Inches to cm

Type inches and read centimetres. The multiplier is 2.54 and it is exact, so the arithmetic is never what goes wrong in this direction — the input is. The number you were given is often a nominal name rather than a measured size, it may arrive as feet and inches, and it may be written as a fraction. All three are valid entries here: 5'10", 3 1/2 in and 62 in all work. Underneath the converter you get the full reference table, a second one covering every sixteenth of an inch, and the cases where converting the figure you were quoted hands you a confidently wrong answer.

When the inch figure is a name rather than a measurement

A piece of American timber sold as a 2×4 does not measure two inches by four. It measures 1.5 by 3.5 inches, which is 3.81 by 8.89 centimetres. The nominal figure describes the piece before it was dried and planed on all four sides, and the trade kept the old name. Convert the name instead of the size and you are out by more than a centimetre on one edge and more than two on the other, which is the difference between a frame that fits the opening you measured and one that does not.

There is a term for what happened when that timber went metric: a soft conversion. The object stays physically identical and only the label changes, which is why it is sold as 38 × 89 mm in metric countries rather than as a tidy 40 × 90. A hard conversion is the opposite — the product is redesigned around round metric numbers, so nothing converts neatly back into inches. Knowing which of the two you are looking at tells you immediately whether an ugly decimal is a sign of a mistake or exactly what you should expect.

The working rule is short. If the inch figure came off a tape measure, convert it and trust the answer. If it came off a product name, a size chart or a trade catalogue, find out what it actually measures before converting anything, because 2.54 will faithfully turn a wrong number into a wrong number with four decimal places on the end of it.

Feet and inches, and what gets lost on the way back

Heights are the most common reason to convert in this direction, and they arrive as two numbers rather than one. You can type them the way they are written: 5'10" is read as 70 inches and comes back as 177.8 cm. Five feet is 152.4 cm exactly, six feet is 182.88 cm, and a foot on its own is 30.48 cm rather than the 30 that people round it to in conversation.

What is worth knowing is how coarse the imperial version is. A whole inch is a 2.54 cm bucket, so 5'10" covers everyone from 177.8 cm up to just under 180.34 cm. Somebody who measures 178 cm and somebody who measures 180 cm report the same height, and converting 180 cm back gives 5 feet 10.87 inches, which anyone writing it on a form rounds to 5'11". The imperial format simply cannot record what the centimetre figure knows.

That is why a passport application, a medical chart and a driving licence can disagree about the same person by more than a centimetre with nobody having made an error. If a form will be checked against a measured value, measure in centimetres and convert nothing — a conversion added on top of a rounded figure keeps the rounding and adds its own.

Fractions, because that is how imperial sizes are actually written

Imperial sizes are written as fractions rather than decimals, so the box takes them that way. 3/8 in is 0.9525 cm, and a mixed number works too: type 3 1/2 in and you get 8.89 cm. That matters more than it sounds, because the lazy way to read "3 1/2" is to delete the space, which turns it into thirty-one halves and produces a perfectly plausible 39.37 cm instead of 8.89. We would rather parse it properly than hand back a figure that is wrong by more than a factor of four and looks entirely normal.

A fraction is an exact quantity, so the untidiness lives only in the display. One sixteenth of an inch is exactly 0.15875 cm, and that is where the second table below comes from: it lists every sixteenth up to a whole inch, because those are the marks that exist on the tool the number came off. A sixteenth being a little over one and a half millimetres is a useful thing to carry around when a tolerance is quoted as a fraction.

Engineering drawings made in the United States often use decimal inches instead — 0.375 rather than 3/8 — for this exact reason: a decimal cannot be misread as something else, and it makes the intended precision visible. If you were handed a decimal inch figure, type it straight in. If you were handed a fraction, type the fraction; converting it yourself first only inserts a rounding step that this page does not need.

Luggage: three dimensions that do not convert one at a time

A carry-on allowance of 22 × 14 × 9 inches is 55.88 × 35.56 × 22.86 cm. Wherever you see 56 × 36 × 23 cm quoted alongside it, that is the same allowance rounded up to whole centimetres rather than a slightly more generous one, and the rounding is precisely why the two sets of numbers describe the same bag.

Checked bags are usually limited by a single total instead: 62 inches for length plus width plus height together. Convert the total and you get 157.48 cm. Convert each side separately and round each one to a whole centimetre first, and the sum can move by around a centimetre and a half in either direction — which is most of the margin you were working with. The allowance is one number, so convert the one number you were given.

Two other things about that measurement are worth saying plainly. It is the outside envelope, so wheels, feet and a retracted handle all count, and an overpacked soft bag is measured as it stands at the gate rather than as it was printed on the label. The inch figure on the suitcase in the shop window is a different kind of number again — that one is a product name, and the first section applies to it.

Sizes where the inch is a difference rather than a length

A US bra size is two measurements in one label and only one of them behaves like a length. The number is a band size in inches. The letter is not a size at all: it encodes the difference between the bust and the band measurement, in inches, with A meaning roughly one inch, B two, C three and D four. So D always means 10.16 cm more than the band, whatever the band happens to be, which is why a 32D and a 38D are not the same cup and why converting the letter on its own into centimetres has no meaning.

Hat sizes work by a different trick. A US hat size is the head circumference in inches divided by pi, which makes it effectively a diameter. A 7 1/4 hat corresponds to a circumference of about 22.78 inches, and that is 57.9 cm — which is very close to how a European hat is labelled, because that system simply states the circumference in centimetres. Two numbers describing the same head, one of them quietly divided by pi.

The general shape is worth carrying around: an inch figure attached to a body-related product is often derived rather than measured, so find out what it was derived from before converting it. Garment charts add a second wrinkle on top of that, because a chart usually lists the body it is designed to fit while the flat garment measures larger by whatever ease the design calls for. Two different inch figures for the same item, and neither of them is wrong.

Screens and printing, where an inch is defined rather than measured

In CSS, one inch means exactly 96 pixels. It is not a measurement of your display and it does not adapt to it — 1in in a stylesheet is 96 CSS pixels on every device, and it coincides with a physical inch only when the display happens to run at 96 pixels per inch. The whole family of absolute CSS units is pinned to that same ratio, so 1in, 2.54cm, 25.4mm, 6pc and 72pt are all the same length by definition and none of them promises anything about the glass in front of you.

Printing is where the inch is still doing real work. Resolution is quoted in dots per inch, so a photograph destined for a 4 × 6 inch print at 300 dpi needs 1200 × 1800 pixels. That print is 10.16 × 15.24 cm, while the 10 × 15 cm paper a lab outside the United States hands you is 3.94 × 5.91 inches. The two are close enough to be sold as the same product and different enough that something has to be cropped, which is why a print occasionally loses a few millimetres off an edge you were not expecting to lose.

The same mismatch runs through document sizes, image placement in layout software and anything that has to come out at a real physical size. If the output has to hit an exact measurement, set the size in the unit the output device actually uses and convert once, at the beginning — not at every step, where each rounding gets another chance to accumulate.