Feet to metres
One foot is exactly 0.3048 metres, so multiply feet by 0.3048. Six feet is 1.8288 m, a ten-foot ceiling is 3.048 m, and a cruising altitude of 35,000 feet is 10,668 m. The factor follows from the inch being defined as 2.54 cm and there being twelve inches to the foot.
How to convert feet to metres
Feet and inches together need one extra step: divide the inches by twelve and add them on. Five feet nine inches is 5 + 9/12 = 5.75 feet, which is 1.7526 m. Aviation is the one field that stayed on feet worldwide for altitude while using metric for almost everything else, which is why flight levels are quoted in hundreds of feet.
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 was measured; it was agreed.
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 It was withdrawn at the end of 2022 in favour of the international foot.
Historical standardisation. Most of the world flies on feet for altitude, with China, Russia and a few others using metres, which is a genuine source of conversion errors in aviation.