How this instrument works
An inch is a twelfth of a foot, and its length was a local matter for most of history — Britain's imperial standard and America's separately calibrated inch differed by a few parts per million right up until 1 July 1959, when six English-speaking nations agreed a common international yard of 0.9144 m. Divide that yard by 36 and one inch lands at exactly 25.4 mm, or 2.54 cm, with no measurement uncertainty left in it whatsoever.
The inch also lives a second life far from tape measures: in printing and page layout. Modern desktop-publishing software, following a standard Adobe set in the 1980s, defines a point as exactly 1⁄72 of an inch and a pica as 1⁄6 of an inch, so a 72-point headline is precisely one inch tall. Screen resolutions carry the same habit forward as pixels per inch, or PPI, tying digital design straight back to this same physical unit.
Centimetres, by contrast, are the everyday metric submultiple: rulers, sewing patterns, growth charts and rainfall gauges are marked in them almost everywhere metric measurement is used. The two units meet constantly in commerce that crosses borders — screen diagonals, pizza sizes, tyre widths and pipe fittings are still quoted in inches worldwide even in fully metric countries, which is exactly the gap this converter closes.
- Type your measurement into the Inches (in) field; it opens at 12, one full foot, as a reference point.
- Read your result on the Centimetres (cm) line, recalculated to six decimal places with every keystroke.
- For a tape-measure fraction, convert to a decimal first: 3⁄8 in is 0.375, 5⁄8 in is 0.625, then multiply by 2.54.
- Working on a technical drawing? Multiply your centimetre answer by 10 for millimetres — ISO drawings never use centimetres.
- Reversing direction? Divide your centimetre figure by 2.54, or nudge Inches (in) until Centimetres (cm) matches.
Worked example — a 12-inch pizza on a metric menu
American pizzerias size their pies in inches measured across the diameter, and a 'large' commonly means 12 inches. Type 12 into Inches (in) and Centimetres (cm) reads 30.48 — the figure a European franchise would print on its own menu for the same pizza, since 12 × 2.54 = 30.48 with nothing rounded away at any step.
That number is worth remembering on its own, because 30.48 cm is also exactly one foot — 12 inches makes a foot by definition, so any 12-inch object always converts to 30.48 cm regardless of what it happens to be. A 10-inch pizza, by comparison, comes to 25.4 cm, a difference of over 5 cm across the diameter even though it sounds like a small step down in the menu.
Questions
Is one inch exactly 2.54 centimetres?
Yes, exactly, by international agreement rather than by measurement. On 1 July 1959, standards bodies in the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa fixed a common yard at 0.9144 m, which makes an inch precisely 25.4 mm — 2.54 cm — with zero uncertainty. Nothing is being physically compared here, so any rounding you see in a result comes purely from display precision, never from the factor itself.
Why do fonts and screens still measure in points and inches?
Because desktop publishing inherited its measurement system from centuries-old printing conventions rather than adopting metric units. A modern PostScript point is fixed at exactly 1⁄72 of an inch, a standard Adobe set in the 1980s, so a 72-point line of text is one inch tall by definition. Screen and print resolution measured in pixels or dots per inch carries that same inch straight into digital design, which is why so many creative tools still default to it.
How do I convert a tape-measure fraction like 5⁄8 inch?
Turn the fraction into a decimal, then multiply by 2.54. Five divided by eight is 0.625, and 0.625 × 2.54 gives exactly 1.5875 cm. Because binary fractions like halves, quarters, eighths and sixteenths always divide cleanly, none of these conversions ever produce a repeating decimal — 1⁄2 in is 1.27 cm, 1⁄4 in is 0.635 cm, and 1⁄16 in is 0.15875 cm, each one exact.
Are American and British inches the same length?
Yes, since 1959. Before that agreement, America's inch derived from an 1893 order tying it to 39.37 inches per metre exactly, giving 25.4000508 mm, while Britain's imperial standard measured roughly 25.399977 mm — a difference too small to see but large enough to matter in precision engineering. One legacy unit, the US survey inch, kept the old American value until NIST formally retired it at the end of 2022; every inch measured today is the same 25.4 mm regardless of country.
Should I round my answer to centimetres or millimetres?
Millimetres, whenever a machinist or a technical drawing is involved — ISO drafting convention avoids centimetres entirely, since a stray cm figure on a drawing reads as a tenfold dimensioning error waiting to happen. Multiply inches by 25.4 for millimetres directly, or shift a centimetre answer one decimal place. Save centimetres for human-scale, non-technical uses: clothing, furniture, height, paper sizes.
Why are pizza, TV and tyre sizes still given in inches in metric countries?
Because those industries originated in inch-based manufacturing and never re-cut their tooling or their marketing once metrication arrived elsewhere. A 55-inch television is a 55-inch television on every continent — 139.7 cm corner to corner — and tyre codes like 205/55 R16 mix millimetre section widths with an inch-sized rim diameter in a single string. Pizza sizing followed American franchises abroad and stuck for the same reason: habit outlasts legislation.