Feet and Inches Calculator (Add, Subtract, Convert)

Working in feet and inches without slipping a digit

Adding two measurements like 5 ft 8 in and 3 ft 9 in is the kind of arithmetic that looks trivial until you are standing on a job site doing it in your head. The trick is that inches roll over at 12, not 10. Add the inches (8 + 9 = 17), then carry one foot for every 12 inches: 17 inches becomes 1 foot 5 inches, so the answer is 9 ft 5 in. Subtraction works the same way in reverse, borrowing 12 inches from a foot when the top inch value is too small. That borrow step is where mental math goes wrong most often.

Why convert to decimal feet

Estimating software, spreadsheets, and most drafting tools want a single decimal number, not a feet-and-inches pair. Decimal feet is just total inches divided by 12, so 9 ft 5 in is 9.4167 ft. Quoting lumber, fencing, or trim in decimal feet also makes it far easier to multiply by a price per foot.

The metric side

One inch is defined as exactly 2.54 centimeters, which makes the conversion clean rather than approximate. The calculator returns centimeters and meters alongside the imperial result, which is handy when a spec sheet or an overseas supplier works in metric.

What you get back

Enter two measurements, choose add or subtract, and the result panel shows the answer as feet and inches, as decimal feet, as a total inch count, and in yards and metric units. A negative result simply means the second measurement was larger than the first, and the sign is kept so the direction stays clear.