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.

What is the SI Prefix Table?

Complete table of the 24 SI metric prefixes with name, symbol, power of ten, and full decimal factor. Covers quetta down to quecto, including the 2022 additions ronna, ronto, quetta, and quecto. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

SI Prefix Table — Gera Tools

Name: SI Prefix Table
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

All 24 prefixes from largest to smallest

Prefix	Symbol	Power	Name (approx.)
quetta	Q	10³⁰	Nonillion
ronna	R	10²⁷	Octillion
yotta	Y	10²⁴	Septillion
zetta	Z	10²¹	Sextillion
exa	E	10¹⁸	Quintillion
peta	P	10¹⁵	Quadrillion
tera	T	10¹²	Trillion
giga	G	10⁹	Billion
mega	M	10⁶	Million
kilo	k	10³	Thousand
hecto	h	10²	Hundred
deca	da	10¹	Ten
(base)	—	10⁰	One
deci	d	10⁻¹	Tenth
centi	c	10⁻²	Hundredth
milli	m	10⁻³	Thousandth
micro	µ	10⁻⁶	Millionth
nano	n	10⁻⁹	Billionth
pico	p	10⁻¹²	Trillionth
femto	f	10⁻¹⁵	Quadrillionth
atto	a	10⁻¹⁸	Quintillionth
zepto	z	10⁻²¹	Sextillionth
yocto	y	10⁻²⁴	Septillionth
ronto	r	10⁻²⁷	Octillionth
quecto	q	10⁻³⁰	Nonillionth

The 2022 additions

In November 2022, the General Conference on Weights and Measures (CGPM) formally adopted four new prefixes: ronna (10²⁷), quetta (10³⁰), ronto (10⁻²⁷), and quecto (10⁻³⁰). The main driver was practical: total global data storage is now measured in zettabytes, and within years it will be in yottabytes. Without the new prefixes, scientists and engineers working in data and genomics were starting to invent informal non-standard prefixes, which threatened coherence of the system.

Quetta and ronna also fill a real need in particle physics and cosmology, where mass scales of the observable universe and sub-atomic particle interactions span extremes that previously required scientific notation even with prefixes.

Case-sensitivity rules

Prefix symbols are case-sensitive and this matters enormously:

M (capital M) = mega = 10⁶
m (lower m) = milli = 10⁻³
G (capital G) = giga = 10⁹
k (lower k) = kilo = 10³

Writing Mw instead of mW in a specification changes the meaning by nine orders of magnitude. In engineering drawings and data sheets, use the correct case as given in this table.

The micro prefix symbol is the Greek lowercase mu (µ). When typing in plain ASCII, u is widely used as a substitute (for example uF for microfarad on components), but the correct SI symbol is µ.

Prefixes that do not follow the thousand-step rule

Hecto (10²), deca (10¹), deci (10⁻¹), and centi (10⁻²) are historical survivors that do not step in factors of one thousand. They persist because centimetre, decimetre, hectolitre, and hectare are so embedded in everyday life that removing them would cause more confusion than they create. In scientific writing, prefer the kilo/milli group and above; the in-between prefixes are mainly used in consumer contexts.

Notes

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. Remember that prefixes never stack: nanometre is correct, but millimicrometre is not. If a quantity is outside the range of a single prefix, use scientific notation rather than combining two prefixes.