The Feet and Inches Calculator adds, subtracts, multiplies and divides lengths written the way tradespeople actually write them, such as 7' 3-1/2" or 21' 7-3/8". Every operation converts each length to total inches, runs the arithmetic, then formats the result back to feet, inches and a fraction at a chosen rounding denominator. Decimal feet, centimetres, metres and millimetres are shown alongside the primary feet-and-inches result.
Add and subtract feet and inches
Addition and subtraction both work by converting every length to a single unit, total inches, before combining them, since feet and inches cannot be added directly the way two decimal numbers can. Once the totals are added or subtracted, the result converts back to feet and inches, carrying or borrowing across the 12-inch boundary as needed.
Total inches = (Feet × 12) + Inches (+ fraction)
Result = Sum or difference of total inches, reformatted to feet + inchesSubtracting 3' 7" from 10' 0" cannot borrow inches directly from a zero, so the calculator converts both lengths to inches first: 120 in minus 43 in equals 77 in, which reformats to 6' 5". Working in total inches removes the borrowing and carrying steps that make feet-and-inches arithmetic error-prone when done by hand on paper.
Multiply and divide by a factor
Multiplying or dividing a length by a plain number, rather than by another length, is the operation behind scaling a dimension or splitting a run into equal parts. The calculator converts to total inches, applies the factor, and reformats the result, exactly as with addition and subtraction.
Result = (Total inches × factor), reformatted
Result = (Total inches ÷ factor), reformattedMultiplying 7' 6" by 3 converts to 90 inches, multiplies to 270 inches, and reformats to 22' 6", useful for finding the total length of three identical framing members cut from the same specification. Dividing 100' by 7 converts to 1,200 inches, divides to about 171.43 inches, and reformats to 14' 3-7/16" once the remainder is rounded to the nearest sixteenth, the kind of division that comes up when splitting a fixed run into a specific number of equal sections.
Set the rounding fraction
Every division or multiplication that does not land on a whole number of inches needs a rounding rule, since a tape measure and a speed square read in fractions, not in decimals carried to several places.
The calculator supports rounding to the nearest 1, 1/2, 1/4, 1/8, 1/16, 1/32 or 1/64 of an inch, then reduces the fraction to its simplest form.
| Denominator | Typical use |
|---|---|
| 1/2, 1/4 | Rough framing, concrete layout |
| 1/8, 1/16 | Standard finish carpentry, trim, stairs |
| 1/32, 1/64 | Cabinetry, fine joinery |
A result of 0.5 inch divided by 2 gives exactly 0.25 inch. At a rounding denominator of 1/64, that converts to 16/64, which reduces to 1/4, so the reported result is 0' 0-1/4" either way; the finer denominator only matters when a division does not land on such a clean fraction. Rounding always happens after the arithmetic is complete, on the final result, never on an intermediate step, so accumulated rounding error from repeated mid-calculation rounding never enters the answer.
Add 21' 7-3/8" and 10' 3/4"
Adding a length of 21 feet 7-3/8 inches to a length of 10 feet 3/4 inch gives a total of 31 feet 8-1/8 inches. Every step below works in total inches, the same path the calculator runs internally. 1. Convert the first length to inches. 21' 7-3/8" = (21 × 12) + 7.375 = 252 + 7.375 = 259.375 in
2. Convert the second length to inches. 10' 3/4" = (10 × 12) + 0.75 = 120 + 0.75 = 120.75 in
3. Add the two totals. 259.375 + 120.75 = 380.125 in
4. Carry into feet. 380.125 ÷ 12 = 31.677 ft, or 31 ft with a remainder of 8.125 in
5. Convert the remainder to a fraction. 0.125 in = 1/8 in, giving a final result of 31' 8-1/8"
Every fraction in a real-world measurement, not just whole eighths, converts to a decimal the same way: divide the fraction's numerator by its denominator and add it to the whole inch count before the feet-and-inches conversion runs. This is why the parser accepts inputs like 7-3/8" directly rather than requiring a pre-converted decimal.
Write feet and inches correctly
Feet and inches have standard symbols, the prime and double prime marks, that are distinct from apostrophes and quotation marks even though most keyboards substitute the closest available character.
A foot mark is a single prime ('), and an inch mark is a double prime ("); together they format a length as 7'3" with no space before either mark and no period after either.
A fraction of an inch is joined to the whole inch count with a hyphen, as in 7'3-1/2", rather than a space, since a space could be misread as a separate measurement. Values under a foot omit the foot mark and figure entirely, written as a bare inch measurement such as 9-1/2" rather than 0'9-1/2". The calculator's own output follows this convention exactly, so a result can be copied directly onto a cut list or a drawing without reformatting.
Convert feet and inches to decimals
Decimal feet are the format most calculators, spreadsheets and CAD programs expect, and converting to decimal requires dividing the inch portion of a measurement by 12 and adding it to the whole foot count. This is the same conversion used at the start of nearly every area and volume calculation on this site.
Decimal feet = Feet + (Inches ÷ 12)A length of 7' 3" converts to 7 + (3 ÷ 12) = 7 + 0.25 = 7.25 ft. A length that includes a fraction, such as 7' 3-1/2", first converts the fraction to a decimal inch, 3.5, then divides: 7 + (3.5 ÷ 12) = 7.2917 ft. Decimal inches, rather than decimal feet, are the more useful format for a machinist or a fine-woodworking layout, and simply skip the division step: 7' 3-1/2" is 87.5 decimal inches, since (7 × 12) + 3.5 = 87.5.
