How it's calculated
total in = ft × 12 + in, then ft = ⌊total ÷ 12⌋, in = total mod 12
ft = feet, in = inches. Convert each measurement to total inches (12 in per foot), add or subtract, then split the result back into feet and remaining inches.
Common questions
Does it handle the inch carry automatically?
Yes. Adding 8 in and 7 in rolls over to 1 ft 3 in for you, so you never hand-carry the 12s.
What is the inch to centimetre factor used?
The exact NIST factor of 1 inch equals 2.54 cm.