Feet and Inches Calculator
Add or subtract feet-and-inches measurements and get the answer in feet, inches and metric.
Working
| First length | 8′ 6 1/2″ = 102.5 in |
| Second length | 3′ 9 1/4″ = 45.25 in |
| Sum in inches | 147.75 in |
| Back to feet | 12′ 3 3/4″ |
Why this is fiddly by hand
Feet and inches is a base-12 system sitting inside a decimal one, so you cannot just add the columns. The reliable method is to flatten both measurements to inches, do the arithmetic, then convert back:
8′ 6½″ = 102.5 in and
3′ 9¼″ = 45.25 in
102.5 + 45.25 = 147.75 in = 12′ 3¾″
Fractions on a tape measure
Tapes are marked in halves, quarters, eighths and sixteenths, so the “round to nearest” option exists to give you an answer you can actually find on the tape. Sixteenths are standard for carpentry; thirty-seconds and sixty-fourths turn up in machining.
Measure twice. Rounding to the nearest sixteenth introduces up to 1/32 in of error per measurement, which compounds across a run of cuts.