| Reference | Feet (ft) | Centimeters (cm) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.974409 ft | 29.7 cm |
| Average adult human height | 5.57743 ft | 170 cm |
| A football pitch (length) | 344.488 ft | 10500 cm |
| A marathon | 138435 ft | 4219500 cm |
| Height of Mount Everest | 29032.2 ft | 884900 cm |
The foot is a unit of length equal to exactly 0.3048 metres, or twelve inches. Its plural is feet and its symbol is ft, though a single prime is used in technical drawing and a straight apostrophe informally.
Units based on the human foot appear across the ancient world, and their lengths varied considerably. The Roman pes measured about 296 millimetres and was divided into twelve unciae, an arrangement English inherited directly. Medieval Europe supported dozens of local feet, and a merchant crossing a few borders might encounter several. The English foot was standardised in stages, but its exact modern value dates only from the International Yard and Pound Agreement of 1959.
Aviation is the foot's most significant surviving international domain. Aircraft altitude is reported in feet almost everywhere, and flight levels are expressed in hundreds of feet, so FL350 means 35,000 feet. Russia and China historically used metres, and China's transition to feet for most flight levels in 2011 removed a genuine safety hazard at the boundaries of adjoining airspace. Vertical separation standards are defined in feet, and reduced vertical separation minima allow 1000 feet between aircraft above 29,000 feet.
Construction in the United States works in feet and inches throughout, and lumber, ceiling heights and room dimensions all follow. Broadcasting and film retain the foot for lens focus scales and for measuring film stock. Water depth in diving is given in feet in American practice and metres elsewhere, a divergence that dive computers accommodate by offering both.
Twelve as a divisor is the foot's practical strength. It divides evenly by two, three, four and six, which suits carpentry and layout work where thirds and quarters are common. A decimal unit divides cleanly only by two and five.
Water and timber keep two of its derivatives alive. The fathom, six feet, measured the depth of water for as long as a sounding line was thrown by hand, and old charts are still marked in fathoms even where new ones use metres. The board foot, a volume of one foot square by one inch thick, remains the trading unit of sawn timber in North America, so a lumber yard quotes prices per thousand board feet. Both illustrate a general habit of the imperial system: rather than adding prefixes, it names each new quantity after the job it does, which makes the units memorable to those who use them and opaque to everyone else.
One foot equals 12 inches, 0.3048 metres, or 30.48 centimetres exactly.
The centimetre is one hundredth of a metre, written cm. Among the metric subdivisions it is the one people reach for most readily in daily life, occupying the range where objects are small enough to hold but too large to measure comfortably in millimetres.
Body measurement is its most common application. Height, waist, chest and inside leg are recorded in centimetres across most of the world, and clothing is sized accordingly. Paediatric growth charts plot height and head circumference in centimetres against age. Rainfall over long periods, snow depth and the dimensions of furniture and luggage are all reported the same way.
The unit had a formal role in science for nearly a century. The centimetre-gram-second system, adopted by the British Association for the Advancement of Science in 1874, took the centimetre as its base unit of length. CGS units such as the erg, the dyne and the gauss were standard in physics until the metre-kilogram-second system displaced them, and SI formally superseded CGS in 1960. Some CGS units persist in astronomy and in parts of electromagnetism.
Volume follows naturally. A cubic centimetre, written cm3 or cc, equals exactly one millilitre. Engine displacement is often quoted in cubic centimetres, particularly for motorcycles, and medical syringes are marked the same way.
Despite its usefulness, the centimetre sits awkwardly in engineering practice. Technical drawings prefer millimetres precisely to avoid mixing units that differ by a factor of ten, since a misplaced decimal point between the two is a plausible and expensive error.
Map scales often make the unit explicit. A 1:25,000 map means one centimetre on the paper represents 25,000 centimetres on the ground, or 250 metres, so four centimetres cover a kilometre. Walkers and orienteers use this relationship constantly, and it is one of the clearer illustrations of why a decimal system is convenient: converting between the two scales requires only moving a decimal point.
Two well-known wavelengths fall in this range and are named by it. Neutral hydrogen radiates at 21 centimetres, a line predicted in 1944 and detected seven years later, and because hydrogen fills the galaxy that single wavelength has mapped the spiral arms of the Milky Way and the rotation curves that first indicated dark matter. Domestic microwave ovens work at 12.2 centimetres, chosen from a band set aside for industrial and medical use rather than for any special resonance with water. In both cases the centimetre is the natural unit because the wave is the size of a hand.
One centimetre equals 10 millimetres or 0.01 metres, and is very close to 0.3937 inches.