Length Converter
Convert meters, feet, miles, kilometers — accurate to 8 decimal places.
About the Length Converter
The length converter translates between nine units of distance: millimetre, centimetre, metre, kilometre, inch, foot, yard, mile, and nautical mile. The conversion factors are all defined exactly in terms of the metre, the SI base unit of length since 1960 (per the SI Brochure, 9th edition, §2.3.1).
The metre itself has been redefined three times. The original 1793 definition was one ten-millionth of the distance from the equator to the North Pole along the Paris meridian. The 1960 definition used the wavelength of a krypton-86 emission line. The current 1983 definition, reaffirmed in 2019, fixes the speed of light in vacuum at exactly 299,792,458 m/s and defines the metre as the distance light travels in 1/299,792,458 of a second.
All imperial units are now defined in terms of SI: the international yard is exactly 0.9144 metre by the 1959 International Yard and Pound Agreement, which makes one foot exactly 0.3048 m and one inch exactly 0.0254 m. The statute mile is exactly 1609.344 metres (5280 feet times 0.3048). The nautical mile was originally defined as one minute of latitude; since 1929 it has been exactly 1852 metres per the International Hydrographic Organization.
The converter handles any real input, including negatives (meaningful in coordinate geometry) and zero. It uses exact conversion factors throughout, so results are accurate to IEEE 754 double-precision limits.
How It Works
The converter uses the two-step method: convert the input to metres using the source unit's factor, then convert from metres to the target unit by dividing by its factor.
base_in_metres = value * factor[from]
result = base_in_metres / factor[to]The factor table is:
mm = 0.001 m cm = 0.01 m m = 1 m
km = 1000 m in = 0.0254 m ft = 0.3048 m
yd = 0.9144 m mi = 1609.344 m nmi = 1852 mEach factor is exact per the SI Brochure and the 1959 International Yard and Pound Agreement. The two-step method (through metres) avoids the need to store an N-by-N table of pairwise conversions, which would have 81 entries for nine units. Using metres as the pivot also makes the formulas transparent: if you ever want to verify a result by hand, you only need to know the metre value of each unit.
The tool reports the result to 10 significant digits and also shows the SI (metre) value in scientific notation. This makes the magnitude of the result obvious even when the target unit makes it hard to read: converting 0.000001 inch to miles gives a result of about 1.578e-11, which is much clearer in scientific notation than as a decimal with leading zeros.
Worked Examples
Converting 1 mile to kilometres: the input in metres is 1 * 1609.344 = 1609.344 m, and the result is 1609.344 / 1000 = 1.609344 km. This is the origin of the popular approximation "1 mile is about 1.6 km." The conversion factor is exact, so the result is precise to the limits of double-precision arithmetic.
Converting 1 inch to centimetres: the input in metres is 1 * 0.0254 = 0.0254 m, and the result is 0.0254 / 0.01 = 2.54 cm. This is exact, since both factors are defined exactly. The common approximation "1 inch is about 2.5 cm" is therefore accurate to within 2 percent.
Converting 100 nautical miles to statute miles: the input in metres is 100 * 1852 = 185,200 m, and the result is 185,200 / 1609.344 = 115.078 miles. This explains why a ship's speed in knots (nautical miles per hour) does not directly equal statute miles per hour: 1 knot equals 1.15078 mph.
When to Use This Tool
Use the length converter when you need to:
- Convert running distances between kilometres and miles for international race entries.
- Translate woodworking measurements between metric and imperial units.
- Compute ship and aircraft distances in nautical miles versus kilometres.
- Convert screen sizes quoted in inches to centimetres for European buyers.
- Translate GPS coordinates expressed in metres into more readable units.
- Verify that a recipe or instruction using one unit system matches a tool calibrated in another.
- Teach the relationships between metric and imperial units in a science class.
Limitations & Disclaimer
This converter uses exact conversion factors defined in SI. It does not handle astronomical units (AU, light-years, parsecs), obsolete units (furlongs, chains), or specialised units (angstroms). Results are subject to IEEE 754 floating-point precision for very large or very small magnitudes. For surveying applications in the US, note that the US survey foot differs from the international foot by 2 parts per million. See our disclaimer for full details.
Frequently Asked Questions
Why is the metre the SI base unit of length?
Because the SI system, established in 1960, defines seven base units from which all other units are derived. The metre is the base unit of length, defined since 1983 as the distance light travels in vacuum in 1/299,792,458 of a second. This definition fixes the speed of light at exactly 299,792,458 m/s.
Why are imperial units defined in terms of the metre?
Because the 1959 International Yard and Pound Agreement standardised the international yard as exactly 0.9144 metre. This means the foot is exactly 0.3048 m and the inch is exactly 0.0254 m. Imperial units are no longer defined independently; they are derived from SI units.
What is the difference between a statute mile and a nautical mile?
A statute mile is 5280 feet, or exactly 1609.344 metres, used for land distances in the US and UK. A nautical mile is exactly 1852 metres, used for air and sea navigation because it corresponds to one minute of latitude on a nautical chart. One nautical mile equals about 1.15078 statute miles.
Why does the converter show results to 10 significant digits?
Because the conversion factors are exact, the result is limited only by the precision of double-precision floating-point arithmetic, which is about 15 to 17 significant digits. Ten significant digits is more than enough for any practical application while keeping the output readable.
Does the converter handle negative values?
Yes. Negative lengths are meaningful in coordinate geometry, where they represent positions to the left of or below an origin. The conversion factor is the same regardless of sign, so a negative input produces a negative result of the same magnitude.
Are there any length units not in this converter?
Yes. Astronomical units (AU), light-years, and parsecs are used for astronomical distances; furlongs, chains, and rods are obsolete surveying units; the angstrom is used in spectroscopy. This converter focuses on the nine units most commonly used in everyday and engineering contexts.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team