How this instrument works
A centimetre is one hundredth of a metre, a survivor of the centimetre-gram-second system that Britain's Association for the Advancement of Science put on a formal footing in 1874. SI keeps it as a permitted submultiple, and metric countries reach for it whenever millimetres feel fussy and metres feel coarse: height on an identity card, a waistband, a sheet of A4 measured 21.0 cm across.
An inch spent centuries as customary length — three barleycorns end to end, in one medieval English statute — before industry lost patience with folklore. Gauge-block maker Carl Edvard Johansson sized his sets around an inch of 25.4 mm so that a single box could serve metric and inch workshops alike, and standards bodies in Britain and America adopted that value during the early 1930s. Law caught up in 1959, when six English-speaking nations fixed a common yard at 0.9144 m; that one number makes an inch 25.4 mm, or 2.54 cm.
Exact means exact. No uncertainty hides behind 2.54 and no digits trail away after it, so this conversion is arithmetic rather than measurement, good to as many places as you care to write down. Reverse it and neatness moves into a fraction: 1 cm is 50⁄127 in, whose decimal form 0.3937007874… runs on forever and is never once wrong.
- Type your measurement into the Inches (in) field — it opens at 10 as a starting point.
- Read your answer on the Centimetres (cm) line, carried to six decimal places and recomputed as you type.
- Tape-measure fractions need decimals first: 3⁄8 in is 0.375, 7⁄16 in is 0.4375, 1⁄32 in is 0.03125.
- Dimensioning a part? Multiply that centimetre result by 10 — ISO drawing practice uses millimetres, never cm.
- Going from centimetres back to inches? Divide by 2.54 instead: 30 cm returns 11.811024 in.
Worked example — a 10-inch tablet on a metric shelf
A tablet sells as a 10-inch model, and joinery for its shelf is dimensioned in centimetres. Enter 10 in the Inches (in) field and your Centimetres (cm) line reads 25.4. Only a decimal point has moved, because 10 × 2.54 terminates and nothing needs rounding away.
Remember that 25.4 cm describes a diagonal, not a width: at 16:10 proportions that panel spans roughly 21.5 cm by 13.5 cm. Build to the diagonal figure and you leave yourself room. Write it as 254 mm if a drawing goes anywhere near a workshop, since millimetres are what a machinist expects to read.
Questions
Is 2.54 exact, or rounded for convenience?
Exact, and legally so. On 1 July 1959 standards laboratories of six countries — United States, United Kingdom, Canada, Australia, New Zealand and South Africa — agreed a common yard of 0.9144 m. Divide by 36 and an inch falls out at 25.4 mm, which is 2.54 cm, carrying no uncertainty whatever. Precision in your result is therefore limited only by how many digits you choose to display.
Does a US inch differ from a British imperial inch?
Not since 1959: both measure 25.4 mm, which is why an inch needs no country label. What did outlive that agreement was America's survey foot, about two parts per million longer and kept alive for land records until NIST and NOAA retired it at the end of 2022. Ordinary inch work was never affected — a survey inch would have differed by roughly 50 nanometres, some thousand times finer than any tape measure can resolve.
Why do metric countries still buy some things by inches?
Because a handful of industries standardised on inch sizing and never re-cut their tooling. Screen diagonals stay in inches worldwide, so a 55-inch television is 139.7 cm corner to corner. Tyre codes mix systems inside a single string: 205/55 R16 gives section width in millimetres and rim diameter in inches. Plumbing threads, bicycle wheels and rack-mount equipment behave much the same way, which makes this conversion a daily chore rather than a curiosity.
Should I convert to centimetres or to millimetres?
Millimetres, if a drawing or a machinist is involved. ISO engineering practice dimensions parts in millimetres, and centimetres almost never appear on a technical drawing, where a stray cm figure reads as a tenfold error waiting to happen. Multiply inches by 25.4 for mm, or shift a centimetre answer one place left. Centimetres suit human-scale things instead: clothing, furniture, body height, paper.
How do I handle fractions like 5⁄8 in?
Turn each fraction into a decimal, then multiply. 5 ÷ 8 is 0.625, and 0.625 × 2.54 gives 1.5875 cm precisely. Binary fractions behave beautifully here, since halving a terminating decimal always terminates: 1⁄2 in is 1.27 cm, 1⁄4 in is 0.635 cm, 1⁄8 in is 0.3175 cm, 1⁄16 in is 0.15875 cm. Nothing repeats anywhere, so a fraction converted properly is never an approximation.
Is multiplying by 2.5 near enough for rough estimates?
It runs about 1.6% short — fine for a shopping guess, poor for anything cut to size. Somebody of 5 ft 10 in, or 70 inches, stands 177.8 cm by an exact factor and only 175 cm by that lazy one, an error of nearly 3 cm. Mental arithmetic can do better: double your figure, halve it, add both, then add four per cent. For 70 inches that reads 140 + 35 = 175, plus 2.8, landing on 177.8 exactly.