Feet and inches calculator
Show the working — with your numbers
The calculator does exact arithmetic on what you type, then rounds only the tape reading to the fraction you choose. It does not allow for saw kerf, the hook on the tape, or marks that were read wrong — measure twice.
Type up to four measurements in feet, inches and fractions, choose plus or minus for each, and optionally multiply or divide the total — to split a length into equal parts, say. You get the answer as a tape reading to 1/8, 1/16 or 1/32 in, in decimal feet and decimal inches, and in millimetres and metres.
Worked example
Imperial. 12′ 6 3/8″ + 4′ 9 5/8″ = 16′ 15 8/8″ = 17′ 4″ (the 3/8 + 5/8 carries one inch, and 16 inches carries a foot). Take off 2′ 7 3/4″ and you have 14′ 8 1/4″, which is 176.25 in. Divide by 3 for three equal pieces: 4′ 10 3/4″ each (58.75 in, before the saw kerf).
Metric. 3,810 mm + 1,450 mm − 790 mm = 4,470 mm (4.47 m, about 14′ 8″ on an imperial tape). Divide by 3: 1,490 mm each.
On the job
Split a length, then take off the kerf. Three pieces from a 14′ 8 1/4″ board need two cuts. With a 1/8 in blade, subtract 1/4 in before dividing: 176 ÷ 3 = 58 2/3 in, a hair under 58 11/16″ each.
Stack from one end. When several pieces run end to end, add their lengths and mark each joint from the same starting point instead of re-hooking the tape at every mark. The total here is the mark for the last joint.
Metric drawings, imperial tape. Type the drawing's millimetres, read the feet-and-inches line, and round to the sixteenth your tape shows. 1,490 mm is 58.66 in — 58 11/16″.
What went wrong?
Quick referencefor tape math
- Inches ↔ mm
- 1 in = 25.4 mm exactly · 1 ft = 304.8 mm · 1 m = 3′ 3 3/8″
- Fractions
- 1/16 = 0.0625 in (1.59 mm) · 1/8 = 0.125 (3.18 mm) · 1/4 = 0.25 (6.35 mm)
- Decimal feet → inches
- decimal × 12 — 0.25 ft = 3″ · 0.5 ft = 6″ · 0.75 ft = 9″
- Decimal inch → sixteenths
- decimal × 16, round — 0.8 in × 16 = 12.8 → 13/16″
- Carry and borrow
- 12 inches to the foot, both ways
The formula
inches = feet × 12 + inches + fraction · answer = Σ(±measurements) × or ÷ k
Every measurement becomes decimal inches (or millimetres) before any arithmetic, so a carry or a borrow can never go wrong. Only the final answer is rounded — to the tape fraction you choose — and turned back into feet and inches.
Questions
- How do I add feet and inches?
- Add the feet, add the inches, then carry every 12 inches into a foot. 5′ 8″ + 3′ 7″ is 8′ 15″, and 15 inches is 1′ 3″, so the answer is 9′ 3″. Fractions add the same way — 3/8 + 5/8 is one whole inch to carry.
- How do I subtract feet and inches when the inches are smaller?
- Borrow a foot as 12 inches, not 10. 10′ 2 1/4″ − 3′ 8 1/2″: borrow to make 9′ 14 1/4″, then 14 1/4 − 8 1/2 = 5 3/4, and 9 − 3 = 6, so 6′ 5 3/4″. Borrowing ten is the classic tape-math slip.
- How do I turn decimal feet into feet and inches?
- Keep the whole feet, multiply the decimal by 12 for inches, then multiply what is left of the inches by 16 and round for sixteenths. 7.65 ft is 7 ft and 0.65 × 12 = 7.8 in; 0.8 × 16 = 12.8, about 13/16. So 7′ 7 13/16″.
- How many millimetres are in a foot and an inch?
- An inch is exactly 25.4 mm, so a foot is 304.8 mm. 17′ 4″ is 208 in × 25.4 = 5,283.2 mm, or 5.283 m. Switch the calculator to metric and every figure is carried both ways.
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