Unit Converters

Every length conversion is a multiplication by a fraction that equals 1. A kilometre and 1,000 metres are the same distance, so (1,000 m ÷ 1 km) is just another way of writing 1. Multiply by it and the number changes while the length stays exactly the same.

The trick is to write the fraction so the unit you want to get rid of sits in the denominator, where it cancels. This method is called dimensional analysis. The converter chains two such fractions through metres: the first takes your unit to metres, the second takes metres to the unit you asked for.

Length is how far apart two points are along a straight line. The metre is the SI base unit, and since 1983 it has been fixed by the speed of light: it is the distance light travels in a vacuum in 1/299,792,458 of a second.

The imperial and US units on this page are tied to the metre by exact definitions. Since the international agreement of 1959, one yard is exactly 0.9144 m and one inch is exactly 25.4 mm. Everything else follows: 1 ft = 12 in = 0.3048 m, and 1 mi = 1,760 yd = 5,280 ft = 1,609.344 m. At the small end the list runs down to the picometre and the ångström, and at the large end up to the astronomical unit, the light-year and the parsec.

• Picometre (pm)1 pm = 1e-12 mOne trillionth of a metre. The Bohr radius of the hydrogen atom is about 52.9 pm.
• Angstrom (Å)1 Å = 1e-10 m0.1 nm, or 100 pm. Used for atomic distances and X-ray wavelengths.
• Nanometre (nm)1 nm = 1e-9 mOne billionth of a metre. Visible light runs from roughly 380 to 750 nm, and green light sits near 550 nm.
• Micrometre (µm)1 µm = 0.000001 mOne millionth of a metre, or a thousandth of a millimetre. A human hair is roughly 70 µm across and a red blood cell about 7 µm.
• Mil (thou) (mil)1 mil = 0.0000254 mOne thousandth of an inch. Used for coating thickness and wire. Not the same as a millimetre.
• Millimetre (mm)1 mm = 0.001 mOne thousandth of a metre. Used for small parts, rainfall and paper sizes.
• Centimetre (cm)1 cm = 0.01 mOne hundredth of a metre, or 10 millimetres. Used for body height and clothing in most countries.
• Inch (in)1 in = 0.0254 mExactly 25.4 mm since the 1959 international agreement. Twelve inches make a foot.
• Decimetre (dm)1 dm = 0.1 mOne tenth of a metre, or 10 cm. Rarely used on its own, but a cubic decimetre is exactly one litre.
• Foot (ft)1 ft = 0.3048 mExactly 0.3048 m, or 12 inches.
• Yard (yd)1 yd = 0.9144 mExactly 0.9144 m, or 3 feet. Used for sports fields and fabric.
• Metre (m)The SI base unit. Defined from the speed of light: the distance light travels in 1/299,792,458 of a second.
• Fathom (ftm)1 ftm = 1.8288 mSix feet, the traditional unit for water depth at sea.
• Decametre (dam)1 dam = 10 mTen metres. Seldom used in practice, but it completes the metric ladder between the metre and the hectometre.
• Chain (ch)1 ch = 20.1168 m66 feet, the surveyor's chain. There are 10 chains in a furlong and 80 in a mile.
• Hectometre (hm)1 hm = 100 mOne hundred metres. The hectare is a square hectometre, which is where that unit gets its name.
• Furlong (fur)1 fur = 201.168 m220 yards, one eighth of a mile. Horse races are still run over furlongs.
• Kilometre (km)1 km = 1,000 mOne thousand metres. The road-distance unit in most of the world.
• Mile (mi)1 mi = 1,609.344 mExactly 1,609.344 m, which is 5,280 feet or 1,760 yards. The statute mile, used on US and UK roads.
• Nautical mile (nmi)1 nmi = 1,852 mClose to one minute of arc of latitude. A speed of one knot is one nautical mile per hour.
• League (lea)1 lea = 4,828.032 mThree statute miles here. Historical leagues varied from place to place.
• Astronomical unit (au)1 au = 149,597,870,700 mFixed in 2012 at exactly this many metres. Close to the average Earth to Sun distance.
• Light-year (ly)1 ly = 9.46073e15 mThe distance light covers in one Julian year of 365.25 days. A unit of distance, not of time.
• Parsec (pc)1 pc = 3.085678e16 mAbout 3.26 light-years. The distance at which one astronomical unit spans one arcsecond.