DATUM

Feet and Inches Calculator — add, subtract, running dimensions

Add and subtract feet, inches and sixteenths as a running dimension string, with a quantity on every row and the residual left by rounding to 1/16.

Add and subtract a string of feet-and-inches dimensions the way a take-off reads, with a quantity on every row — and see the rounding the sixteenth-inch answer hides.

Six rows, added top to bottom. A row with no length, or a quantity of zero, is ignored. Enter a negative quantity to subtract that dimension from the running total.

exact 0 1/4 1/2 3/4 1 in —
One inch, divided into the sixteenths a tape actually carries. The triangle is where the exact total falls; the heavy tick is the mark you would read it off at. One sixteenth is 1.5875 mm, so the gap between them is the most a single reading can be wrong by.
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As a dimension string—
Exact total—
Total inches—
Nearest 1/16 in on a tape—
Rounding residual—
Metric equivalent—

Until the end of 2022 the United States had two feet, and this page uses the surviving one. The international foot is 0.3048 m exactly, fixed by the 1959 refinement of the yard; the U.S. survey foot was 1200/3937 m, two parts per million longer. NIST and NOAA retired the survey foot in a 2020 Federal Register notice, and it should not have been used for anything since 31 December 2022. Two parts per million is an eighth of an inch in a mile — invisible on a building, decisive on a boundary. Every figure below is the international foot, so a dimension taken off a plat drawn in survey feet before 2023 is very slightly not the dimension you will set out.

Feet and inches carries in two bases at the same time, which is why the arithmetic goes wrong by hand. The fraction carries at sixteen and the inches carry at twelve: 7/16 plus 5/8 is 17/16, which is 1‑1/16, which then has to be added to the whole inches, which may in turn carry a foot. Doing that six times in a column is where a take‑off goes quietly wrong. This page keeps every entry as an exact number of inches, adds them once, and only then converts back — and it prints the string it actually added, so you can check the terms rather than trust the answer.

Round once, at the end. A single reading rounded to the nearest sixteenth is out by at most 1/32 in. Round six readings one at a time and then add them, and the worst case is six of those — 3/16 in, nearly five millimetres, in a string a wall is built to. Rounding the total once caps the error at 1/32 in no matter how many terms there are. That is what the residual row is for: it is the size of the single rounding this page performed, and if it is zero, no rounding happened at all.

A tape cannot hold a metric dimension. One sixteenth is 25.4÷16 = 1.5875 mm exactly, so the marks on an imperial tape fall at intervals no metric dimension lands on. A 100 mm setting‑out dimension is 3.937008 in; the nearest mark is 3‑15/16 in, which is 100.0125 mm. That is harmless once and cumulative if you keep doing it, which is the argument for entering the millimetres and letting the rounding happen at the total. Switch the units select and the row fields become metres and millimetres; the scale above still shows an imperial inch, labelled in millimetres, so you can see what a sixteenth costs.

What this will not do: read a typed dimension. There is no box to type 5′ 6 1/2″ into, on purpose. On a real drawing set, 5′6 means five feet six inches to one trade and five feet six tenths to another; 6-1/2, 6 1/2 and 6.5 all appear on the same sheet meaning the same thing, and 6-1/2″ next to a hyphenated range means something else again. A parser has to guess, and a quantity calculator that guesses is worse than one that asks. The fields are separate so that nothing is inferred.

1 in = 25.4 mm exactly — National Bureau of Standards, Refinement of Values for the Yard and the Pound, 24 FR 5348, effective 1 July 1959 (F.R. Doc. 59‑5442): “The value for the inch, derived from the value for the Yard effective July 1, 1959, is exactly equivalent to 25.4 millimeters.” The same notice fixes 1 yard = 0.9144 m exactly, from which 1 ft = 0.3048 m.

The U.S. survey foot is retired — NIST and NOAA, Deprecation of the United States (U.S.) Survey Foot, 85 FR 62698, 5 October 2020: use of the survey foot (1200/3937 m) was to be discontinued after 31 December 2022, superseded by the international foot for all applications. This page uses the international foot only.

Everything else here is arithmetic rather than a standard: the sixteenth‑inch rounding, the quantity multiplier and the residual are properties of the numbers you enter, not of any published figure. The fraction shown is always reduced, so 12/16 reads 3/4.