Feet and Inches Calculator — add and subtract
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.
Feet and Inches Calculator — add, subtract, running dimensions
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.
Quick answer: inch fractions in decimals and millimetres
| Inches | Decimal | Millimetres |
|---|---|---|
| 1/16" | 0.0625 | 1.59 mm |
| 1/8" | 0.125 | 3.17 mm |
| 1/4" | 0.25 | 6.35 mm |
| 3/8" | 0.375 | 9.52 mm |
| 1/2" | 0.5 | 12.7 mm |
| 5/8" | 0.625 | 15.88 mm |
| 3/4" | 0.75 | 19.05 mm |
| 1" | 1 | 25.4 mm |
| 1 ft | 12 | 304.8 mm |
1 inch is 25.4 mm exactly by definition, so these are exact. A running dimension like 10'-6 3/8" is 126.375 in, or 3,209.9 mm.
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.
| As a dimension string | — |
|---|---|
| Exact total | — |
| Total inches | — |
| Nearest 1/16 in on a tape | — |
| Rounding residual | — |
| Metric equivalent | — |
| Row 1 | — |
|---|---|
| Row 2 | — |
| Row 3 | — |
| Row 4 | — |
| Row 5 | — |
| Row 6 | — |
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. It is invisible on a building and 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. It also 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. That is 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.
What I check on a job
Making the dimension string close. Before I trust a total from this page, I make the string close. On a plan, the running dimensions along a wall should add up to the overall dimension printed above them, and when the sum here misses that overall, the miss tells me more than the answer does. It is rarely the arithmetic. It is usually one term measured to a different reference: one dimension to face of stud, the next to finished face, a third to a centerline, all on one line. The calculator adds what it is given, so I mark the reference against each dimension on the drawing before I enter it, and I only use the quantity field for pieces that genuinely repeat. The other trap is a dimension marked to verify in the field. I leave that row out, total the rest, and treat the remainder against the overall as the number to confirm on site, not one to build from. A string that closes on paper still has to close in the room.
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.
Worked example
Add 12'-6", twice 3'-4", and take away 1'-9". What is the string?
- 12'-6" = 150 in; 3'-4" × 2 = 80 in; 1'-9" = 21 in.
- 150 + 80 − 21 = 209 in.
- 209 ÷ 12 = 17 ft with 5 in left over.
17'-5" exactly — no sixteenths in this one, so the rounding residual is zero. Mixed strings with fractions are where the tape and the arithmetic drift apart.
6 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How do you add feet and inches with fractions?
Convert every row to sixteenths of an inch, add or subtract them, then carry back to feet, inches and a fraction. That is what the calculator does, and it shows the exact decimal beside the rounded string.
What is the rounding residual?
The difference between the exact sum and the nearest sixteenth. It is usually under 1/32 inch; it is shown so you know it exists.
Which foot does the calculator use, international or U.S. survey?
The international foot, 0.3048 m exactly. The U.S. survey foot (1200/3937 m, two parts per million longer) was retired by NIST and NOAA at the end of 2022, so it should not be used for new work.
How big is one sixteenth of an inch in millimetres?
1.5875 mm. That is also the most a single tape reading can be off by when the exact total falls between two marks, which is why the calculator prints the exact decimal beside the rounded string.