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What factor does the SI prefix micro (symbol μ) stand for?

10 to the minus 6, one millionth.

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  1. One thousandth (10^-3)
  2. One billionth (10^-9)
  3. One millionth (10^-6) ✔

Why: The small prefixes step by a factor of a thousand: milli is 10^-3, micro is 10^-6, nano is 10^-9. Micro uses the Greek letter mu.

Huh, didn't know that: Micron is an obsolete name for micrometer; the proper unit symbol is μm.

The dad joke
I bought a microwave, but it only waves one millionth as much.
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References

Answer: One millionth (10^-6). The small prefixes step by a factor of a thousand: milli is 10^-3, micro is 10^-6, nano is 10^-9. Micro uses the Greek letter mu.

  1. NIST: Metric (SI) Prefixes www.nist.gov/pml/owm/metric-si-prefixes
    microgram μ 10 -6 millionth
    Checked 2026-10-11.
  2. NIST: Writing with SI Units www.nist.gov/pml/owm/writing-si-metric-system-units
    The word "micron" is an obsolete term for the quantity "micrometer."
    Checked 2026-10-11.

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