# mm to km Converter URL: https://speedycalculator.com/calculators/conversion/mm-to-km/ Last updated: 2026-10-25 > Convert millimetres (mm) to kilometres (km): 1 millimetre = 1 × 10^-6 km, so multiply by 1 × 10^-6 or divide by 1,000,000. Both fields are editable, so you can also type in kilometres and get millimetres back. ## Inputs - Millimetres (mm) ## Results - Kilometres ## Formula ``` 1 mm = 1 × 10^-6 km 1 km = 1,000,000 mm km = mm × 1 × 10^-6 ``` ## Explanation 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 MillimetresKilometresKilometresMillimetres1,000 mm0.001 km1 km1,000,000 mm10,000 mm0.01 km2 km2,000,000 mm100,000 mm0.1 km5 km5,000,000 mm1,000,000 mm1 km10 km10,000,000 mm5,000,000 mm5 km20 km20,000,000 mm10,000,000 mm10 km50 km50,000,000 mm100 km100,000,000 mm1,000 km1 × 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). ## Worked example Inputs — Millimetres: 1,000,000 mm. Results — **Kilometres = 1 km**. ## FAQ **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. ## Sources - [NIST Special Publication 811 – Guide for the Use of the International System of Units](https://www.nist.gov/pml/special-publication-811) - [NIST Handbook 44 Appendix C – General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)