Convert feet and inches to metric
Metric conversion runs through the same fixed factors used across every other calculator on this site: one inch equals exactly 2.54 centimetres, and one foot equals exactly 0.3048 metres. Neither figure is an approximation; both are defined constants under the international agreement that standardised the inch and the foot.
Centimetres = Total inches × 2.54
Metres = Total feet × 0.3048
Millimetres = Centimetres × 10A length of 7' 3-1/2", or 87.5 decimal inches, converts to 87.5 × 2.54 = 222.25 cm, or 2.2225 m, or 2,222.5 mm. Construction drawings produced for an international audience, or hardware specified by a manufacturer outside the United States, commonly need this conversion run in the opposite direction as well, from a metric spec back into feet and inches for a US-based crew.
Divide a run into equal spaces
Splitting a fixed total length into a specific number of equal spaces is the calculation behind stud spacing, joist layout and baluster spacing, and it differs from a plain division because the result usually needs rounding to a buildable fraction rather than an exact decimal.
A stair railing run of 118 inches divided into 8 equal baluster spaces gives 118 ÷ 8 = 14.75 inches per space, which is already a clean 3/4-inch fraction and needs no further rounding: 14-3/4" per space. A run that does not divide evenly, such as 120 inches into 7 spaces, gives 17.142857 inches per space, which rounds to 17-1/8" at a sixteenth-inch denominator; the small rounding difference across all 7 spaces, a fraction of an inch total, is usually absorbed at one end of the run rather than distributed invisibly across every space.
Frequently asked questions
How do I add feet and inches together?
Convert both lengths to total inches, add the two totals, then convert the sum back to feet and inches, carrying into a new foot every time the inches reach 12. Adding 21' 7-3/8" and 10' 3/4" gives 259.375 + 120.75 = 380.125 inches, which reformats to 31' 8-1/8".
How do I subtract feet and inches when the inches don't fit?
Convert both lengths to total inches first, then subtract, rather than trying to borrow a foot as 12 inches by hand. Subtracting 3' 7" from 10' 0" converts to 120 minus 43, or 77 inches, which reformats to 6' 5" without any separate borrowing step.
How do I multiply a length in feet and inches by a number?
Convert the length to total inches, multiply by the factor, then reformat the result. Multiplying 7' 6" by 3 converts to 90 inches, multiplies to 270 inches, and reformats to 22' 6".
How do I convert feet and inches to decimal feet?
Divide the inch portion by 12 and add it to the whole foot count. A length of 7' 3" converts to 7 + (3 ÷ 12), or 7.25 decimal feet.
How do I convert feet and inches to centimetres or metres?
Convert to total inches first, then multiply by 2.54 for centimetres, or convert to total feet and multiply by 0.3048 for metres. A length of 7' 3-1/2", or 87.5 inches, converts to 222.25 cm, or 2.2225 m.
What do the foot mark and inch mark (' and ") mean?
The single prime (') represents feet and the double prime (") represents inches, written directly after the number with no space, as in 7'3". A fraction of an inch attaches with a hyphen rather than a space, as in 7'3-1/2", to avoid it being read as a separate measurement.
How do I round a stair or layout measurement to the nearest 1/16 inch?
Convert the measurement to a decimal, multiply by 16, round to the nearest whole number, then divide by 16 and reduce the resulting fraction. A decimal remainder of 0.42857 inch, multiplied by 16, gives 6.857, which rounds to 7, giving 7/16 inch.
How do I divide a wall into equal stud spacing?
Divide the total wall length by the target number of spaces to find the exact spacing, then round to a buildable fraction. A 118-inch run split into 8 equal spaces gives exactly 14.75 inches, or 14-3/4", per space.
Why does 11-15/16" plus 1/16" equal 1 foot exactly?
Because 11-15/16 inches converts to 11.9375 decimal inches, and adding 0.0625 (1/16) brings the total to exactly 12.0 inches, which carries into a full foot with zero inches remaining: 1' 0". This carrying behaviour is identical to carrying a 1 when two decimal numbers sum past a round number.
How accurate is a tape measure reading to 1/32 inch in real construction?
A quality tape measure can be read consistently to about 1/32 inch by an experienced user, though the tape's own manufacturing tolerance and the flex in a long, unsupported reading both introduce small additional error. Rough framing is normally laid out to the nearest 1/8 or 1/16 inch, while finish carpentry and cabinetry commonly work to 1/32 or 1/64 inch where the tighter fit is visible in the finished piece.
Summary
The Feet and Inches Calculator adds, subtracts, multiplies and divides trade-format lengths by converting every value to total inches, running the arithmetic, and reformatting the result with carrying or borrowing handled automatically.
A rounding denominator from 1 down to 1/64 of an inch controls how the final fraction is expressed, and results convert to decimal feet, centimetres, metres and millimetres alongside the primary feet-and-inches figure.
This parser underlies every other calculator on this site that accepts a dimension in feet and inches, so a format like 7' 3-1/2" works identically whether it is entered here or on a concrete, tile or stair calculation.