How many SI prefixes are there?

There are 24 prefixes covering 20 orders of magnitude in each direction, from quetta (ten to the thirty) down to quecto (ten to the minus thirty). They let you express extremely large and extremely small quantities compactly.

What are the newest SI prefixes?

Ronna, quetta, ronto, and quecto were adopted by the CGPM in November 2022. They were added partly to handle the scale of modern data storage, where global data volumes are now measured in zettabytes and yottabytes.

Why is the micro prefix symbol the Greek letter mu?

Micro is symbolised by the Greek lowercase mu because m was already taken by milli. When mu is unavailable, ASCII contexts sometimes substitute u, but the correct SI symbol is the mu character.

Are prefix symbols case-sensitive?

Yes, strictly. Capital M means mega (a million) while lowercase m means milli (a thousandth). Mixing case changes the value by a factor of a billion, so case matters enormously in scientific and engineering writing.

Can prefixes be combined or stacked?

No. SI rules forbid compound prefixes, so you cannot write millimicro to mean nano. You pick the single prefix nearest the magnitude you need and use it once per unit.

SI Prefix Table — Gera Tools

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SI prefixes are a system of multipliers that attach to any SI unit to scale it up or down by powers of ten. They let you write a million metres as a megametre or a billionth of a second as a nanosecond without trailing strings of zeros.

How it works

Each prefix represents a fixed power of ten. A prefix symbol is written directly in front of a unit symbol with no space, and the combination is treated as a single new unit. Filtering this table by name, symbol, or exponent finds any of the 24 prefixes. The factor column shows the full decimal expansion so the scale is unambiguous:

kilo  (k)  = 10^3   = 1 000
mega  (M)  = 10^6   = 1 000 000
milli (m)  = 10^-3  = 0.001
micro (µ)  = 10^-6  = 0.000 001

The prefixes were chosen so that successive engineering prefixes (kilo, mega, giga, …) step in factors of one thousand, which matches how scientific notation and computing capacities are usually grouped.

Notes and examples

Case is load-bearing: 5 Mm is five megametres (5 000 000 m) while 5 mm is five millimetres (0.005 m) — a difference of a billion. The 2022 additions quetta, ronna, ronto, and quecto extended the range to cope with the explosion in data measurement; total global data storage is now naturally expressed in zettabytes. Remember that prefixes never stack: nanometre is correct, but millimicrometre is not.