In science, “micro” means one millionth. As an official prefix in the metric system, micro represents a factor of 10⁻⁶, meaning one unit divided into a million equal parts. The symbol is the Greek letter μ (mu), and it comes from the Greek word mikrós, meaning “small.” But micro shows up across science in ways that go well beyond that strict numerical definition.
The Exact Value of Micro
The International System of Units (SI) defines micro as exactly one millionth, or 0.000001, of a base unit. One micrometer (μm) is one millionth of a meter. One microgram (μg) is one millionth of a gram. One microsecond (μs) is one millionth of a second. The prefix sits between milli (one thousandth, 10⁻³) and nano (one billionth, 10⁻⁹) on the scale of SI prefixes.
To picture how small a micrometer actually is: a human hair is roughly 70 to 110 μm wide, depending on how it’s measured. That means a single micrometer is about one hundredth the width of a hair. You’d need a microscope to see anything at this scale, which is exactly why the tool and the prefix share the same Greek root.
Where You’ll See Micro in Everyday Science
The prefix appears constantly in biology, medicine, chemistry, and environmental science. In each case, “micro” signals that something operates at a scale too small to see with the naked eye but still measurable with standard laboratory tools.
A standard compound microscope can resolve details down to about 200 nanometers, or 0.2 μm. That’s enough to see most bacteria, which typically measure 1 to 4 μm in length. E. coli, for example, is about 2 μm long and roughly 1 μm wide. The entire field of microbiology is named for this scale: it studies organisms too small to see without magnification, from bacteria and fungi to single-celled parasites.
In nutrition and medicine, the microgram (μg or mcg) is a standard dosing unit. Your body needs milligram quantities of some vitamins and minerals but only microgram quantities of others. Vitamin D, vitamin B12, and folic acid are all measured in micrograms. The distinction matters: a milligram is 1,000 times larger than a microgram, so confusing the two could mean getting far too much or too little of a nutrient.
Microplastics and Environmental Science
One of the most common places people encounter “micro” in the news is microplastics. These are plastic particles generally defined as 5 millimeters or smaller, though the exact lower boundary varies by organization. The U.S. FDA and the International Organization for Standardization set the lower limit at 1 μm, classifying anything smaller as a nanoplastic. Other agencies, including the EPA and the European Union, use a more inclusive definition that extends all the way down to 1 nanometer.
The name “microplastic” reflects the general principle: these particles exist at a scale where they’re difficult or impossible to see individually, yet they accumulate in water, soil, and living tissue in measurable quantities. The prefix tells you immediately that you’re dealing with something very small but not at the atomic or molecular level.
Micro in Computing
The term “microchip” and “microprocessor” both use the prefix to describe the miniaturized circuits etched onto silicon. Early microprocessors in the late 1960s had features around 20 μm wide. By the mid-1980s, that shrank to 1 μm. The industry crossed from the micrometer scale into the nanometer scale around 1987, when feature sizes dropped below 1 μm to 800 nm. Today’s most advanced chips have features measured at 2 or 3 nanometers, thousands of times smaller than the original “micro” chips, but the name stuck.
Micro vs. Macro
In many scientific fields, “micro” is paired against “macro” to distinguish between small-scale and large-scale analysis. The most familiar example is economics. Microeconomics studies the decisions of individual consumers, households, and businesses: how a single firm sets prices, how a household allocates its budget. Macroeconomics looks at entire economies, tracking national output, employment rates, and inflation across countries or regions.
The same logic applies in biology (microbiology vs. macrobiology), ecology (micro-habitats vs. macro-ecosystems), and physics. In every case, “micro” points you toward the small, individual, or local, while “macro” points toward the large, aggregate, or systemic.
When Micro Doesn’t Mean Exactly One Millionth
Outside the strict SI definition, “micro” is also used loosely in science to mean “very small” without implying an exact factor of 10⁻⁶. A microhabitat is a tiny, localized environment, like the underside of a rock or the bark of a tree. A microclimate is a small area where weather conditions differ from the surrounding region. Microsurgery refers to surgery performed under magnification on very small structures like nerves or blood vessels. In these cases, the prefix signals scale and precision rather than a specific mathematical value.
The key distinction is context. When micro appears before a unit of measurement (meter, gram, second, liter), it always means exactly one millionth. When it appears before a broader scientific term, it means “at a small scale” in a more general sense. Both uses trace back to the same Greek root, and both carry the same core idea: something too small to observe or appreciate without special tools or focused attention.