Feet to metres
A foot is exactly 0.3048 metres, which follows from twelve inches of exactly 2.54 cm each. Six feet is 1.8288 m, a ten-foot ceiling is 3.048 m, and 35,000 feet is 10,668 m. In the other direction a metre is 3.2808 feet, so the "about three feet" everyone uses is roughly eight and a half per cent short.
How to convert feet to metres
Feet come with inches attached and this converter takes one number, so the inches have to be folded in first. Divide them by twelve and add: five feet nine inches is 5 + 0.75 = 5.75 ft, which is 1.7526 m, or 175.26 cm. The three worth memorising are 3 in = 0.25, 6 in = 0.5 and 9 in = 0.75; everything else is the inches over twelve.
Aviation is the one industry that stayed on feet worldwide while going metric for nearly everything else, and the reason is safety rather than sentiment. Vertical separation only works if every aircraft in a block of airspace is reading the same scale, so a mixed system is worse than either system alone. Flight levels are quoted in hundreds of feet, which makes FL350 an altitude of 35,000 ft, or 10,668 m. China, Mongolia and North Korea still assign cruising levels in metres, and the boundaries where the two systems meet are handled with published conversion tables rather than mental arithmetic. Russia moved onto feet in November 2011.
There were also two American feet until very recently. The US survey foot was 1200/3937 of a metre, longer than the international foot by two parts per million, and it survived because state plane coordinate systems had been computed with it since the 1930s. Two parts per million is nothing across a room. Across a coordinate quoted in the millions of feet, which is ordinary in a state plane grid, it is about two feet on the ground. NIST and NOAA retired it at the end of 2022, and surveys after that date use the international foot.
What people use it for
- Converting a room or ceiling height from an American listing
- Reading a cruising altitude or a flight level in metres
- Turning a height in feet and inches into metres for a form
- Checking a trail’s elevation gain given in feet
- Costing rope, cable or hose sold by the foot
Questions
Nine inches is 0.75 of a foot, so enter 5.75. The answer is 1.7526 metres, or 175.26 cm.
Because the inch was defined as exactly 2.54 cm in 1959, and twelve of those make 30.48 cm. Nothing here was measured; it was agreed.
3.28084 feet, or 3 feet 3.37 inches. Rounding it to three feet is about eight and a half per cent low, which is far too coarse for anything structural.
A slightly longer foot used in American land surveying, differing by two parts per million. It was withdrawn at the end of 2022 in favour of the international foot.
Because vertical separation depends on everyone using the same scale, and the historical standard was feet. Changing it would mean changing every altimeter, chart and procedure at once.
A flight level of 35,000 feet, which is 10,668 metres. Flight levels are pressure altitudes quoted in hundreds of feet against a standard setting rather than against local pressure.
The same factor applies, because a difference in height is still a length. Three thousand feet of climb is 914.4 m.
Six feet, so 1.8288 m. It is a depth unit from sounding lines; most modern charts print metres instead, and mixing the two on one chart is a known navigational hazard.