Length Converter — Metric & Imperial

UK road signs use miles and yards, but most other measurements are metric. This converter handles metres, centimetres, millimetres, kilometres, feet, inches, yards and miles, which makes it useful for property listings quoted in square footage, for DIY jobs and for running distances. It keeps full precision internally and also rounds to practical decimal places for everyday use, so you can take either the exact figure or the one you would actually say out loud.

The core equivalents are fixed by definition rather than by convention. One inch has been exactly 2.54 cm since 1959, one foot is 12 inches or 30.48 cm, one yard is 3 feet or 0.9144 m, and one mile is 1,760 yards, 5,280 feet or 1.60934 km. Going the other way, 1 km is 0.621 miles. Following a height through shows the pattern: 5 ft 10 in is 70 inches, and 70 × 2.54 gives 177.8 cm, or 1.778 m. That is also why British heights are spoken as 5'10" but written as 178cm in medical records.

Britain metricated between 1965 and 1995, then carved out exemptions that survive today, chiefly road signs, pints of beer and cider, and milk in glass bottles. The EU stopped demanding metric road signs in 2009. Construction has been metric only since the 1980s, which is why estate agents dual-quote a room as 14'6" × 12' and 4.42m × 3.66m, or an older 8x10 room as 2.44m × 3.05m. Land surveying went metric in 1995, driving licences have used kg for vehicle weights since 2014, and pilots and mariners still work in nautical miles, where 1 NM is 1.852 km.

Familiar distances give the numbers some shape. Land's End to John o'Groats is 874 miles (1,407 km) by road, London to Edinburgh 405 miles (652 km), and the Dover to Calais crossing only 21 miles (34 km). The London Marathon covers 26.2 miles (42.2 km), an athletics track is 400m, and a Premier League pitch runs 100-110m by 64-75m. The M25 loops 117 miles (188 km) and the Underground network holds 250 miles (402 km) of track. Measured upward instead, the Big Ben tower stands 96m (315 ft) against the Shard at 310m (1,016 ft). Motorway driver location signs mark every 1/10 mile, and the country holds 50,000 miles of public road and 15,500 miles of railway track.

Trades work in millimetres and never in centimetres, because a decimal point in the wrong place turns into an expensive cut. A board is 2400mm, not 2.4m. A standard UK internal door is 1981mm tall, which is 78 inches, and either 762 or 838mm wide, with ceilings at 2.4m in new builds and 2.5m or more in older houses. A brick is 215×102.5×65mm, and timber sold as 2×4 is a nominal name for a piece measuring 38×89mm once planed. A 5m tape suits DIY and an 8m tape the trades, a laser measure at £30-£100 holds ±1.5mm over 30m, and a surveyor's total station manages ±0.5mm over 100m.

Below the millimetre the prefixes keep going. One micron (μm) is 0.001 mm, and a human hair is about 70μm across. One nanometre (nm) is 0.001 μm, and the DNA double helix measures 2nm. Above the kilometre the units stretch just as far in the other direction. The Moon averages 384,400 km (238,855 miles) from Earth and the Sun 150 million km (93 million miles), while a light year, the distance light travels in one year, is 9.461 × 10^15 m or 5.879 × 10^12 miles. The Hubble telescope resolves detail down to 0.05 arcseconds.