Feet and Inches Calculator: Add, Subtract, Convert

Estimate only. Check product labels and local building rules before you buy or build.

Quick answer

To add feet and inches, turn each length into inches (feet × 12 + inches + fraction), add them, then divide by 12: the whole number is feet and the remainder is inches. For example, 5 ft 7 3/8 in + 3 ft 9 3/4 in = 67.375 in + 45.75 in = 113.125 in, which is 9 ft 5 1/8 in. One inch is exactly 2.54 cm, so the same length is about 287.34 cm.

How to use the feet and inches calculator

Pick the operation: add or subtract two lengths, multiply or divide one length by a number, or convert a single length.
Enter each length as feet, inches and a fraction from the list. Leave feet empty to work in inches only; inches can be more than 12 or have decimals, such as 7.25.
For multiply and divide, enter a plain number, for example 7 to split a length into 7 equal parts.
Choose the smallest mark you will measure to: 1/16, 1/8 or 1/4 in. The result in feet and inches is rounded to that mark.
Read the result in feet and inches, total inches, decimal feet, centimeters, millimeters and meters. The decimal and metric values are the exact result, not the rounded one.

How do you add and subtract feet and inches?

Convert both lengths to inches, do the arithmetic in inches, then convert back. A foot is exactly 12 inches, so 5 ft 7 3/8 in is 5 × 12 + 7 + 3/8 = 67.375 in. Working in inches means you never have to borrow a foot when the inches of the second length are larger, as they are in 6 ft 2 in − 3 ft 9 in.

To turn inches back into feet and inches, divide by 12. The whole number is the feet and the remainder is the inches. Round the remainder to the smallest mark on your tape: multiply the decimal part by 16, round to a whole number, and write it over 16, reduced if you can. One inch is exactly 25.4 mm, so metric values need no rounding until you write them down.

total inches = feet × 12 + inches + fractionresult = first ± second, or length × n, or length ÷ nfeet = whole part of (result ÷ 12)inches = result − feet × 12sixteenths = round(decimal part of inches × 16)decimal feet = result ÷ 12centimeters = result × 2.54

Fractions of an inch to decimals and millimeters

Every value is exact: the decimal is the fraction worked out, and the millimeters are the decimal × 25.4.

Sixteenths of an inch as decimal inches and millimeters
Fraction	Decimal inches	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
1	1	25.4

Inches to decimal feet chart

Decimal feet are inches ÷ 12, rounded to four decimals. Centimeters are inches × 2.54 and are exact.

Whole inches as decimal feet and centimeters
Inches	Decimal feet	Centimeters
1	0.0833	2.54
2	0.1667	5.08
3	0.25	7.62
4	0.3333	10.16
5	0.4167	12.7
6	0.5	15.24
7	0.5833	17.78
8	0.6667	20.32
9	0.75	22.86
10	0.8333	25.4
11	0.9167	27.94
12	1	30.48

Worked examples

Adding two wall lengths for baseboard

Two walls that meet in a corner measure 5 ft 7 3/8 in and 3 ft 9 3/4 in. In inches that is 5 × 12 + 7 + 0.375 = 67.375 in and 3 × 12 + 9 + 0.75 = 45.75 in, so together they make 113.125 in.

113.125 ÷ 12 = 9 with 5.125 in left over, so the total is 9 ft 5 1/8 in. The same length is 9.4271 decimal feet, 287.34 cm or 2,873.4 mm.

Dividing a 10 ft 3 in wall into 7 equal spaces

10 ft 3 in is 123 in, and 123 ÷ 7 = 17.5714 in. On a tape marked in sixteenths, 0.5714 × 16 = 9.14, so the nearest mark is 9/16 and each space is 17 9/16 in, or 1 ft 5 9/16 in.

Seven spaces of 17 9/16 in add up to 122 15/16 in, which is 1/16 in short of the wall. To keep the error from adding up, measure every mark from the same starting end: the second mark is at 2 × 17.5714 = 35.1429 in, which rounds to 35 1/8 in, and the third at 52.7143 in, which rounds to 52 11/16 in.

How accurate is a tape measure?

