Feet and Inches Calculator
Add, subtract, multiply, or divide lengths in feet and fractional inches, or convert decimal feet or centimeters. Enter your measurements and choose how precisely to display the inch fraction.
Calculate or Convert Lengths
Feet and Inches Formula
Where:
- \(f_A,f_B\) — whole feet in lengths A and B.
- \(i_A,i_B\) — whole inches in lengths A and B.
- \(n_A,n_B\) — numerators of the selected sixteenth-inch fractions, from 0 to 15.
- \(T_A,T_B\) — the lengths expressed as total inches; \(T\) is the result of a length operation.
- \(k\) — the multiplier or divisor; \(r\) is the unitless ratio of length A to length B.
Converting each length to inches first makes the selected arithmetic operation straightforward. The feet-and-inches display is rounded to the chosen fraction precision; the decimal and metric results use the unrounded value.
Feet and Inches – Calculation Example
Example: add the default lengths, 5 ft 8 1/2 in and 2 ft 3 1/4 in, using the default precision of the nearest 1/16 inch.
\[T_A=5(12)+8+\frac{8}{16}=68.5\text{ in},\qquad T_B=2(12)+3+\frac{4}{16}=27.25\text{ in}\]
\[T=68.5+27.25=95.75\text{ in}=7\text{ ft }11\frac{3}{4}\text{ in}\]
Result: 7 ft 11 3/4 in, equivalent to 95.75 in, approximately 7.979167 decimal ft, 243.205 cm, or 2.43205 m.
How to use the calculator
- In Calculator tab, choose Add, subtract, multiply or divide to work with lengths, or Convert decimal feet or centimeters to convert a single value.
- For length arithmetic, choose an Operation. Enter Length A using its Feet, Whole inches, and Inch fraction fields. Enter Length B in the corresponding fields when adding, subtracting, or calculating a ratio.
- For multiplication or division by a number, enter it in Multiplier or divisor. For conversion instead, enter Decimal length and select Input unit: Decimal feet or Centimeters.
- Choose Display fraction precision, from the nearest 1/2 inch to the nearest 1/16 inch. Read the feet-and-inches result alongside the total inches, decimal feet, centimeters, and meters. A length ÷ length operation shows a unitless ratio instead.
Understanding feet, inches, and length ratios
A foot contains 12 inches, so feet-and-inches measurements do not behave like decimal numbers. For example, 5 ft 8 in is \(5+8/12\) feet, not 5.8 feet. Expressing both measurements in total inches before calculating avoids that common mistake. The calculator then carries each full group of 12 inches into feet when it formats the result.
The Inch fraction field lets you enter a measurement in sixteenths of an inch, including familiar values such as 1/2, 1/4, and 3/4 inch. This is useful when adding cut lengths, finding the difference between two dimensions, or scaling a measurement. Subtraction can produce a negative length, which indicates how far length A falls below length B.
Multiplying a length by a number scales it; dividing by a number splits or reduces it. Dividing length A by length B answers a different question: how many times as long is A as B? Because both measurements are converted to inches before division, their units cancel. A ratio of 2 means A is twice as long as B; it is not a result of 2 inches or 2 feet.
The conversion tab also accepts decimal feet or centimeters directly. Decimal feet may be convenient for calculations, while feet and fractional inches are often easier to mark on a tape measure. For a separate metric-to-imperial conversion, see the meters to feet converter.
Precision and practical limits
Display fraction precision changes only the feet-and-inches display. If an exact calculation lands between the available marks, that display rounds to the nearest selected fraction; total inches, decimal feet, centimeters, and meters retain the calculated value. They may therefore appear slightly different from a conversion made using the rounded feet-and-inches display. Choosing 1/16 inch gives a finer display than choosing 1/2 inch, but it does not make the original measurement more accurate.
Enter non-negative whole numbers in the Feet and Whole inches fields. You can enter 12 or more whole inches; the result automatically carries them into feet. A divisor cannot be zero, and length B must be greater than zero when calculating a ratio. The calculator limits entered lengths and the magnitude of length results to one billion inches.
Unit conversion does not account for measurement tolerance, saw-kerf loss, or rounding required by a particular project. Check those separately before cutting or ordering materials. If you are turning drawing measurements into real-world lengths, a map scale calculator may also help; for standardized sheet dimensions, use the paper size converter.
Frequently Asked Questions
How do I subtract feet and inches by hand when the inches do not subtract evenly?
Borrow 1 foot as 12 inches before subtracting. For example, to calculate 6 ft 2 in minus 3 ft 9 in, rewrite 6 ft 2 in as 5 ft 14 in. Then subtract the feet and inches separately: 5 − 3 = 2 ft and 14 − 9 = 5 in. The difference is 2 ft 5 in.
How do I convert decimal feet or centimeters to feet and inches by hand?
For decimal feet, keep the whole-number feet and multiply the decimal part by 12. Thus, 4.75 ft is 4 ft plus 9 in. For centimeters, divide by 2.54 to get total inches, then divide by 12 to separate feet from remaining inches. Keep extra decimal places until the end if you plan to round to a tape-measure fraction.
How do I enter a tape-measure mark such as 7/8 inch?
Enter the feet and whole inches as usual, then select 7/8 in the Inch fraction field. A tape-measure fraction describes only the portion beyond the whole-inch mark: a reading of 3 ft 4 7/8 in means 3 feet, 4 whole inches, and another 7/8 inch. Do not enter 4.875 in the Whole inches field.
What fraction precision should I choose for a woodworking measurement?
Choose a fraction precision that matches the marks you can reliably measure and cut. Nearest 1/16 inch may suit a carefully marked tape measure, while nearest 1/8 or 1/4 inch may be more practical for rough layouts. A finer setting changes the displayed rounding, not the accuracy of the dimensions you entered or the tolerances of your tools.
Can I use this calculator to find square feet or square inches?
No. Multiplying a length by a plain number gives another length, not an area. To find area, multiply two lengths expressed in the same unit: for example, 3 ft × 4 ft = 12 square feet. You can use this calculator to convert each dimension to decimal feet first, then multiply those values separately.
What do zero or negative results mean for a length calculation?
A zero difference means the two entered lengths are equal; a negative difference means the first is shorter than the second. Multiplying by a negative number can also produce a signed result, although physical material lengths are normally reported as non-negative magnitudes. If a result is very close to zero, its rounded feet-and-inches display may show zero while the decimal output retains a small difference.
Why might equal cut lengths not add up to the original board length?
Each saw cut removes material, called kerf, so dividing a board’s length by the number of pieces can overstate the usable length per piece. Subtract the total expected kerf before dividing. For example, if four pieces require three cuts at 1/8 in each, reserve 3/8 in for those cuts. Allow extra material for squaring ends or trimming mistakes.
I want to divide 3 3/8" by two, as a check so I can do more calculations. I set accuracy to the nearest 1/16". Your result was 1 3/4" . Here is my math:
3" divided by 2 is 1 1/2"
3/8" divided by two is 3/16"
1 1/2" + 3/16" = 1 8/16" + 3/16" = 1 11/16"
not 1 3/4"
where is the disconnect?