SI Prefixes Explained: When to Use k, M, G, m, µ, and n
What kilo, mega, giga, milli, micro, and nano actually mean, how to convert across prefixes, and why kilobytes confuse the kilo prefix with 1024.
What is an SI prefix?
An SI prefix attaches to a base unit to scale it by a power of ten. Instead of writing a long string of zeros, a short symbol carries the magnitude, and the same set of prefixes works across every SI unit -- length, mass, voltage, frequency, and more. Mixing up the exponent shifts a value by a factor of 10, 1,000, or worse, so it's worth knowing each one precisely.
Prefixes for large numbers: k, M, G
Kilo (k) is 10 to the 3rd power (x1,000), mega (M) is 10 to the 6th (x1,000,000), and giga (G) is 10 to the 9th (x1,000,000,000). A kilometer is 1,000 meters; MHz shows up in radio and Wi-Fi frequency bands; GHz is the common unit for CPU clock speeds and fast wireless links. Each step up adds 3 to the exponent, which makes the pattern easy to track.
Prefixes for small numbers: m, µ, n
Milli (m) is 10 to the -3 (one-thousandth), micro (µ) is 10 to the -6 (one-millionth), and nano (n) is 10 to the -9 (one-billionth). A millimeter is a thousandth of a meter; micrometers commonly describe cell size or fine particles; nanometers describe visible-light wavelength (roughly 400-700 nm) and semiconductor feature size.
How to convert across prefixes
Converting between prefixes just means taking the ratio of their multipliers. 1 GHz (10^9) is 1,000 times larger than 1 MHz (10^6), so 1 GHz equals 1,000 MHz. Likewise, 1 mm (10^-3) is 1,000 times larger than 1 µm (10^-6), so 1 mm equals 1,000 µm. The difference between the two exponents tells you exactly how many zeros shift.
How to choose the right prefix
In practice, picking a prefix that keeps the numeric part roughly between 0.1 and 1,000 keeps a value readable. Conventions vary by field: network speeds are usually quoted in Mbps or Gbps, capacitor values in pF, nF, or µF, and distances in m or km. The same physical quantity can favor a different prefix depending on context, which is worth remembering as convention rather than a hard rule.
A common mix-up with prefixes
A frequent misunderstanding involves the "kilobyte" (KB): the SI kilo prefix is always exactly x1,000, but computing has long used "kilobyte" informally to mean 1,024 (2 to the 10th power), since memory addressing is binary. The international standard resolves this by naming the binary version Ki (kibi) instead. Separately, confusing lowercase m (milli) with uppercase M (mega) introduces an error of nearly a billion-fold.
Frequently asked questions
How many Hz is 1 GHz?
Giga (G) is 10 to the 9th power, so 1 GHz equals 1,000,000,000 Hz.
How many µm is 1 mm?
Milli is 10^-3 and micro is 10^-6, a difference of 1,000x, so 1 mm equals 1,000 µm.
Is the "k" in kilobyte 1,000 or 1,024?
Both appear in practice. The SI kilo prefix is strictly 1,000x, but computing often uses 1,024 (2^10) informally; the international standard names that binary version Ki (kibi) to distinguish it.