below give the rule, worked examples, a profile of each unit, a comparison, the places where the conversion is needed, and the slips that cause costly errors.
The rule for convert in to cm
Multiply by 2.54
To convert in to cm, multiply the number of inches by 2.54. The factor has been exact since the international agreement of 1959, when the inch was fixed at 25.4 millimetres. Ten inches is 25.4 cm, a hundred is 254 cm, and a tenth of an inch is 0.254 cm, which is 2.54 mm.
Worked examples
A 9-inch cake tin has a diameter of 22.86 cm, so a European 23 cm tin is the nearest match. A 15-inch laptop is 38.1 cm across the diagonal. A 32-inch waist is 81.28 cm, and a 6-inch nail is 15.24 cm long. A 1 by 6 board, in lumber terms, is actually ¾ inch thick and 5½ inches wide, or 1.9 cm by 14 cm. Each is a single multiplication, which is all it takes to convert in to cm.
Mental shortcuts
Double the inches and add half of the inches, which is the same as multiplying by 2.5, then add a further two per cent if you want to be closer. For 12 inches: 24 + 6 = 30, and a pinch more gives 30.5 cm, against an exact 30.48. For everyday decisions, treat an inch as 2½ centimetres, about the width of your thumb, and the answer will be within 2 per cent.
When to round
If the original is a nominal size, such as a 2 by 4, there is no point in keeping decimals. If the original is a measured length, keep as many digits as the measurement had.
The inch in short
A thumb and a treaty
The inch is one twelfth of a foot and one thirty-sixth of a yard. Its name comes from the Latin for a twelfth, and its length was once taken from a thumb, later from three barleycorns. Today it is fixed at exactly 25.4 millimetres, by an agreement signed in 1959 by the English-speaking countries. That treaty is the reason the page can show an exact figure.
Where you see it
American and British packaging, clothing sizes, screen diagonals, nails and screws, pipe fittings, tyre and wheel rims, photograph and frame sizes, and the thickness of metal sheets are all given in inches. In computing, the inch is the base of display density, pixels per inch, and of printer resolution, dots per inch.
The centimetre in short
One hundredth of a metre
A centimetre is a hundredth of a metre. It is the working unit of everyday length in countries that use the metric system: tailors, teachers, doctors and shop assistants all think in centimetres. The metre itself is defined from the speed of light, and the centimetre follows by dividing by a hundred, so it is the same everywhere in the world.
Where you see it
Height charts, furniture catalogues, fabric widths, shoe sizes in some countries, rulers and school exercise books are marked in centimetres. A centimetre is roughly the width of a fingernail, and ten of them make a decimetre. Its symbol is cm.
Inch against centimetre
An inch is 2.54 times a centimetre, so a figure in centimetres is bigger than the same length in inches: 10 in is 25.4 cm, while 10 cm is only 3.94 in. That is the whole difference between a screen sold as "24 inch" and one sold as "61 cm". When you compare products from the two worlds, convert in to cm first and judge afterwards.
Write the unit that your reader expects. For a global audience, give both, in the form 24 in (61 cm). In shops, compare dimensions of the packaging, not of the product, if the product must fit through a door.
Where the conversion shows up
Online shopping from the United States
Product pages list size in inches: a duffel bag at 22 × 12 × 12 inches is 55.9 × 30.5 × 30.5 cm, a cabin-size suitcase for most airlines; a rug of 5 × 8 feet is 152 × 244 cm; a picture frame of 8 × 10 inches is 20.3 × 25.4 cm. Before you buy, convert in to cm for every dimension and compare with the space where the product has to go. Include the box for delivery: the courier's measure, not yours, decides whether it fits through the stairwell.
Screens, cameras and sensors
Display sizes are diagonals in inches: 13 inches is 33 cm, 24 inches is 61 cm, 27 inches is 68.6 cm, 32 inches is 81.3 cm, 55 inches is 139.7 cm. Sensor sizes are quoted in odd fractions of inches that do not match real dimensions. The Full HD resolution on a 24-inch screen gives about 92 pixels per inch, and the Retina class of display packs more than 200 into the same inch. Knowing the physical size helps you judge how far to sit from a screen.
