What Is a Micrometer?
A micrometer (symbol µm), also called a micron, is a metric unit of length equal to one millionth of a meter, 10⁻⁶ m. The prefix micro- comes from the Greek mikros, “small”, and is listed by NIST among the SI prefixes. One micrometer is a thousandth of a millimeter. The old name “micron” was officially dropped in 1967, though it lives on in labels for filters and coatings.
Micrometers are the natural scale of biology and fine manufacturing: cell sizes, bacteria, dust and pollution particles, fiber diameters and machining tolerances. The word also names a precision measuring tool, the micrometer caliper, which is pronounced differently (my-KROM-it-er) and usually reads to a thousandth of a millimeter or of an inch.
What Is a Mile?
The mile is the international mile of 1,609.344 meters, defined by the 1959 International Yard and Pound Agreement and descended from the Roman mille passus, a thousand paces. At the micrometer scale it is enormous: 1,609,344,000 µm.
Microscopic Scales vs a Mile
Comparing a mile with things you can only see under a microscope shows just how many orders of magnitude separate them. A red blood cell is about 7.5 µm across, which is 4.66028 × 10⁻⁹ miles. Laying red blood cells edge to edge, you would need about 214,579,200 of them to span one mile.
Other everyday microscopic sizes convert the same way. A human hair is roughly 17 to 180 µm thick, depending on the person, so a hair 100 µm wide is 6.21371 × 10⁻⁸ miles. A typical E. coli bacterium is about 2 µm long, and the fine particles called PM2.5, monitored for air quality, are 2.5 µm or smaller: 1.55343 × 10⁻⁹ miles. An ordinary light microscope can resolve details down to about 0.2 µm.
| Object | Micrometers (µm) | Miles (mi) |
|---|---|---|
| Light microscope resolution limit | 0.2 | 1.24274 × 10⁻¹⁰ |
| E. coli bacterium (length) | 2 | 1.24274 × 10⁻⁹ |
| PM2.5 particle (upper limit) | 2.5 | 1.55343 × 10⁻⁹ |
| Red blood cell | 7.5 | 4.66028 × 10⁻⁹ |
| Fine human hair | 17 | 1.05633 × 10⁻⁸ |
| Sheet of printer paper | 100 | 6.21371 × 10⁻⁸ |
| Thick human hair | 180 | 1.11847 × 10⁻⁷ |
The micrometer is also the unit of infrared light. Thermal cameras see wavelengths of roughly 8 to 14 µm, the range of heat radiated by objects at room temperature, and fiber-optic networks send data on infrared light near 1.55 µm. Both are measured in micrometers because they are about a thousand times longer than the nanometer scale of visible light.
These numbers are so small that writing them as decimals is impractical, which is why scientific notation is used. Each power of ten in the exponent is one “zero”: a result of 6.2 × 10⁻¹⁰ miles means the decimal point is moved ten places to the left.
How Many Micrometers in a Mile?
There are exactly 1,609,344,000 micrometers in a mile, 1.6 billion. To convert miles to micrometers, multiply by that number; a tenth of a mile is 160,934,400 µm.