Length Converter
Convert between metres, feet, inches, miles, kilometres and more, with the precision and rounding issues that cause real errors.
Last reviewed by the Radiatus Cloud team
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Exact conversions, not approximations
Since 1959 the international inch is defined as exactly 25.4 millimetres, so imperial-to-metric length conversion is exact rather than measured. One foot is exactly 0.3048 metres and one mile is exactly 1609.344 metres. Using 2.54 for the inch is not a rounded approximation; it is the definition.
The survey foot problem
The United States maintained a second, slightly different foot for land surveying, where one foot equalled 1200/3937 metres. The difference is about two parts per million, which is invisible on a tape measure and amounts to roughly 3 metres across the width of the United States. It caused real discrepancies in surveying and mapping, and was officially retired at the end of 2022, though legacy data still carries it.
Rounding compounds
Converting a measurement, rounding it, then converting back does not return the original. Round once at the end, not at each step. This matters most in manufacturing and construction, where a chain of rounded conversions can accumulate into a tolerance failure.
Significant figures carry meaning
A measurement of 3 feet converted to 0.9144 metres implies precision to a tenth of a millimetre that the original almost certainly did not have. Converting does not create accuracy. Carry the same number of significant figures through, or state the tolerance separately.
Where conversion errors have consequences
The Mars Climate Orbiter was lost in 1999 because one team supplied thrust in pound-force seconds while the receiving software expected newton seconds. A Boeing 767 ran out of fuel mid-flight in 1983 after a conversion error between pounds and kilograms. Unit errors are not a trivial category, and the fix is always the same: state units explicitly at every interface rather than assuming a default.
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Frequently Asked Questions
Is the inch to millimetre conversion exact?
Yes. Since 1959 the international inch is defined as exactly 25.4 millimetres, so imperial-to-metric length conversion is exact by definition rather than measured.
What is the US survey foot?
A slightly different foot used for land surveying, equal to 1200/3937 metres, about two parts per million larger. Negligible on a tape measure, roughly 3 metres across the United States. Retired at the end of 2022.
Why does converting back not give the original number?
Because rounding at each step compounds. Round once at the end rather than after every conversion, which matters most in manufacturing and construction tolerances.
How many decimal places should I keep?
No more than the original measurement justified. Converting 3 feet to 0.9144 metres implies tenth-of-a-millimetre precision the original did not have. Conversion cannot create accuracy.
Do unit errors really cause problems?
Yes, expensively. The Mars Climate Orbiter was lost in 1999 to a pound-force versus newton mismatch, and a Boeing 767 ran out of fuel in 1983 after a pounds-to-kilograms error.
Privacy & Security
Converted locally.
About This Tool
This tool runs entirely in your browser. No data is sent to any server, ensuring complete privacy. Simply use the interface above to get started — no registration or login required.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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