Cooking, baking and the home workshop
American recipes call for 8-inch, 9-inch and 12-inch pans, which are 20, 23 and 30 cm. A pizza of 14 inches is 35.6 cm. In a workshop, drill bits come in fractions: ¼ inch is 6.35 mm, ½ inch is 12.7 mm, ¾ inch is 19.05 mm. Plumbing parts are labelled in nominal inches that do not match the true diameter, so measure the actual pipe before you buy.
Cars, bikes and tyres
Rims are quoted in inches: a 16-inch rim is 40.6 cm, a 17-inch rim 43.2 cm. Mountain bikes use 27.5 and 29 inches, 69.9 and 73.7 cm, for wheel diameter. Tyre widths are in millimetres but aspect ratios and rim sizes combine units. In the United States, car ground clearance and roof height are listed in inches, which matters if you plan to take the vehicle to a car park with a limit of 2.1 m, or 82.7 inches.
A short history of the inch
From thumbs to treaties
Early peoples measured with the body: a thumb, a hand, a foot, a pace. The Romans divided the foot into twelve unciae, the origin of both inch and ounce. In medieval England, a statute of the fourteenth century declared the inch to be the length of three barleycorns, round and dry, laid end to end. The definitions drifted over time, and by the nineteenth century the British and American inches differed by a few parts in a million. In 1959 the national standards laboratories settled the matter: one inch, 25.4 mm. When you convert in to cm today, you apply a number that engineers in six countries chose to share.
Inches in everyday English
The inch lives on in idioms: giving someone an inch and having them take a mile, being within an inch of your life, inching forward. It appears in the names of things, such as the 3.5-inch floppy disk and the 5.25-inch drive bay, and in sizes like a 4-inch nail. The inch has also found a home in the digital world, where the phrase "inches" is shorthand for the size of a phone or tablet.
Why metric countries still use it
Electronics are the main reason. Display panels, hard drives and component packages were standardised when the American industry led, and the sizes stuck. Even in countries that went metric generations ago, a monitor is "27 inches" and a tyre "17 inches". A technician who works in millimetres must therefore switch units several times a day.
Taking the measurement well
Straight and flat
Most measurement errors come from the way the tape is held, not from arithmetic. Keep the tape straight and flat against the object, avoid sag over long distances, and read the scale with your eye directly above the mark. When two people measure, the one at the far end should hold the tape taut and at the same height as the near end. A string or a chalk line helps over uneven ground. Before you convert in to cm, be sure the original figure is something you would bet on.
Measure twice, write once
Take each measurement twice and write the result with its unit as soon as you have it. If the two readings differ by more than a millimetre, measure a third time. Photograph the tape against the object as a record. A minute spent on this saves a trip back to the shop.
Measuring well
Choosing the right tool
A tape measure marked in both inches and centimetres is the cheapest solution to this whole problem. Look for a tape with both scales on opposite edges, so that you can read a length in one unit and check it in the other without arithmetic. For small parts, a calliper with a switch between inches and millimetres reads to 0.01 mm, or 0.0004 inch. A laser distance meter measures a room in seconds and can display in either unit.
Reading fractions of an inch
American tapes divide the inch into halves, quarters, eighths and sixteenths. A reading of 5 and 3/8 inches means 5.375 inches, or 13.65 cm. Learn the sixteenths and you can read any tape quickly: 1/16 is 0.0625 inch or 1.59 mm, 1/8 is 3.18 mm, 1/4 is 6.35 mm, 1/2 is 12.7 mm. Whenever you convert in to cm from a fraction, convert the fraction to a decimal first.
Avoiding cumulative errors
If a plan has several consecutive dimensions, add them in inches and convert the total, rather than converting each and adding the rounded results. Rounding each part to the nearest millimetre can shift a long run by several millimetres, enough to make a shelf not fit.
