How this instrument works
A centimetre is one hundredth of a metre, and since 1983 that metre has been defined by how far light travels in 1⁄299792458 of a second. Of all decimal submultiples, this one earned its permanent place on workbenches and in shops: school rulers, dressmaking tape, hospital growth charts, rainfall gauges and road-bike frames are quoted in centimetres, while decimetres and decametres quietly vanished from ordinary speech.
An inch descends from uncia, a Roman twelfth — twelve of them made a pes, and French still calls an inch pouce, meaning thumb. For most of its life its length was a local matter. Britain's imperial inch measured about 25.399977 mm; America's, fixed by an 1893 Mendenhall Order declaring one metre equal to 39.37 inches exactly, worked out at 25.4000508 mm. Two nations, two inches, three parts in every million apart.
That split closed on 1 July 1959, when standards laboratories in six English-speaking countries adopted an international yard of 0.9144 m — which makes an inch precisely 25.4 mm and a centimetre precisely 50⁄127 inch. Converting between these two units became arithmetic rather than measurement: no tolerance, no drift, nothing to calibrate. One nuisance survives. Fifty over 127 has no terminating decimal, so 0.393700787402 is that ratio cut at twelve places, wrong by about one part in a trillion.
- Type your measurement into the Centimetres (cm) field. A value of 10 is preloaded so you can watch the factor at work.
- Read the Inches (in) line, shown to six decimals and recomputed with every keystroke.
- Going backwards? Multiply your inch figure by 2.54 for centimetres — that direction is exact and needs no long factor at all.
- For a tape or steel rule, multiply whatever sits after your decimal point by 16 and round, giving sixteenths of an inch.
Worked example — a 10 cm knitting gauge
Knitting and crochet patterns quote gauge over a 10 cm square: 22 stitches and 30 rows to 10 cm, say. Enter 10 in Centimetres (cm) and Inches (in) reads 3.93700787402 — not 4. English-language patterns print “4 in” beside that 10 cm as a courtesy, yet those are two different swatches.
Sixty-three thousandths of an inch separates them, roughly 1.6 percent. Across one gauge square nobody would notice; across a 100 cm sweater body worked to a 4-inch gauge, your finished piece lands about 1.6 cm narrow. Push 3.93700787402 back through 2.54 and you return to 10 out to ten decimal places, with only that final rounding left over.
Questions
Is one inch exactly 2.54 centimetres?
Yes, exactly — by agreement rather than by measurement. Standards bodies of six countries, among them America, Britain, Canada and Australia, fixed an international yard at 0.9144 m on 1 July 1959, which puts an inch at 25.4 mm with zero uncertainty. Nothing physical is being compared here, so a centimetre-to-inch result carries no experimental error whatever. What you see rounded on screen is display precision alone.
Why 0.393700787402 rather than something tidier?
Because 1 ÷ 2.54 equals 50⁄127. With 127 prime, that fraction's decimal expansion — 0.393700787401574803149606… — repeats without ever terminating. Twelve places is where this sheet cuts it, leaving under two parts in every trillion adrift: roughly one nanometre spread over each kilometre of tape. The popular short form 0.3937 goes wrong at your fourth decimal, which machinists working to thousandths spot at once.
Do American and British inches still differ?
Not since 1959. Beforehand America worked from an 1893 legal declaration that one metre equalled 39.37 inches exactly, giving 25.4000508 mm, while Britain's imperial standard bar yielded roughly 25.399977 mm. Both gave way to 25.4 mm. One holdout remained, the US survey inch used for land parcels and state coordinate grids, and NIST retired that at the end of 2022. Anything measured today uses one inch only.
How do I read a decimal inch answer on a tape measure?
Multiply whatever follows your decimal point by 16, then round to a whole number of sixteenths. Ten centimetres gives 3.937 in; 0.937 × 16 = 14.99, so you want 15⁄16, and 3 15⁄16 in sits half a thousandth of an inch over target — far finer than any tape can be read. For a rule graduated in 32nds, multiply by 32 instead.
Why are screens and pipes still sized in inches in metric countries?
Trade convention outlives legislation. Display diagonals, pipe bores, wheel diameters and lumber names travelled with American manufacturing and stuck fast: a 15.6 in laptop panel measures 39.62 cm corner to corner, and nobody advertises it that way. Cycling splits down the middle, sizing road frames in centimetres and mountain frames in inches. Photo labs do too, since 10 × 15 cm prints are not 4 × 6 in, which comes to 10.16 × 15.24 cm.
What do a centimetre and an inch look like without a ruler?
One centimetre sits close to an adult little fingernail's width; one inch runs about as long as your thumb's top joint, which is why French names it pouce. Ruler lengths deserve one warning: 30 cm school rules are not foot rules. Twelve inches comes to 30.48 cm, so that familiar plastic strip falls 4.8 mm short — enough to throw any layout drawn by stepping it end over end.