Micrometers and millimeters
The micrometer (micron, µm) is one millionth of a meter, or a thousandth of a millimeter. Machining tolerances, coating thickness, filter ratings and cell sizes in biology are given in micrometers.
The millimeter is one thousandth of a meter. Engineering drawings, bolt and drill sizes, rainfall, camera lenses and jewelry use millimeters.
Worked example
Convert 50 µm: 50 × 0.001 = 0.05, so 50 micrometers is 0.05 millimeters. Likewise 20 µm = 0.02 mm.
Estimating in your head
The factor is a power of ten, so just move the decimal point 3 places to the left: 50 µm = 0.05 mm.
Going the other way
To convert millimeters to micrometers, divide by 0.001 (or multiply by 1,000).
How this works
mm = µm × 0.001
1 µm = 0.000001 m and 1 mm = 0.001 m, both by definition, so the factor is their ratio: 0.001 exactly. Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many millimeters are in a micrometer?
1 micrometer = 0.001 millimeters, exactly. The value follows from the unit definitions, so it never changes.
How do I convert micrometers to millimeters?
Multiply by 0.001. For example, 20 µm × 0.001 = 0.02 mm.
What is 100 µm in millimeters?
100 µm = 0.1 mm. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- Federal Register 59-5442 (1959) — Refinement of values for the yard and the pound
- Wikipedia — Conversion of units of length
Reviewed Oct 17, 2026 · test vectors fixtures/units/length/micrometers-to-mm/