A metal tape that meets NIST Handbook 44 may be off by up to 1/32 in over the first 6 ft and up to 1/16 in from 7 to 30 ft, too long or too short. The allowance grows to 1/8 in from 31 to 55 ft, 3/16 in from 56 to 80 ft and 1/4 in from 81 to 100 ft.

That is why 1/16 in is the finest rounding the calculator offers: past 6 ft, a finer fraction is smaller than the error the tape itself is allowed. Handbook 44 lets tapes be divided into inches and halves, quarters, eighths and further halvings of the inch, or into tenths and hundredths of a foot, so check which marks your own tape has.

Questions

How many inches are in a foot?

There are exactly 12 inches in a foot, so divide inches by 12 to get feet. 150 in ÷ 12 = 12.5, which is 12 ft 6 in: the whole number is the feet, and 0.5 × 12 = 6 is the inches left over. To go the other way, multiply the feet by 12 and add the inches, so 9 ft 5 in is 9 × 12 + 5 = 113 in.

What is 5/8 of an inch as a decimal and in millimeters?

5/8 in is 0.625 in, or 15.875 mm. Divide the top number by the bottom number for the decimal, 5 ÷ 8 = 0.625, then multiply by 25.4 for millimeters, because one inch is defined as exactly 25.4 mm. The fraction chart above lists every sixteenth, from 1/16 in (1.5875 mm) to 15/16 in (23.8125 mm).

How do you round a decimal to the nearest 1/16 inch?

Multiply the decimal part by 16, round to a whole number and put it over 16. For 0.3 in, 0.3 × 16 = 4.8, which rounds to 5, so 0.3 in is about 5/16 in. Reduce the fraction when you can: 8/16 is 1/2 and 12/16 is 3/4. When a value falls exactly halfway, the calculator uses the round-half-to-even rule from NIST SP 811 and picks the even sixteenth, so 3/32 in and 5/32 in both round to 1/8 in.

How do you convert feet and inches to centimeters?

Multiply the total inches by 2.54. 5 ft 7 in is 5 × 12 + 7 = 67 in, and 67 × 2.54 = 170.18 cm. The factor is exact: since July 1, 1959 the inch has been defined as 25.4 mm, so a foot is exactly 30.48 cm, or 0.3048 m. Keep the full value until the last step and round only the answer you write down.

What are decimal feet?

Decimal feet write the inches as a part of a foot, so 6 in is 0.5 ft and 9 ft 5 1/8 in is 9.4271 ft. To convert, divide the inches by 12: 5.125 ÷ 12 = 0.4271. NIST Handbook 44 allows tapes graduated in tenths and hundredths of a foot, so check whether a plan or a tape uses decimal feet before you read 9.5 ft as 9 ft 5 in. It is 9 ft 6 in.

Is a foot the same as a U.S. survey foot?

For building work, yes: the two differ by 2 parts per million, about 3 mm in a mile. The foot is exactly 0.3048 m, and the old U.S. survey foot is 1200/3937 m. Since January 1, 2023, NIST says the survey foot should be avoided except for historic and legacy applications, so it only matters when you compare long distances against old land surveys.

Sources

National Institute of Standards and Technology (NIST), NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement (checked October 19, 2026)12 inches = 1 foot; 1 inch = 2.54 cm and 1 foot = 0.3048 m exactly; the international foot is 0.999 998 survey foot and the mile on it is about 3 mm shorter.
National Institute of Standards and Technology (NIST), NIST Handbook 44 (2026), Section 5.52: Linear Measures (checked October 19, 2026)Tapes divided into inches and binary submultiples, or tenths and hundredths of a foot; metal tape tolerances from 1/32 in (6 ft or less) to 1/4 in (81 to 100 ft).
National Institute of Standards and Technology (NIST), SI Units: Length (checked October 19, 2026)The inch is exactly 25.4 mm, from the yard effective July 1, 1959.
National Institute of Standards and Technology (NIST), U.S. Survey Foot (checked October 19, 2026)Survey foot = 1200/3937 m; avoided from January 1, 2023 except for historic and legacy applications.
National Institute of Standards and Technology (NIST), NIST Guide to the SI, Appendix B: Conversion Factors (checked October 19, 2026)Rounding rule B.7.1: a value exactly halfway is rounded to the even digit.

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