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In SI symbols, how is MW different from mW?

MW is a megawatt (a million watts); mW is a milliwatt (a thousandth of a watt).

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  1. They mean the same; case does not matter
  2. MW is a million watts; mW is a thousandth ✔
  3. MW is a thousandth of a watt; mW is a million

Why: Case matters in SI prefixes. Capital M is mega, 10 to the 6th. Lowercase m is milli, 10 to the minus 3rd. Mixing them up is an error of a billion times.

Huh, didn't know that: Prefixes for big multiples use capitals, except kilo (k), hecto (h) and deka (da).

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My power plant is mega. My phone charger is milli. Same letter, very different attitude.
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References

Answer: MW is a million watts; mW is a thousandth. Case matters in SI prefixes. Capital M is mega, 10 to the 6th. Lowercase m is milli, 10 to the minus 3rd. Mixing them up is an error of a billion times.

  1. NIST: Metric (SI) Prefixes www.nist.gov/pml/owm/metric-si-prefixes
    prefixes for multiples (larger quantities or whole units) use uppercase symbols with the exception of three: kilo (k), hecto (h), and deka (da)
    Checked 2026-10-11.
  2. NIST: Metric (SI) Prefixes www.nist.gov/pml/owm/metric-si-prefixes
    mega M 10 6 million
    Checked 2026-10-11.

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