Twelve inches to the foot
The foot has been divided into twelve inches since Roman times, and both are now pinned to the metre: an inch is exactly 2.54 cm and a foot exactly 30.48 cm. Because 12 does not divide neatly into tens, decimal feet look odd — 70 inches is 5.8333… feet — which is why the calculator also gives the feet-and-inches form (5 ft 10 in) that people actually use.
Reading decimal feet
The fraction after the decimal point is twelfths, not tenths: 0.5 ft is 6 inches, 0.25 ft is 3 inches and 0.1 ft is 1.2 inches. Survey and construction plans often use decimal feet for exactly this reason, so 12.75 ft on a drawing is 12 ft 9 in, not 12 ft 7.5 in.