Feet-Inch Calculator

Guide to feet, inches and fractions

Tape measures in the US are marked in feet, inches and fractions of an inch, usually down to 1/16". That's easy to read but awkward to add, subtract or divide, which is why this calculator turns everything into inches, does the math, and turns the answer back into feet, inches and a fraction.

Reading the fractions

Each inch is split in half (1/2), then quarters (1/4, 3/4), eighths (1/8, 3/8, 5/8, 7/8) and sixteenths. Longer tick marks are the bigger fractions. A reading of 7 and 3 ticks past the 1/4 mark on a 1/16 tape is 7 7/16". Always reduce: 8/16 = 1/2, 12/16 = 3/4.

Converting to decimals

Inches → decimal feet: divide inches by 12. Fraction → decimal: divide the top by the bottom. So 5' 7 3/8" = 5 × 12 + 7 + 0.375 = 67.375 in = 5.615 ft. To go back, multiply the decimal part of an inch by 16 and round: 0.3" × 16 = 4.8, which is 5/16". Our feet-inch to decimal chart lists the common values.

Worked example: adding two measurements

5' 7 3/8" + 2' 9 5/8": add feet (7 ft), add inches (16 in), add fractions (3/8 + 5/8 = 1 in). 16 + 1 = 17 in = 1' 5", so the total is 8' 5". Typed into the calculator as 5' 7 3/8 + 2' 9 5/8, the answer is the same.

Dividing a board evenly

To split 8' 5" into 3 equal pieces: 101 in ÷ 3 = 33.667 in = 2' 9 11/16" (to the nearest 1/16). Remember each saw cut removes a kerf of about 1/8" on a typical circular saw blade, so subtract kerfs first if the pieces must be exact.

Stairs

Divide the total rise by your maximum riser height and round up to get the number of risers. Then divide the rise by that count for the actual riser height. With a 105" rise and a 7¾" maximum: 105 ÷ 7.75 = 13.5, so 14 risers of 7½". There is one fewer tread than risers, so 13 treads at 10" gives a total run of 130". The IRC model residential code commonly used in the US sets a 7¾" maximum riser and a 10" minimum tread, but always check your local code.

Rafters and board feet

For a roof pitch of P in 12, rafter length = run × √(144 + P²) ÷ 12. A 12 ft run at 6/12 is 12 × √180 ÷ 12 = 13.42 ft before overhang. Board feet = thickness (in) × width (in) × length (ft) ÷ 12, using nominal sizes as lumber yards usually do, so a 2×6 that's 10 ft long is 10 board feet.

Rounding to the right precision

Choose the fraction to suit the job. Rough framing is usually fine to 1/8", while trim, cabinets and furniture often need 1/16" or better. The calculator rounds only the final answer. All the in-between math is done at full precision, so adding ten measurements doesn't pile up rounding errors the way repeated tape-reading does. If you're laying out repeated spacing, such as studs 16" on centre or balusters, calculate each mark from the starting point rather than measuring from the previous mark. That stops small errors from adding up along the run.

Common mistakes

More: inches to feet conversion, area calculator, tile calculator.