Paper, print and photography
Paper sizes
North America uses Letter paper, 8.5 × 11 inches or 21.6 × 27.9 cm, and Legal, 8.5 × 14 inches, while most of the world uses A4, 21 × 29.7 cm. The two are close but not identical: Letter is a little wider and shorter. Printing a document made for one on the other can cut margins, so check the print preview. Posters and photographs follow the same pattern: 8 × 10 inches is 20.3 × 25.4 cm, 11 × 14 is 27.9 × 35.6 cm, and 16 × 20 is 40.6 × 50.8 cm.
Print resolution
Print resolution is measured in dots per inch. A photograph printed at 300 dpi needs 300 pixels for every inch, 118 for every centimetre. A 20 × 30 cm print therefore needs 2,362 × 3,543 pixels, about 8.4 megapixels. Knowing both units lets you decide whether a camera file is big enough for a given print.
Small habits that make the job easier
Keep a conversion card
Write a small table on a card or in your phone: 1 in is 2.5 cm, 6 in is 15 cm, 12 in is 30.5 cm, 24 in is 61 cm, 36 in is 91.4 cm. With five anchors you can interpolate almost any length, and you will seldom have to convert in to cm from scratch. The card is especially useful in shops, where you often need a quick judgement before you decide to buy.
Check two ways
When the stakes are high, such as ordering a kitchen worktop or a made-to-measure curtain, convert once on this page and then convert back with the reverse page. If you return to the number you started with, the result is right. Also measure again with a tape marked in centimetres. A second measurement catches slips that arithmetic alone cannot.
Say both units aloud
In a team that spans two measuring cultures, say both units whenever you give a dimension: "twelve inches, thirty centimetres". It costs a second and prevents the kind of misunderstanding that appears only after a part has been cut. Whoever has to convert in to cm later will thank you for writing the unit next to every number in the notes.
More values for convert in to cm
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 in = 0.254 cm
- 0.5 in = 1.27 cm
- 3 in = 7.62 cm
- 4 in = 10.16 cm
- 7 in = 17.78 cm
- 15 in = 38.1 cm
- 20 in = 50.8 cm
- 75 in = 190.5 cm
- 200 in = 508 cm
- 500 in = 1,270 cm
- 2,000 in = 5,080 cm
- 10,000 in = 25,400 cm
Mistakes when you convert in to cm
Dividing by 2.54
Going from inches to centimetres means a larger number, so multiply. Dividing by 2.54 is the conversion in the other direction. A quick test: 10 inches should be about 25 cm, not 4.
Confusing nominal and actual sizes
A "2 by 4" board is not 2 inches by 4 inches; it is about 1.5 by 3.5 inches. Pipes and fittings use nominal sizes as well. Measure the real object, then convert in to cm on the real figure.
Forgetting feet and inches
A height of 5 ft 9 in is 69 inches, which is 175.3 cm. If you convert only the inches and forget the feet, the answer is badly wrong. Turn feet into inches first by multiplying by twelve.
Quick answers about inches and centimetres
How many centimetres are in an inch?
Exactly 2.54. To convert in to cm, multiply by that number.
How many inches are in a centimetre?
0.393701 inches, so a centimetre is a little under 0.4 inch.
What is 12 inches in centimetres?
30.48 cm, the length of a standard ruler.
What is 6 feet in centimetres?
72 inches, which is 182.88 cm.
Is the figure exact?
Yes. The inch is defined as exactly 2.54 cm, so no rounding is involved until the display.
Is my number sent anywhere?
No. The calculation runs in your browser and nothing is uploaded.
Related pages and a last word
If you work with lengths regularly, these conversions are close by: Convert in to ft, Convert in to dm, Convert dm to cm, Convert m to cm, Convert cm to m, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L. The reverse of this page is Convert cm to in.
In short, to convert in to cm multiply by 2.54, check that the answer is larger than the starting figure and round to the precision of your data. Your number stays on your device: the arithmetic is done locally, with no account and nothing to delete afterwards.