Micrometer (μm)
The micrometer (also called the micron) is a metric unit of length equal to one-millionth of a meter or one-thousandth of a millimeter. Abbreviated as "µm", it is the standard unit for measuring cells, bacteria, semiconductor features, and fine particles. At this scale, objects are invisible to the naked eye and require optical or electron microscopes to observe.
Definition
One micrometer equals exactly 10-6 meters, 0.001 millimeters, 1,000 nanometers, 10,000 ångströms, or approximately 0.0000394 inches. There are 1,000 micrometers in a millimeter and 1,000,000 micrometers in a meter. The deprecated symbol µ (micron) is no longer recommended; the correct SI symbol is µm.
History
The micrometer became a practical unit as microscopy advanced in the 19th century. The term "micron" (symbol µ) was commonly used until 1967, when the General Conference on Weights and Measures officially deprecated it in favor of "micrometer" (symbol µm) to conform with SI naming conventions. Despite this, "micron" persists in everyday usage in many industries, particularly semiconductor manufacturing and filtration. The micrometer became critically important in the late 20th century as integrated circuit feature sizes shrank below 1 µm.
Common Uses
Semiconductor manufacturing specifies transistor gate lengths, process nodes, and feature sizes in micrometers (or nanometers for advanced nodes). Biological cell diameters are measured in micrometers: red blood cells are about 7 µm, white blood cells 10-15 µm. Air filtration ratings (MERV, HEPA) reference particle sizes in micrometers. Coating thicknesses in the automotive and aerospace industries use micrometers. Thread counts and fiber diameters in textiles are specified in micrometers. Pollen grains range from about 10 to 100 µm.
Did You Know? Facts About Micrometer
- Human hair is 50-75 micrometers in diameter, while spider silk is only about 3-5 µm.
- Bacteria typically measure 1-10 µm, while most viruses are 0.02-0.3 µm (20-300 nanometers).
- The smallest object visible to the naked eye is about 40 µm - anything smaller requires a microscope.
- Modern semiconductor chips have features as small as 0.003 µm (3 nm), though the industry still calls older processes by their micrometer node (e.g., "0.18 micron process").
- PM2.5 particulate matter is defined as particles with a diameter of 2.5 µm or smaller - small enough to enter the lungs and bloodstream.