How to Convert Millimetres to Kilometres (mm to km)
Length conversions come up when reading overseas furniture specs, cycling the Round Island Route in kilometres while a foreign app shows miles, or checking marine distances in the Singapore Strait in nautical miles. This guide shows how to convert millimetres into kilometres by hand, with examples and a quick-reference table.
The conversion
To convert millimetres to kilometres, multiply the value by 1.000000e-06. To go the other way, divide by the same number (or multiply by 1,000,000). Because the factor is tiny, scientific notation keeps the numbers manageable.
Kilometres = millimetres × 1.000000e-06
Worked examples
- 3 mm = 3.000000e-6 km
- 12.5 mm = 1.250000e-5 km
- 75 mm = 7.500000e-5 km
Millimetres to Kilometres table
| Millimetres | Kilometres |
|---|---|
| 1 mm | 1.000000e-6 km |
| 2 mm | 2.000000e-6 km |
| 5 mm | 5.000000e-6 km |
| 10 mm | 1.000000e-5 km |
| 20 mm | 2.000000e-5 km |
| 25 mm | 2.500000e-5 km |
| 50 mm | 5.000000e-5 km |
| 100 mm | 1.000000e-4 km |
| 250 mm | 0.00025 km |
| 500 mm | 0.0005 km |
| 1,000 mm | 0.001 km |
Common questions
Is mm to km exact?
The factors used here are the internationally defined values where they exist; results are rounded for display only.
How do I convert back?
Use the Kilometres to Millimetres converter.
Mistakes to avoid
Dropping the units
A bare number without a unit is easy to misread later. Always write the unit with the result.
Mixing similar-sounding units
US and imperial gallons, pints and tons are different sizes, and decimal kilobytes differ from binary kibibytes.
Multiplying when you should divide
Converting to a smaller unit gives a bigger number, and converting to a larger unit gives a smaller one. If your answer moves the wrong way, you have inverted the factor.
Background
Many conversion factors are exact by definition. The inch has been defined as exactly 2.54 centimetres since 1959, and the pound as exactly 0.45359237 kilograms, so conversions involving them can be as precise as you need.
Writing the units next to every number while you work is the simplest way to avoid mistakes. If the units do not cancel to leave the one you want, the calculation is set up wrongly.
For everyday use, three or four significant figures are usually enough. Engineering, science and trade documents may need more, and rounding should happen only at the end of a calculation.
For any value, use the Millimetres to Kilometres Converter or browse all length converters.
Run your own numbers: Millimetres to Kilometres Converter