How to use the feet and inches calculator
- Type the first length as feet, inches and a fraction of an inch. 5 ft 7 3/8 in goes in as 5, 7 and 3/8.
- Add more lengths. For each one, choose Add or Subtract. Press Add another length for as many as you need.
- Optional: multiply the total, for example by 3 for three identical pieces, or divide it, for example by 4 to split a board into four equal parts.
You do not have to tidy the numbers first. 18 inches is fine, and so is 2.5 in the feet box. The result updates as you type and is shown five ways: feet and inches with a fraction, the same in the short 8' 2 5/8" style, decimal feet, total inches, and centimeters and meters. Copy result gives you the sum with every step.
How feet and inches are added
Mixing feet, inches and fractions is where mistakes creep in, so the calculator turns everything into inches first. A foot is 12 inches (NIST lists "12 inches (in) = 1 foot (ft)"), so each length is feet x 12 + inches + the fraction. Then it adds and subtracts, and turns the total back into feet and inches.
Example: 5 ft 7 3/8 in plus 3 ft 9 3/4 in, minus 1 ft 2 1/2 in. Press Try an example to see it.
| Length | Working | Inches |
|---|---|---|
| 5 ft 7 3/8 in | 5 x 12 + 7 + 0.375 | 67.375 |
| + 3 ft 9 3/4 in | 3 x 12 + 9 + 0.75 | 45.75 |
| - 1 ft 2 1/2 in | 1 x 12 + 2 + 0.5 | -14.5 |
| Total | 67.375 + 45.75 - 14.5 | 98.625 |
98.625 inches is 8 feet (96 inches) with 2.625 inches left over, and 0.625 is 5/8. So the answer is 8 ft 2 5/8 in. In decimal feet that is 98.625 / 12 = 8.2188 ft, and in metric 98.625 x 2.54 = 250.51 cm, or 2.505 m.
When you subtract by hand you sometimes have to borrow a foot. 5 ft 2 in minus 3 ft 8 in: 2 inches is less than 8, so borrow 12 inches from the feet to make 4 ft 14 in, then 4 ft 14 in minus 3 ft 8 in is 1 ft 6 in. Working in inches avoids the borrowing: 62 - 44 = 18 inches, which is 1 ft 6 in.
Multiplying and dividing lengths
Multiply when you need several pieces of the same length. Three shelves of 2 ft 7 1/2 in each are 31.5 x 3 = 94.5 inches, or 7 ft 10 1/2 in of board.
Divide to split a length into equal parts. 10 ft 5 in divided by 3 is 125 / 3 = 41.667 inches. That is not a whole number of sixteenths, so the calculator rounds it to the nearest 1/16 inch and says so: 3 ft 5 11/16 in. Decimal feet, total inches and the metric figures are worked out from the exact value, not the rounded one.
When you cut a board into parts, the saw blade turns a thin strip into sawdust at every cut, so the real pieces come out slightly shorter. Take the blade's width off before you divide if it matters.
Fractions of an inch as decimals and millimeters
US tape measures often mark inches in halves, quarters, eighths and sixteenths. This table turns each sixteenth into a decimal inch and into millimeters, using 1 inch = 25.4 mm exactly.
| Fraction | Decimal inch | Millimeters |
|---|---|---|
| 1/16 | 0.0625 | 1.5875 |
| 1/8 | 0.125 | 3.175 |
| 3/16 | 0.1875 | 4.7625 |
| 1/4 | 0.25 | 6.35 |
| 5/16 | 0.3125 | 7.9375 |
| 3/8 | 0.375 | 9.525 |
| 7/16 | 0.4375 | 11.1125 |
| 1/2 | 0.5 | 12.7 |
| 9/16 | 0.5625 | 14.2875 |
| 5/8 | 0.625 | 15.875 |
| 11/16 | 0.6875 | 17.4625 |
| 3/4 | 0.75 | 19.05 |
| 13/16 | 0.8125 | 20.6375 |
| 7/8 | 0.875 | 22.225 |
| 15/16 | 0.9375 | 23.8125 |
To turn decimal feet back into feet and inches, keep the whole feet, multiply what is left by 12 for inches, then multiply the leftover part of an inch by 16 for sixteenths. 8.21875 ft is 8 ft and 0.21875 x 12 = 2.625 in, and 0.625 x 16 = 10 sixteenths, which is 5/8.
Inches to centimeters
The inch has an exact metric size. NIST Handbook 44 lists 1 inch (in) as "2.54 centimeters (exactly)", and a foot as 0.3048 m. So the calculator multiplies the total inches by 2.54 for centimeters and divides by 100 for meters. No rounding happens until the figures are shown: centimeters to 2 decimal places and meters to 3.
Questions and answers
How do you add feet and inches?
Turn each length into inches (feet x 12 + inches), add them, then divide by 12: the whole number is feet and the remainder is inches. 5 ft 7 in + 3 ft 9 in is 67 + 45 = 112 inches, which is 9 ft 4 in.
How do you subtract feet and inches?
Turn both into inches, subtract, and turn the answer back. 5 ft 2 in minus 3 ft 8 in is 62 - 44 = 18 inches, or 1 ft 6 in. On paper, borrow 12 inches from the feet when the inches are too small.
How do I convert decimal feet to feet and inches?
Keep the whole feet and multiply the decimal part by 12. 8.25 ft is 8 ft and 0.25 x 12 = 3 in, so 8 ft 3 in. If the inches have a decimal, multiply that part by 16 to get sixteenths.
How many centimeters are in an inch?
Exactly 2.54. A foot is 12 x 2.54 = 30.48 cm, and 1/16 inch is 1.5875 mm.
Why does the answer say it is rounded?
Results in feet and inches are shown to the nearest 1/16 inch, a sixteenth being a common mark on US tape measures. When the exact answer falls between two sixteenths, for example after dividing, the calculator rounds to the nearer one and tells you. The decimal and metric figures stay exact.
Sources
Checked on 7 October 2026. If a rule has changed, please tell us.