A kilometre is one million millimetres. This large jump is handy for engineering totals, such as summing cable or pipe lengths recorded in millimetres.
How to use the mm to km Converter
- Type the number of millimetres into the Millimetres field. Decimals such as 500,000 are fine.
- Read the answer in Kilometres – it updates as you type, with no button to press.
- To convert the other way, type into Kilometres and the millimetres are recalculated.
- Press Copy results to copy both values with the page title, or Copy link to share this exact conversion.
This page converts only mm ⇄ km. Need a different unit? Use the general length converter, where you can choose any “from” and “to” unit.
How it works
Both units measure length, so they are always in the same ratio: one millimetre equals 1 × 10^-6 kilometres. The calculator multiplies the number of millimetres you type by 1 × 10^-6 to give kilometres, and when you type into the kilometres field it divides by the same factor to give millimetres. Because the ratio never changes, doubling the millimetres always doubles the kilometres. Results are calculated at full precision and rounded only for display (up to eight decimal places, with very large or small values shown in scientific notation).
How to convert millimetres to kilometres
- Write down the value in millimetres, for example 1,000,000 mm.
- Multiply by the conversion factor 1 × 10-6 (because 1 millimetre = 1 × 10-6 km).
- 1,000,000 × 1 × 10-6 = 1 km.
- To go back from kilometres to millimetres, divide by 1 × 10-6 – or multiply by 1,000,000.
mm to km conversion table
| Millimetres | Kilometres | Kilometres | Millimetres |
|---|---|---|---|
| 1,000 mm | 0.001 km | 1 km | 1,000,000 mm |
| 10,000 mm | 0.01 km | 2 km | 2,000,000 mm |
| 100,000 mm | 0.1 km | 5 km | 5,000,000 mm |
| 1,000,000 mm | 1 km | 10 km | 10,000,000 mm |
| 5,000,000 mm | 5 km | 20 km | 20,000,000 mm |
| 10,000,000 mm | 10 km | 50 km | 50,000,000 mm |
| 100 km | 100,000,000 mm | ||
| 1,000 km | 1 × 109 mm |
What is a millimetre?
A millimetre is one thousandth of a metre. Engineering drawings, construction plans, rainfall, screws, bolts and jewellery are all dimensioned in millimetres.
What is a kilometre?
A kilometre is 1,000 metres. It is the standard unit for road distances, maps, running races and travel in almost every country except the U.S. and UK, where miles are still used on road signs.
Common mistakes to avoid
Mixing the international foot (0.3048 m) with the old u.s. survey foot, confusing nautical and statute miles, and writing decimal feet such as 5.5 ft as “5 ft 5 in” (it is 5 ft 6 in).
Formula
1 mm = 1 × 10^-6 km1 km = 1,000,000 mmkm = mm × 1 × 10^-6Worked example
Inputs — Millimetres: 1,000,000 mm.
Results — Kilometres = 1 km.
Frequently asked questions
How many mm in a km?
1,000,000.
How many km is 5,000,000 mm?
5 km.
How do I convert millimetres to kilometres?
Multiply the number of millimetres by 1 × 10^-6. For example, 1,000,000 mm × 1 × 10^-6 = 1 km.
How do I convert kilometres back to millimetres?
Divide by 1 × 10^-6 (or multiply by 1,000,000), or simply type the value into the kilometres field above.
What is 1,000 mm in kilometres?
1,000 mm = 0.001 km.
Is this conversion exact?
Yes. The factor comes from the official definitions of both units (NIST and the SI), so the only difference you may see is rounding in the last displayed digit.
Can I convert to other units on this page?
This page converts only between millimetres and kilometres. To pick any length unit, use the general length converter linked under Related calculators.
What formula does the mm to km Converter use?
1 mm = 1 × 10^-6 km.