Add and subtract feet and inches

How do I add feet and inches?

Add the inches together and the feet together, then carry every 12 inches over as a foot. For 5 ft 7 3/8 in + 6 ft 8 1/2 in: the inches make 7 3/8 + 8 1/2 = 15 7/8 in, which is 1 ft 3 7/8 in; the feet make 5 + 6 + 1 = 12. The total is 12 ft 3 7/8 in.

How do I subtract feet and inches?

Subtract the inches first. If the top number has fewer inches, borrow a foot and add 12 inches to it. For 10 ft 2 in − 4 ft 7 1/4 in: borrow to get 9 ft 14 in, then 14 − 7 1/4 = 6 3/4 in and 9 − 4 = 5 ft. The answer is 5 ft 6 3/4 in.

How do I divide a length into equal parts?

Turn the length into inches, divide, and turn the answer back into feet and inches. An 8 ft board cut into 3 equal pieces: 96 in ÷ 3 = 32 in, or 2 ft 8 in each. A 10 ft wall split into 3 bays: 120 ÷ 3 = 40 in, or 3 ft 4 in. Leave room for the saw kerf, about 1/8 in per cut.

How do I convert feet and inches to decimal feet?

Turn everything into inches and divide by 12. 5 ft 7 3/8 in is 60 + 7.375 = 67.375 in, and 67.375 ÷ 12 = 5.615 ft. For centimeters, multiply the inches by 2.54: 67.375 × 2.54 = 171.1 cm.

What do the small marks on a tape measure mean?

Most US tapes mark every 1/16 inch. The longest marks between inches are 1/2, then 1/4, 1/8 and the shortest 1/16. Count sixteenths from the last inch and reduce: 6/16 is 3/8 and 12/16 is 3/4. This calculator rounds its answer to 1/16 in unless you pick 1/8 or 1/32.

How do I convert inches to feet and inches?

Divide by 12: the whole number is the feet and the remainder is the inches. 89.625 inches is 7 ft 5.625 in, which the calculator rounds to 7 ft 5 5/8 in. Going the other way, multiply the feet by 12 and add the inches.

Why does my answer change when I round to 1/8 instead of 1/16?

Because the result is rounded to the nearest mark. Splitting 14 ft 2 in into 7 equal parts gives 24.286 in each, which is 2 ft 5/16 in to the nearest 1/16 and 2 ft 1/4 in to the nearest 1/8 in. Choose the finest division you can actually measure.

Working in inches

The calculator converts both lengths to inches, does the arithmetic and converts the answer back to feet, inches and a fraction. An inch is exactly 25.4 millimeters and a foot is exactly 12 inches, so no conversion error creeps in. Rounding happens only once, at the end, to the nearest 1/16, 1/8 or 1/32 in. Because the rounding is applied to the final value, adding many short pieces in one calculation is more accurate than rounding each piece first.

Worked example: a countertop run and a wall

A run of 8 ft 4 1/2 in plus a run of 3 ft 9 3/4 in is 12 ft 2 1/4 in. The difference between them is 4 ft 6 3/4 in. Six boards of 5 ft 3 1/2 in laid end to end measure 31 ft 9 in. A 14 ft 2 in wall divided into 5 equal bays gives 2 ft 10 in per bay. Each of these is rounded to the nearest 1/16 in.

Common mistakes

The most common is treating the inches like decimals: 3 ft 9.75 in is not 3.975 ft, because 9.75 in is 0.8125 ft (9.75 ÷ 12), giving 3.8125 ft. Another is forgetting to carry 12 inches into a foot after adding. A third is ignoring the saw kerf when dividing a board into pieces, about 1/8 in per cut, which adds up over many cuts. Finally, a tape reading of 3/16 is not the same as 3/8; reduce a count of sixteenths before reading it.

Sources: NIST SP 811, Guide for the Use of the International System of Units, Appendix B.8 (the international inch is exactly 25.4 mm, the foot exactly 0.3048 m). Related: decimal to fraction calculator, square footage calculator, stair calculator and fraction calculator.