Length converter guide
The metre (m) is the SI base unit of length. This converter expresses the source in metres and divides by the target unit’s metres-per-unit factor.
The foot option is the international foot, exactly 0.3048 m, not the former U.S. survey foot. The nautical mile is exactly 1852 m. Astronomical units are offered for scale, but measured astronomical distances can carry uncertainty beyond the unit definition.
How to convert length units
- Enter the length: Provide a finite number in the selected source unit.
- Choose the source: Select the exact metric, international customary, nautical, or astronomical unit.
- Choose the target: Select the output unit or use Swap to reverse the conversion.
- Convert: Select Convert length to calculate through metres.
Formula and variables
Metres provide the common base for every supported unit pair.
target length = source length × (metres per source unit) ÷ (metres per target unit)- Lₛ — Source length
- The signed finite length entered by the user.
- fₛ — Source factor
- Metres in one source unit.
- fₜ — Target factor
- Metres in one target unit.
Convert miles to kilometres
Express one international mile in kilometres.
- Amount
- 1
- From
- mi
- To
- km
- 1 mi = 1609.344 m
- 1609.344 m ÷ 1000 m/km
Result: 1 mi = 1.609344 km.
The conversion is exact because the international mile is defined from the exact international foot.
Understanding your results
Defined versus measured values
Exact unit factors do not make a measured distance exact. Round the final result to match the precision of the original measurement.
Astronomical scale
Scientific notation is often more readable for metres per light-year or parsec; the displayed exponent does not imply measurement precision.
Assumptions
- The foot is the international foot of exactly 0.3048 m.
- The mile is the international mile of exactly 1609.344 m.
- The international nautical mile is exactly 1852 m.
- The astronomical unit is 149,597,870,700 m and the parsec factor is derived from the astronomical unit and π.
Limitations
- The former U.S. survey foot is not included.
- The calculator does not account for map projection, terrain, geodesics, or route distance.
- Very large or very small results use floating-point arithmetic.
- A unit conversion does not reduce uncertainty in a measured distance.
Common mistakes
- Mixing the international foot with the former U.S. survey foot.
- Treating a nautical mile as an ordinary mile.
- Using a map distance without accounting for projection or terrain.
- Rounding exact factors before completing the conversion.
- Assuming a light-year is a unit of time.
Practical use cases
Travel, construction, and science
Compare metric and international customary specifications without replacing exact factors with rounded rules of thumb.
Navigation and astronomy
Translate nautical miles or compare au, light-years, parsecs, and metres across large scales.
Planning and decision guide
Record the length standard
For older U.S. surveying records, confirm whether the source used the former U.S. survey foot. This calculator intentionally uses the international foot.
Frequently asked questions
How many kilometres are in one mile?
One international mile is exactly 1.609344 km.
How many metres are in one foot?
One international foot is exactly 0.3048 m.
Is a nautical mile the same as a mile?
No. One international nautical mile is 1852 m; one international mile is 1609.344 m.
Does this use the U.S. survey foot?
No. It uses the international foot. The former U.S. survey foot had a slightly different factor.
Is a light-year a time unit?
No. A light-year is a length: the distance light travels in vacuum during one Julian year.
Sources and review
- NIST Guide to the SI, Appendix B.9: Factors for Units Listed by Kind of Quantity or Field of Science — National Institute of Standards and Technology. Accessed 2026-08-29.
- Revised Unit Conversion Factors: U.S. Survey Foot — National Institute of Standards and Technology. Accessed 2026-08-29.
Reviewed 2026-08-29.