How to add and subtract feet and inches
Work in inches, then convert back: that avoids every carrying mistake. The calculator does exactly this:
- Turn each length into inches: 8 ft 6-1/2 in = 8 × 12 + 6.5 = 102.5 in.
- Add or subtract: 102.5 + 31.75 = 134.25 in.
- Divide by 12 for the feet (11) and keep the remainder as inches (2.25 in = 2-1/4 in).
Subtraction works the same way. When the inches would go negative, borrow 12 from the feet: 10 ft 2 in − 3 ft 8 in = 9 ft 14 in − 3 ft 8 in = 6 ft 6 in.
Fractions of an inch
A tape measure divides the inch into 16ths, so results are rounded to the nearest 1/16 in by default; switch to 1/8 or 1/32 under More options. The table converts the common marks:
| Fraction | Decimal inch | Millimetres |
|---|---|---|
| 1/16 | 0.0625 | 1.59 |
| 1/8 | 0.125 | 3.18 |
| 3/16 | 0.1875 | 4.76 |
| 1/4 | 0.25 | 6.35 |
| 5/16 | 0.3125 | 7.94 |
| 3/8 | 0.375 | 9.52 |
| 7/16 | 0.4375 | 11.11 |
| 1/2 | 0.5 | 12.7 |
| 5/8 | 0.625 | 15.88 |
| 3/4 | 0.75 | 19.05 |
| 7/8 | 0.875 | 22.23 |
Multiplying and dividing lengths
- Multiply by a number for repeated parts: 12 treads of 10-1/2 in = 126 in = 10 ft 6 in.
- Divide by a number to split a length evenly: a 12 ft wall in 5 equal bays = 28.8 in = 2 ft 4-13/16 in.
- How many fit divides one length by another and gives the leftover: 12 ft ÷ 16 in = 9 spaces exactly, the reason a 12 ft wall takes 10 studs (see the stud calculator).
Inches to feet calculator: conversion table
To convert inches to feet, divide by 12: the whole number is feet and the remainder is inches. Decimal feet are the inches divided by 12 without splitting.
| Inches | Feet and inches | Decimal feet | Millimetres |
|---|---|---|---|
| 16 | 1′ 4″ | 1.3333 | 406 |
| 24 | 2′ 0″ | 2 | 610 |
| 30 | 2′ 6″ | 2.5 | 762 |
| 36 | 3′ 0″ | 3 | 914 |
| 48 | 4′ 0″ | 4 | 1,219 |
| 60 | 5′ 0″ | 5 | 1,524 |
| 72 | 6′ 0″ | 6 | 1,829 |
| 92.625 | 7′ 8-5/8″ | 7.7188 | 2,353 |
| 96 | 8′ 0″ | 8 | 2,438 |
| 100 | 8′ 4″ | 8.3333 | 2,540 |
| 120 | 10′ 0″ | 10 | 3,048 |
| 144 | 12′ 0″ | 12 | 3,658 |
Feet and inches to decimal feet and metric
Decimal feet are inches divided by 12; one inch is exactly 25.4 mm, so one foot is 304.8 mm. Plans and estimates often mix the two: a roof rafter of 14′ 5-9/16″ is 14.46 ft. Toggle the panel to m · mm to type metres and millimetres and still get feet and inches back. For lumber orders the linear feet calculator totals lengths, and the stair calculator uses the same tape-style input.
Common feet and inches mistakes
Tape-measure math goes wrong in a few predictable places:
- Reading decimal feet as inches. 5.5 ft is 5 ft 6 in, not 5 ft 5 in.
- Forgetting to carry. 6 ft 8 in + 3 ft 6 in is 9 ft 14 in, which is 10 ft 2 in.
- Rounding at every step. Keep full fractions and round once at the end.
- Misreading the tape. 7/16 and 7/8 sit close together on a tape; count the marks from the nearest inch.
Frequently asked questions
How do you add feet and inches?
Convert both to inches, add, then divide by 12: the whole number is feet and the remainder is inches.
How many inches is 5 ft 7 in?
5 × 12 + 7 = 67 inches, or 170.2 cm.
How do you convert inches to feet?
Divide by 12. 100 in ÷ 12 = 8 ft with 4 in left over, or 8.333 decimal feet.
What is 0.75 ft in inches?
9 inches. Multiply decimal feet by 12.
How do I write 7 3/4 inches as a decimal?
7.75 in. Three quarters is 0.75; one sixteenth is 0.0625.