μm
micrometer
Lengthexact by definition
The micrometre is 10⁻⁶ of a metre, an exact decimal prefix on the SI base unit. It is the scale of cells, dust and machining tolerances: a red blood cell is about 7 μm across and a human hair is 50 to 100 μm thick. The factor is exact by definition.
Watch out: Air-quality figures such as PM2.5 and PM10 refer to particle diameters of 2.5 μm and 10 μm. They are size thresholds, not concentrations, and the concentration that goes with them is quoted separately in μg/m³.
| 1 μm | 0.000001 m |
Where the unit came from
The same length was long called the micron, with symbol μ. That name was dropped from official use by the CGPM in 1967, though it survives in filtration and machining, where "5 micron" filters and "micron finish" are still ordinary trade language.
Solvers that use length