foot
Lengthexact by definition
The foot is exactly 0.3048 metres, twelve inches, fixed by the international yard and pound agreement of 1959. It is the primary unit of building dimensions and altitude in the United States, and of aviation altitude nearly everywhere. The factor is exact by definition.
Watch out: The US survey foot is 1200/3937 metre, about 2 parts per million longer than the international foot. On a 100 ft room it is meaningless, but on a state plane coordinate of 2,000,000 ft it is a 4 ft error. The United States retired the survey foot at the end of 2022, so all new work uses the international foot, but legacy survey data and older state plane coordinates may still carry the old definition and must be labelled.
| 1 ft | 0.3048 m |
The foot is one of the oldest units still in use, appearing in Greek, Roman and every medieval European measurement system, always at roughly the length of a human foot but never at the same value twice. Local feet differed from town to town well into the nineteenth century, which is exactly the problem national standards were created to end.
About the foot
Aviation is the one place where the foot is genuinely global. Flight levels, cloud bases, terrain clearance and vertical separation are quoted in feet in almost every country, including those that use metres for everything else, with China, Russia and North Korea as the notable exceptions. The reason is inertia with real safety weight behind it: instrumentation, procedures and pilot training were standardised on feet early, and mixed units in vertical separation would be a hazard rather than an inconvenience. A "flight level 350" aircraft is at 35,000 ft on a standard-pressure altimeter setting.
The other place feet refuse to leave is construction in the United States, where the foot-and-inch system survives because it subdivides well by hand. A 16 in stud spacing divides a 4 ft sheet exactly three times, and a 4 ft by 8 ft panel is the size it is because that grid works. The trap for anyone converting is the survey foot. Two parts per million sounds negligible until it is applied to a coordinate in the millions, where it moves a point by several feet, so surveying software insists you declare which foot a dataset uses.