How to Convert cm to Inches (and Back)

To convert centimetres to inches, divide the number of centimetres by 2.54. To convert inches to centimetres, multiply by 2.54. That factor is exact by definition, not a rounded approximation: one inch is defined as exactly 2.54 centimetres, so the only error in your answer is the error in your original measurement.

Key Takeaways

• cm to inches: divide by 2.54. Inches to cm: multiply by 2.54. Nothing else is needed.
• 2.54 is exact, agreed by international treaty in 1959. It is not a rounded figure like 3.28084 feet per metre, which genuinely is truncated.
• 1 cm = 0.3937 in and 1 m = 39.37 in. Both of those decimals repeat forever, so they are the rounded side of the relationship.
• The common mental shortcut of multiplying centimetres by 0.4 is always 1.6% too big - fine for a shelf, wrong by nearly 3 cm at adult height.
• For height, convert to total inches first, then divide by 12: the remainder is your inches.
• A foot is 30.48 cm, not 30 cm. That single rounding is behind most bad conversions.

Worked both ways, using 168 cm and 30 inches:

• 168 / 2.54 = 66.14 in
• 30 x 2.54 = 76.2 cm

The division is the part people fumble, because 2.54 is awkward in your head. If you are converting a batch of measurements rather than one, a unit converter removes the arithmetic entirely and keeps every figure to the same number of decimal places, which matters more than it sounds once you start cutting material to those numbers.

This is worth understanding, because it is the reason you never need more than one conversion factor and never need to worry about which country's inch you are using.

Until 1959 the inch was not the same length everywhere. The United States, the United Kingdom and the Commonwealth countries each maintained their own physical standards, and they disagreed in the sixth decimal place. The US inch worked out at roughly 2.540005 cm; the British one was slightly different again. For everyday work this made no difference, but for surveying, engineering and international trade it was an irritation.

The International Yard and Pound Agreement of 1959, signed by the US, UK, Canada, Australia, New Zealand and South Africa, fixed the yard at exactly 0.9144 metres. Divide that by 36 and you get an inch of exactly 0.0254 metres, or 2.54 centimetres, or 25.4 millimetres. Since then, every inch in the English-speaking world is defined in terms of the metre.

The imperial system is metric underneath

That last point catches people out. The inch has no independent physical standard any more. The metre is defined by fixing the speed of light in vacuum at 299,792,458 metres per second, as set out by the International Bureau of Weights and Measures, and the inch is then defined as a fixed fraction of the metre. When you divide by 2.54 you are not approximating across two rival systems; you are applying a definition.

One legacy exception has now closed. The United States kept a separate survey foot, very slightly longer than the international foot, for land surveying and mapping. NIST and the National Geodetic Survey retired it at the end of 2022, so US surveying has moved to the same international foot as everyone else. Unless you are reading a land deed drafted decades ago, 2.54 is the only number you need.

If the broader history of why two systems coexist at all interests you, the metric versus imperial units guide covers how the split happened and why the US, UK and Canada each ended up with a different compromise.

Decimal inches are what you want for spreadsheets and screen sizes. The nearest sixteenth is what you want at a workbench, because that is the finest mark on most imperial tape measures. Every row below is computed from the exact factor and then rounded once.

A few of these are worth committing to memory. 10 cm is just under 4 inches (3.937 in, or 4 inches minus a sixteenth). 30 cm is just under a foot (11.81 in). 1 metre is 39.37 inches, which is 3 feet 3.37 inches - noticeably more than a yard, and the reason a metre stick will not fit in a 36-inch case.

Notice how quickly the shortfall grows. At 10 cm the gap between the true answer and a round 4 inches is 1.6 mm and nobody cares. At 200 cm the same proportional error is 3.2 cm, which is the difference between a wardrobe fitting under a ceiling and not.

Going the other way is easier, because multiplying by 2.54 is kinder than dividing by it, and because every answer is exact rather than a repeating decimal.

There is a decomposition that makes this trivial in your head, and it is exact rather than approximate. Since 2 + 0.5 + 0.04 = 2.54:

Double the inches, add half the inches, then add 4% of the inches.

For 32 inches: 64 + 16 + 1.28 = 81.28 cm. That is not a shortcut with an error term, it is the exact answer arrived at in three easy steps. For 55 inches: 110 + 27.5 + 2.2 = 139.7 cm.

Exact anchors

These are the pairs where both sides land on tidy numbers. They are useful as sanity checks - if your answer is on the wrong side of the nearest anchor, you have made an error somewhere.

Most people carry one of two rules in their head: divide centimetres by 2.5 (the same thing as multiplying by 0.4), or multiply inches by 2.5. Both work, both are biased in a known direction, and knowing the size of the bias is what makes them safe to use.

Dividing by 2.5 instead of 2.54 makes the answer 1.6% too large, every single time. Multiplying inches by 2.5 instead of 2.54 makes the answer about 1.6% too small (1.57%, precisely), every single time. The percentage never changes; only the absolute size of the mistake grows.

That 180 cm row is the one to remember. The shortcut turns a 180 cm person into exactly 6 feet. They are actually 5 ft 10.9 in. If you are converting your own height for a form, a visa application or a dating profile, the lazy rule will promote you by more than an inch.

Two shortcuts that are actually accurate

The first is a one-line correction to the rule you already use: divide by 2.5, then take 1.6% off the answer. For 180 cm, dividing by 2.5 gives 72; 1.6% of 72 is about 1.15; 72 - 1.15 = 70.85 inches, against a true 70.87. That is close enough to hang a picture by.

