Feet to Inches Converter

feet to inches — quick reference

The formula, worked line by line

One multiplication by an exact integer. There is no constant to argue about and no rounding to disclose, because the foot has been twelve inches for as long as both words have meant anything.

It is worth saying as a rule rather than as an example, because the failure is one of habit rather than understanding. People know perfectly well that a foot is twelve inches, and still type 5.9 for five foot nine, because that is how the height is said aloud.

inches = feet x 12
6 x 12 = 72 in
5 ft 9 in = 5 x 12 + 9 = 69 in
5.75 x 12 = 69 in
5.9 x 12 = 70.8 in - not five foot nine

Fractions of an inch

Below the inch, imperial measurement goes binary rather than decimal: halves, quarters, eighths, sixteenths. A tape measure marked in sixteenths resolves to 1.5875 mm, which is finer than most people assume and coarser than a metric rule marked in millimetres.

Where decimal feet are correct

How to use the feet to inches converter

Multiply by twelve. The foot is twelve inches by definition, so the conversion is exact and there is nothing to round. The risk on this page is not in the arithmetic at all — it is in what gets typed into the box, because a decimal number of feet and a feet-and-inches height look almost identical and mean different things.

Somebody converting a height usually holds a figure like five foot nine. Typed as 5.9 that becomes 70.8 inches, because nine tenths of a foot is 10.8 inches. The intended answer was 69. The calculator did nothing wrong and the result is nearly two inches too tall.

The fix is to convert the height to a single number before typing anything. Whole feet times twelve, plus the loose inches. Five times twelve is sixty, plus nine is sixty-nine. That figure is unambiguous, survives any form, and cannot be misread the way a decimal can.

Decimal feet are not a mistake in themselves — surveying and construction in the United States use them deliberately, because arithmetic on tenths beats arithmetic on twelfths. The mistake is mixing the two notations in one workflow, which is how a column of figures ends up containing both 5.75 and 5.09 meaning the same kind of thing.

Entering a height without losing two inches

Any form with a single feet box forces a choice, and the safe one is to not use it for a height at all. Convert to total inches first. It takes one multiplication and one addition, and the resulting number cannot be misinterpreted by the form, by a spreadsheet, or by whoever reads it next.

Spreadsheets make this worse rather than better. A column containing 5.9 and 5.10 as heights will sort 5.10 first, because numerically it is smaller, and will average to something meaningless. Storing total inches avoids both problems and can be formatted for display later.

Why twelve, and where the inch itself comes from

Twelve divides evenly by two, three, four and six. That is the entire case for it, and before decimal arithmetic was routine it was a strong one: a third of a foot is exactly four inches, where a third of a metre is 33.33 centimetres. Halves, thirds and quarters all come out whole.

The inch itself is now defined against the metre rather than against anything physical. The 1959 international agreement fixed the yard at exactly 0.9144 metres, which makes the inch exactly 2.54 centimetres and the foot exactly 0.3048 metres. Nothing about the modern foot derives from a human foot.

Every feet value, worked out

68 common feet figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.

Questions people ask