Convert in to cm

An inch has been exactly 25.4 mm since 1959, when six countries agreed to stop using slightly different ones. Before that a US inch and a UK inch differed in the seventh significant figure: 25.400 05 mm against 25.400 00.

Centimetres are the everyday metric length — body measurements, paper, furniture — and they are the one metric unit that has no direct customary counterpart, which is why so much converting happens through them.

The factor is 2.54, and almost nobody carries that around. Rounded to 2.5 it is off by 1.6 % — which stays invisible on small numbers and turns into a whole unit somewhere around 100 in.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The inch has been defined in metric terms since the international yard and pound agreement of 1959, and in this page's units that comes out at 2.54 cm to one. The definition is exact by stipulation — it was not measured, and it cannot be refined by measuring it better. The decimal above is the whole of it, with nothing rounded away at the end.

Before that agreement the US and the British versions differed slightly, and old survey figures still carry the older definition. For anything outside land surveying the difference is far below the precision anybody is working to.

The inch has no independent existence any more. Since 1959 it has been defined as exactly 25.4 millimetres by agreement between the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa — a decision to adopt a round metric value rather than to keep measuring the old ones against each other. Before that a US inch and a British inch differed in the seventh decimal place, which mattered only to people making gauge blocks and mattered a great deal to them.

One consequence runs through this whole page: a measurement in inches is already a metric measurement written in a different alphabet. Converting it loses nothing and adds nothing. Any imprecision in the centimetre figure came in with the original tape measure, not with the arithmetic.

American measurements below a few feet almost never arrive as a decimal. A height is five foot ten, a worktop is 8 foot 6, a gap is 2 foot 3 — and none of those is a number you can feed straight into a multiplication. Convert the feet to inches first, add the loose inches, then multiply once: five foot ten is 70 inches and 177.8 cm.

The failure this prevents is treating the inches as a decimal fraction of a foot. Reading 5 ft 10 in as 5.10 ft and multiplying by 30.48 gives 155.4 cm, which is a person twenty-two centimetres shorter than the one being described. The same trap sits in spreadsheets that store heights as 5.10 and sort them below 5.9.

Anything measured in an American workshop or hardware shop is quoted in halves, quarters, eighths and sixteenths rather than in decimals. Divide the fraction out before you convert: a quarter inch is 0.25 × 2.54 = 0.635 cm, three eighths is 0.9525 cm, three sixteenths is 0.47625 cm. The awkwardness of those answers is a signal rather than an error.

It is telling you that the centimetre is the wrong size of unit for this measurement. The same three figures in millimetres are 6.35, 9.525 and 4.7625, which are numbers a metric supplier will recognise on sight. Where the inch figure came from a tool, a fastener or a thickness, convert to millimetres and treat the centimetre as a unit for describing furniture.

Screen diagonals never went metric anywhere. A 55-inch television is 55 inches in Berlin and in Tokyo, and the box may not print a centimetre figure at all. The diagonal is 139.7 cm, but the number that decides whether it fits is the width: a 16:9 panel at that diagonal is about 121.8 cm wide and 68.5 cm high, plus whatever the bezel and the stand add.

The same habit holds for monitors, tablets and phones, and it is why a metric buyer converts screens more often than anything else on this page. Convert the diagonal to get a feel for the size, then find the width and height in the specifications rather than deriving them, because the ratio is not always 16:9 and the manufacturer measures the panel while the cabinet has to hold the frame.

Clothing is where inches survive on a European label. Jeans are sold across the continent as 32 × 34, and both numbers are inches: an 81.3 cm waist and an 86.4 cm inside leg. Shirt collars run the other way — a 15.5-inch collar is 39.4 cm, and European shirts are labelled 39 or 40 outright, so the conversion is the whole of the translation.

Shoes are the exception that catches people out, because neither scale is in inches or centimetres. British and American sizes step by a third of an inch, about 8.5 mm, from different starting points; European sizes step by two thirds of a centimetre, about 6.7 mm. Two scales with different steps and different origins cannot be converted with a factor, which is why every shoe conversion is a lookup table and why the tables disagree with each other.

This is where the conversion earns its keep and where the failure costs money. An American kitchen appliance is 36 inches wide, which is 91.4 cm, and a European kitchen is built around 60 cm units — the American fridge does not fit the gap and no amount of pushing changes that. A US queen mattress is 60 × 80 inches, or 152.4 × 203.2 cm, against a European 160 × 200, so bedding bought on one side of the Atlantic hangs off or falls short on the other.

The habit that prevents it is converting the space before you convert the object. Measure the opening in centimetres, subtract the clearance the fitting instructions ask for, and only then convert the listing. Doing it the other way round produces a centimetre figure that looks close enough to the space you half remember, and the two-centimetre shortfall is not visible until the delivery arrives.

Some pairs of things are almost the same size and are not the same size, and those are the ones worth converting carefully. US Letter is 8.5 × 11 inches, which is 21.59 × 27.94 cm, against A4 at 21.0 × 29.7 cm — Letter is six millimetres wider and eighteen millimetres shorter. A document laid out for one prints with a shifted margin on the other, and a ring binder sized for one lets the other slide.

Doorways are the same story at a larger scale. A standard American internal door is 36 × 80 inches, or 91.4 × 203.2 cm; a common British one is 762 × 1,981 mm, which is 30 × 78 inches to the nearest millimetre. Anything you are carrying through a doorway needs the diagonal of the opening rather than its width, and that is worth converting before the sofa is on the stairs rather than after.