How this instrument works
An inch is a twelfth by etymology as much as by arithmetic. Roman surveyors split their pes into twelve unciae, and that word travelled into English twice — once as inch, once as ounce, the twelfth part of the old troy pound. Twelve outlived every later reform because it splits cleanly: halves, thirds, quarters and sixths of the foot all land on whole inches, which is precisely what carpenters marking out stud walls want.
The modern legal anchor arrived in 1959, when standards bodies in six English-speaking nations agreed to define one yard as 0.9144 metre exactly. That fixed the foot at 0.3048 metre and the inch at 25.4 millimetre. Notice what did not move: twelve. That ratio sits inside customary usage rather than between systems, so re-anchoring everything to metric arithmetic left it untouched, and no revision since has proposed otherwise.
Both units remain in daily service. American and Canadian building trades frame at sixteen-inch centres on eight-foot sheets, ninety-six inches tall; height goes onto driving licences as feet plus inches; screens, pipe bores and bicycle wheels are sold in inches; aviation still reports altitude in feet across most of world airspace. Surveyors are the exception — they work in decimal feet and tenths, which is why survey plans reading 6.50 never mean six feet five.
- Type your length into the Feet (ft) field — 6 ft is preloaded as the starting reference.
- Read your answer on the Inches (in) line; it recomputes as you type, to six decimal places.
- Entering mixed measurements? Turn the inch part into decimals first — 6 ft 3 in becomes 6.25 ft, because 3 ÷ 12 = 0.25.
- Tape reading includes a fraction? Add it as decimals: five-eighths becomes 0.625 in, or 0.052083 ft.
- Going backwards, divide inches by 12; its whole part gives feet and any leftover fraction times 12 returns spare inches.
Worked example — a 6 ft bench rail
A bench rail is cut to 6 ft and has to be drilled at four-inch centres. Enter 6 in the Feet (ft) field and the Inches (in) line returns 72.0 — six lots of twelve, with nothing rounded away. Divide 72 by 4 and your layout falls out at once: eighteen gaps, nineteen hole centres, first and last sitting flush with each end.
Seventy-two is also why 6 ft reads as such convenient height on plans. It covers four full sixteen-inch stud bays plus eight inches, and it is exactly three quarters of a standard 96 in sheet, so that rail can be marked straight off any sheet edge without any second calculation.
Questions
Is a foot exactly 12 inches?
Yes — exactly, by definition rather than by measurement. The inch is defined as one twelfth of the foot within customary usage itself, so this factor carries no uncertainty and needs no tolerance. Whatever precision you feed in comes straight back out: 6 ft is 72 in, 0.001 ft is 0.012 in, and any rounding you see here happens only in how many decimals get displayed.
Why twelve inches rather than ten?
Because twelve divides well. Halves, thirds, quarters and sixths of the foot all come out whole — 6, 4, 3 and 2 inches — whereas decimal feet give broken figures for anything in thirds. Trades that lay out work by folding the dimension in half, then half again, keep that arithmetic in their heads; ten pushes one third of the foot to 3.333. Twelve is Roman inheritance: their pes held twelve unciae, and that divisor outlived the empire that set it.
Does 6.5 feet mean 6 feet 5 inches?
No, and misreading it that way is by far this conversion's most expensive slip. The decimal is a fraction of the foot, so 6.5 ft is half a foot past six — 0.5 × 12 = 6 — giving 6 ft 6 in, one full inch beyond what careless reading suggests. Convert the fractional part deliberately by multiplying it by 12, then, if what remains is not whole, carry on into eighths or sixteenths for tape work.
Do imperial and US customary inches differ?
Not for length. Since 1959 the inch has been 25.4 mm and the foot 0.3048 m on both sides of the Atlantic, and twelve inches to the foot was never in dispute anyway. Those systems part company on volume — the US gallon holds 231 cubic inches against the imperial gallon's 277.42 — but converting feet to inches is safe without first asking which country your figure came from.
What about the US survey foot?
It was retired for new work at the end of 2022, and it never touched this ratio. The survey foot measured 1200/3937 m, roughly two parts per million longer than the international foot, and it was still divided into twelve survey inches. So 6 survey feet is 72 survey inches; only physical length differs, by roughly one ten-thousandth of an inch over that distance. State plane coordinate grids are where that hair mattered.
How do I convert inches back to feet?
Divide by 12. A tidy way by hand is to peel off whole feet first: 100 in is 8 ft plus 4 in left over, because 8 × 12 = 96. As decimals, 100 ÷ 12 = 8.333 ft. Repeating decimals show up going this way and never going out, since dividing by 12 brings in the factor of three that most measurements do not carry.