In the metric scale, a micrometer is smaller than a millimeter. Micrometers are also called microns, and there are 1,000 microns in a millimeter. A human hair is typically 40 to 50 microns wide, or 0.04 millimeter, and a dust mite is usually 400 microns long, or 0.4 millimeter.
In the metric system, the base is the meter and is multiplied or divided by powers of 10 to represent other units. Units smaller than the millimeter are the micron (1 x 10^-6 m), the nanometer (1 x 10^-9 m), the picometer (1 x 10^-12 m) and the femtometer (1 x 10^-15 m).