The second uses the anchors instead of a factor. Split the measurement into round chunks you know: 1 m is 39.37 in and 80 cm is 31.5 in, so 180 cm is 39.37 + 31.5 = 70.87 in. Break 168 cm into 100 + 60 + 8 and you get 39.37 + 23.62 + 3.15 = 66.14 in. Adding memorised values beats dividing by 2.54 in your head, and it does not drift.

In practice, use the rough rule for a quick sense of scale and the exact division for anything you will act on. The pattern is the same one that trips people up converting weight, where 2.2 pounds per kilogram is the rounded stand-in for the exact figure; the kg to pounds conversion guide works through that error in the same way.

Height is the single most common reason anyone converts centimetres, and it needs one extra step because feet and inches are a mixed unit.

The method

• Step 1. Divide the centimetres by 2.54 to get total inches. 175 / 2.54 = 68.90 in.
• Step 2. Divide the total inches by 12. The whole number is your feet: 68.90 / 12 = 5.74, so 5 feet.
• Step 3. The remainder is your inches: 68.90 - (5 x 12) = 8.90 in.
• Result: 175 cm is 5 ft 8.9 in, written 5'9" once rounded.

The common error is stopping after step 2 and reading 5.74 as 5 feet 7 inches. The 0.74 is a fraction of a foot, not a count of inches. Multiply it by 12 before you say it out loud.

Height conversion table (140-200 cm)

Feet and inches are rounded to the nearest whole inch, which is how heights are normally quoted. The middle column gives the unrounded figure if you need it.

Two things fall out of this table. First, five feet is 152.4 cm and six feet is 182.88 cm - not 152 and 183, though those are the usual shorthand. Second, rounding to the nearest inch throws away up to 1.27 cm in either direction, so someone quoted as 5 ft 11 in could be anywhere from about 179 cm to 181.6 cm. If you convert 180 cm to 5 ft 11 in and then convert back, you get 180.34 cm. That drift is harmless for a passport form and unhelpful for anything measured twice.

Screens are measured on the diagonal

A television or monitor size is the diagonal of the viewable panel, not its width. That is why a 55-inch set is nowhere near 55 inches across. The table below gives the diagonal in centimetres and the panel width and height for a standard 16:9 screen, so you can check whether it fits the alcove before it arrives.

Add the bezel and the stand feet to those widths - manufacturers publish the outer dimensions separately, and they are always larger than the panel.

Paper sizes never quite match

The A series is defined in millimetres and the US series in inches, which is why documents sent across the Atlantic print with odd margins. A4 is narrower and taller than Letter.

The practical consequence: A4 is 0.23 inches narrower than Letter but 0.69 inches taller. A Letter-formatted document scaled to A4 loses roughly 6 mm of usable width at the sides.

Luggage

Airlines publish limits in whichever unit suits them, and the two common cabin-bag standards are not the same box. A frequently quoted North American allowance is 22 x 14 x 9 inches, which is 55.9 x 35.6 x 22.9 cm. A frequently quoted European one is 55 x 40 x 20 cm, which is 21.7 x 15.7 x 7.9 inches. The depths are the problem: a bag built to a 9-inch depth is 22.9 cm and will not pass a 20 cm gauge. Checked-bag rules often use a 62-inch total of length plus width plus depth, which is 157.5 cm and is usually advertised as 158 cm. Always check the specific airline before you fly, because these figures vary by carrier, route and fare.

Clothing

Trouser and chest measurements convert cleanly: a 32-inch waist is 81.3 cm, a 34-inch waist is 86.4 cm, a 40-inch chest is 101.6 cm. What does not convert cleanly is the label. Sizing on clothing is a brand's own scale rather than a measurement, so a labelled 32 rarely measures 32 inches. Measure the garment, convert the measurement, and ignore the tag. Shoes are a separate system again - most brands' charts are built on foot length in millimetres, so use the manufacturer's chart rather than converting a UK or US size arithmetically.

The conversion is exact. Almost every error that survives to the finished job is a rounding error you introduced afterwards, and there are four that account for most of them.

Round once, at the end. If you convert 247 cm to 97.24 in, round it to 97, then divide by 3 for shelf spacing, you have baked a quarter-inch error into every shelf. Carry the decimals through the whole calculation and round the final cut.

Never round-trip. Converting cm to inches, rounding to the nearest sixteenth, and then converting back to centimetres compounds two roundings. Keep the original measurement in its original unit as the master figure and convert only for the tool you are holding.

Know what a sixteenth is worth. A millimetre is finer than a sixteenth of an inch, so a metric tape is slightly more precise than an imperial one at the same reading effort. This table is the one to have on your phone in a hardware shop.

Match your precision to the job. Two decimal places in inches is about a quarter of a millimetre, which is finer than any tape measure can be read and far finer than timber behaves. Quoting a wall as 118.11 inches is false precision if you measured it to the nearest centimetre. As a rule: one decimal place in inches for anything structural, two for machining or CAD, none for a screen size or a shoe box.

The last habit is the most useful and the least technical: write the unit down. Most genuinely expensive conversion mistakes are not arithmetic failures at all, but a number that travelled from one document to another without its unit attached, and was then read in the wrong one. The UK government's guidance on weights, measures and packaging law exists partly for this reason: for trade, the unit is not decoration, it is part of the figure. Label every measurement at the moment you take it, and the conversion itself will never be the thing that goes wrong.