Feet and Inches Calculator (add, subtract, multiply, to cm)

Understanding the feet and inches calculator — concept and principle

This calculator adds, subtracts, multiplies and divides lengths given in feet and inches, and shows the result in feet and inches, to the nearest 1/16 inch, in total inches, decimal feet, centimetres, metres and millimetres. Carpenters, DIYers and anyone measuring rooms or furniture in imperial units use it.

Add in inches, then regroup — with the default inputs: 9′ 6.75″.

How it is calculated

Convert each length to inches: total = feet × 12 + inches
Apply the operation: A + B, A − B, A × number or A ÷ number
Split back: whole feet = ⌊total ÷ 12⌋, inches = remainder
Nearest 1/16 inch: round total inches × 16 and reduce the fraction
Decimal feet = inches ÷ 12; cm = inches × 2.54; m = inches × 0.0254; mm = inches × 25.4

Inches above 11 are carried into feet automatically.

The principle

Feet and inches are a mixed-base system: 12 inches make a foot, so ordinary decimal arithmetic on "5.85" feet goes wrong. Working entirely in inches avoids that, and splitting the result back into feet and inches makes it readable. Tape measures are marked in sixteenths, so rounding to the nearest 1/16 inch gives a value you can mark directly. The metric conversion is exact: since 1959 the international inch is defined as 2.54 cm.

Worked example

Defaults: A = 5 ft 8.5 in, B = 3 ft 10.25 in, operation A + B.

A = 5 × 12 + 8.5 = 68.5 in; B = 3 × 12 + 10.25 = 46.25 in.
Sum = 114.75 in.
114.75 ÷ 12 = 9 ft remainder 6.75 in: 9′ 6.75″; nearest 1/16: 9′ 6 3/4″.
Decimal feet = 9.5625.
Metric = 114.75 × 2.54 = 291.47 cm = 2.9146 m = 2,914.6 mm.

How to read the result

Use the 1/16 inch form for marking a cut, decimal feet for area or material calculations, and the metric values when buying products sold in metric sizes. When cutting, remember the saw blade removes material (the kerf), so allow for it.

Common mistakes

Treating 5.8 ft as 5 ft 8 in; it is 5 ft 9.6 in.
Adding inches without carrying 12 into a foot.
Rounding too early in a series of measurements.
Forgetting blade width when dividing a board into equal pieces.

Keep in mind: Rounding to 1/16 inch can accumulate over many pieces. For precise work, keep the exact value and round only when marking.

Frequently asked questions

How do I convert decimal feet to feet and inches?

Keep the whole feet and multiply the decimal part by 12 for inches: 9.5625 ft = 9 ft 6.75 in.

How many centimetres is one inch?

Exactly 2.54 cm.

Can I enter more than 12 inches?

Yes; extra inches are carried into feet.

Sources: NIST Special Publication 811 (2008), Appendix B (1 ft = 0.3048 m; 1 in = 2.54 cm)

Checked 9 Nov 2026. Exact definitions from NIST Special Publication 811 (2008), Appendix B: 1 ft = 0.3048 m and 12 in = 1 ft, hence 1 in = 2.54 cm (international yard and pound agreement of 1959).

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