Length Converter

Length converter for metric, imperial and astronomical units

This length converter turns a distance in one unit into any of the 17 others, from nanometers up to parsecs. Choose the unit you have and the unit you need, and the result is calculated from the exact definition of each unit rather than from rounded rules of thumb.

Typing in the result box converts the other way, from the target unit back to the source. Below the result, a table also gives that length in every supported unit, which helps when you are not sure which unit a drawing or spec sheet expects.

How the units are defined

Every conversion passes through the meter. Each unit is stored as an exact number of meters, so feet to miles is simply feet → meters → miles, with no rounding in the middle.

The imperial units all follow from the 1959 international yard and pound agreement, in which the United States, the United Kingdom and several Commonwealth countries fixed the yard at exactly 0.9144 m.

The metric units (nm, µm, mm, cm, dm, m, km) are powers of ten of the meter, so they are exact by construction.

Example conversions

1 in      = 25.4 mm
6 ft      = 1.8288 m
1 mi      = 1.609344 km
100 m     = 328.0839895 ft
42.195 km = 26.21875746 mi
1 nmi     = 1.150779448 mi

The first three results are exact. Metric-to-imperial conversions usually end in a long decimal because 0.3048 does not divide evenly into a power of ten: one meter is 3.280839895… feet. The marathon distance makes a good check, coming out at just over 26.2 miles.

The US survey foot

Before 1959 the United States defined the meter as exactly 39.37 inches, which makes a foot 1200/3937 m. Land surveyors kept that "survey foot" so existing maps and coordinates would not shift. It is two parts per million longer than the international foot: 1 ftus = 1.000002 ft.

That is about 1 cm per mile (5280 survey feet = 5280.01056 ft), but state plane coordinates run into the millions of feet, where a value of 2,000,000 differs by roughly 4 feet between the two definitions. The survey foot was officially retired at the end of 2022, yet it still appears in older deeds, plats and GIS data, so choose it only when the source says so.

Nautical and astronomical distances

A nautical mile is exactly 1852 m. Ships and aircraft combine it with the knot, one nautical mile per hour, so the speed converter is the natural companion for route planning.

At the far end of the scale are the astronomical unit (exactly 149,597,870,700 m), the light-year (the distance light travels in a Julian year of 365.25 days, 9,460,730,472,580,800 m) and the parsec (648,000/π au, about 3.26 light-years). For stellar distances, the astronomical distance converter adds light-seconds, kiloparsecs and solar radii.

Precision and number input

Significant digits can be set to 3, 4, 5, 6, 8, 10, 12 or 15; the default is 10. At 10 digits a light-year shows as 9460730473000 km, and at 15 you see the full 9460730472580.8 km. Results of 10¹⁵ or more, or below one millionth, switch to exponent notation, so one inch in light-years reads 2.684782118e-18.

Commas are treated as thousands separators and removed, so 1,609.344 reads as 1609.344. A decimal comma is therefore not understood: type 2.5, not 2,5, which would become 25. A hint in the status line points this out when it happens.

Common mistakes

• Using 2.54 the wrong way round. An inch holds 2.54 cm, so divide centimeters by 2.54 to get inches: 180 cm = 70.86614173 in.
• Reading decimal feet as feet and inches. 5.75 ft is 5 ft 9 in, not 5 ft 7.5 in. It equals 175.26 cm.
• Mixing up statute and nautical miles. The nautical mile is about 15% longer.
• Squaring the factor in your head. A square foot is 0.3048² m², not 0.3048 m². Use the area converter for surfaces.

The calculation happens in your browser, so the values you enter are never sent anywhere.