Inches to Centimeters

What this tool is used for

• Converting an imperial drawing dimension for a supplier working in millimetres or centimetres.
• Reading a US product size against a metric tape measure.
• Converting a screen or paper size for a metric specification.
• Checking whether an imperial-sized item fits a metric-labelled space.
• Converting a height in feet and inches for a metric medical form.

Frequently Asked Questions

What is the formula?

Multiply by 2.54 exactly. Because the inch is DEFINED as 2.54 cm, no precision is lost in either direction — unlike conversions such as miles to kilometres, where the factor is exact but rarely written out in full.

How do I convert a feet-and-inches height?

Multiply feet by 12, add the inches, then multiply by 2.54. 5 ft 9 in is 69 in, which is 175.26 cm. Converting the feet and the inches separately then adding is where most errors creep in.

How do fractional inches work?

Imperial measurement uses binary fractions — halves, quarters, eighths, sixteenths — not decimals. 3/8 in is 0.375 in, which is 9.525 mm. Tooling and timber sizes are quoted this way, so decimal input often needs rounding to the nearest 1/16.

Why are pipe and screw sizes not really inches?

Many are nominal, not actual. A "1/2 inch" pipe has neither a 1/2 in inner nor outer diameter — the number is a trade designation inherited from older manufacturing. Converting it to millimetres gives a number that measures nothing.

What is a mil or a thou?

One thousandth of an inch, 0.0254 mm — standard in PCB design and machining tolerances. Beware the ambiguity: in some European contexts "mil" means a millimetre, so the same word can differ by a factor of about 39.

Why is 2.54 exact?

Because the inch was redefined in 1959 as exactly 2.54 centimetres, making it a metric-derived unit. Every imperial length conversion is therefore exact — the imprecision people expect belongs to the older definitions.

Does screen size convert directly?

The diagonal does, but it tells you little on its own — a 27-inch 16:9 display and a 27-inch 21:9 one have different widths and heights. The aspect ratio is needed before the diagonal means anything physical.

Common errors and gotchas

• Converting a fractional inch by eye rather than exactly, where a sixteenth is 0.15875 cm.
• Using 2.5 instead of the exact 2.54, which is a 1.6% error on every measurement.
• Forgetting to convert the feet part when the input is feet and inches.
• Reporting centimetres where the trade works in millimetres, a factor of ten waiting to happen.
• Rounding each dimension separately and then finding the total no longer matches.