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US customary + metric equivalents

Feet and Inches Calculator for Construction Measurements

Add or subtract two tape measurements, scale one length by a factor, and convert the exact answer to decimal feet, total inches, millimetres and metres.

Feet and Inches Calculator

SiteCalculators · mixed construction measurement check

Enter mixed measurements

Keep the sign separate from the feet, inches and fraction components. Values larger than 12 inches are normalised in the answer.

Tape fractions
Operation
Multiplication and division use a unitless factor, so the result remains a length.
First measurement
ft
in
Second measurement
ft
in
Display precision
The exact total-inch value remains available even when the tape-style display is rounded.
For site records, retain the unrounded measurement used in the calculation and state the tape or reporting precision separately.
Measurement limitation: This calculator performs arithmetic only. It does not correct tape sag, temperature, slope, hook position, calibration, drawing scale or construction tolerance.

How feet and inches arithmetic works

Construction dimensions are often written as a combination such as 18 ft 7 3/8 in. Reliable arithmetic starts by converting the entire measurement to one unit. Twelve inches make one foot, so each foot contributes 12 inches. The fraction is then added to the whole-inch component.

total inches = (feet × 12) + whole inches + (numerator ÷ denominator)

Addition or subtraction is performed on total inches. The answer is then divided back into whole feet, whole inches and a reduced fraction. This avoids manual borrowing mistakes when one inch component crosses a 12-inch boundary.

Worked addition with tape fractions

Add 18 ft 7 3/8 in to 6 ft 11 7/8 in. The first length is 223.375 inches and the second is 83.875 inches. Their sum is exactly 307.25 inches. Dividing by 12 gives 25 ft 7 1/4 in, which is 25.6041667 decimal feet or 7,804.15 mm.

The example button loads those values. Changing the display setting to the nearest 1/2 inch would show 25 ft 7 1/2 in, but the result panel would still retain 307.25 exact total inches and report the 0.25-inch rounding difference.

Subtracting and negative answers

Subtraction uses the same single-unit method. If the second measurement is longer, the answer is shown with a negative sign before the complete mixed measurement. Keep the sign attached to the whole length rather than applying it to only the feet or inches component.

Scaling one construction length

Multiplication and division apply a unitless factor to the first length. This is appropriate for operations such as halving a setting-out dimension, dividing a run into equal bays or applying a drawing scale. The calculator deliberately does not multiply two lengths: length multiplied by length is an area and requires a different result unit.

Feet, total inches and metric equivalents

Decimal feet are useful in surveying, machine control and civil drawings, while mixed feet and inches are common on architectural or trade information. One international inch is exactly 25.4 mm, so the metric conversions are deterministic. The usefulness of the displayed precision still depends on the original measurement.

decimal feet = total inches ÷ 12
millimetres = total inches × 25.4
metres = total inches × 0.0254

Frequently asked questions

How do I add feet, inches and fractions?

Convert both measurements to total inches, add them, then divide the result into whole feet, whole inches and a reduced fraction. This calculator performs that arithmetic without discarding the exact decimal result.

Can the calculator show decimal feet and millimetres?

Yes. Every result includes rounded feet and inches, exact total inches, decimal feet, millimetres and metres.

What does rounding to the nearest sixteenth mean?

The displayed mixed measurement is rounded to the closest 1/16 inch. The exact total-inch result is retained separately and the calculator reports any difference introduced by tape-fraction rounding.