SI units and prefixes: from nano to kilo

SI and metric prefixes explained: the full list from quecto to quetta, how to convert milli, micro and nano, and the seven SI base units, with lab examples.

By Zachery Casey · Published 5 November 2026 · Reviewed 9 November 2026 · Editorial policy

Lab measurements move between very large and very small numbers all the time: a 0.2 µm filter, a 250 mL flask, a 10 mM stock solution. SI prefixes are what make those numbers manageable. A prefix in front of a unit multiplies it by a fixed power of ten, so "milli" always means a thousandth and "micro" always means a millionth. This guide sets out the SI base units, the prefixes used in the lab, and how to convert between them without slipping a decimal place.

Terms are defined in the glossary.

The seven SI base units

The International System of Units (SI) is built on seven base units. Every other unit is made by combining them.

Quantity	Unit	Symbol
Time	second	s
Length	metre	m
Mass	kilogram	kg
Electric current	ampere	A
Temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

The list is from the BIPM, the international body that maintains the SI. The kilogram is the only base unit with a prefix already in its name, which is why prefixes for mass attach to the gram instead.

Units built from them

Labs use many units made from the base units:

Unit	What it measures	Made from
mol/L (M)	Concentration	Amount ÷ volume. See molarity
g/mol	Molar mass	Mass ÷ amount. See molar mass
g/mL	Density	Mass ÷ volume
pascal (Pa)	Pressure	Force ÷ area. HPLC pressures are often given in bar (1 bar = 100,000 Pa)
joule (J)	Energy	Used for activation energy in kJ/mol. See the Arrhenius equation
degree Celsius (°C)	Temperature	The same size step as the kelvin, offset by 273.15

The litre isn't an SI unit in the strict sense, but it is accepted for use alongside them. One litre is one cubic decimetre (1 dm³), and one millilitre is one cubic centimetre (1 cm³).

The SI prefixes used in the lab

The full list runs from 10⁻³⁰ to 10³⁰. These are the ones a lab meets day to day (BIPM):

Prefix	Symbol	Factor	In words	Example
giga	G	10⁹	a billion	gigabyte of instrument data
mega	M	10⁶	a million	megapascal (MPa) of pressure
kilo	k	10³	a thousand	kilogram, kilojoule
(none)		10⁰	one	gram, litre, mole
centi	c	10⁻²	a hundredth	centimetre
milli	m	10⁻³	a thousandth	millilitre, milligram, millimolar
micro	µ	10⁻⁶	a millionth	microlitre, micrometre
nano	n	10⁻⁹	a billionth	nanometre, nanomolar
pico	p	10⁻¹²	a trillionth	picomole

Hecto (h), deca (da) and deci (d) also exist but are rarely used in the lab, apart from the decimetre in the definition of the litre.

Show all 24 SI prefixes, quecto to quetta

Prefix	Symbol	Factor
quetta	Q	10³⁰
ronna	R	10²⁷
yotta	Y	10²⁴
zetta	Z	10²¹
exa	E	10¹⁸
peta	P	10¹⁵
tera	T	10¹²
giga	G	10⁹
mega	M	10⁶
kilo	k	10³
hecto	h	10²
deca	da	10¹
deci	d	10⁻¹
centi	c	10⁻²
milli	m	10⁻³
micro	µ	10⁻⁶
nano	n	10⁻⁹
pico	p	10⁻¹²
femto	f	10⁻¹⁵
atto	a	10⁻¹⁸
zepto	z	10⁻²¹
yocto	y	10⁻²⁴
ronto	r	10⁻²⁷
quecto	q	10⁻³⁰

From the BIPM.

How to convert between prefixes

Each step down the main ladder (kilo → none → milli → micro → nano → pico) is a factor of 1,000.

Scale of SI prefixes from kilo (10³) through the base unit, milli (10⁻³), micro (10⁻⁶) and nano (10⁻⁹) to pico (10⁻¹²), with an example conversion under each step — The prefixes a lab uses most, each a factor of 1,000 from the next.

So:

Going to a smaller prefix, multiply by 1,000 per step. The number gets bigger.
Going to a larger prefix, divide by 1,000 per step. The number gets smaller.

Worked examples

Convert	Steps	Calculation	Answer
0.25 L to mL	One step down	0.25 × 1,000	250 mL
2.5 µm to nm	One step down	2.5 × 1,000	2,500 nm
0.0125 g of sodium chloride to mg	One step down	0.0125 × 1,000	12.5 mg
50 mM to M	One step up	50 ÷ 1,000	0.050 M
50 mM to µM	One step down	50 × 1,000	50,000 µM
250 µL to mL	One step up	250 ÷ 1,000	0.25 mL

A quick check: the smaller the unit, the bigger the number. 250 mL and 0.25 L are the same volume. If a conversion makes the number smaller when the unit got smaller, the step went the wrong way.

For concentration units such as mg/mL, µg/mL and ppm, see molarity, mg/mL and dilutions and parts per million. For working out amounts of substance, see moles and Avogadro's number.

Writing the micro symbol: µ, u and mc

The SI symbol for micro is the Greek letter µ. Where it can't be typed, some documents and instruments write a plain "u" instead, as in "uL" for microlitres. A few labels use "mc". Neither is an SI symbol. When reading a document, check which convention it uses, and write µ in your own records where you can.

Common mistakes

Mixing up m and M. Lower-case m is milli (a thousandth). Capital M is mega (a million), and in chemistry M on its own also means mol/L. A "10 mM" solution is a thousand times weaker than a "10 M" one.
Stacking prefixes. Write nm, not mµm. Only one prefix goes on a unit.
Prefixing the kilogram. A thousandth of a gram is a milligram (mg), not a "microkilogram".
Dropping a step. Going from mL to nL is two steps (× 1,000,000), not one.
Squared and cubed units. 1 cm³ is (0.01 m)³ = 10⁻⁶ m³, not 10⁻² m³. The prefix is cubed along with the unit.
Too many figures. Converting doesn't make a number more precise. 0.25 L becomes 250 mL, not 250.000 mL. See accuracy, precision and significant figures.

For measuring volumes accurately, see pipettes and volumetric glassware. For the calculators that do these conversions for you, see the molarity calculator and dilution calculator. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.

All 15 guides in this topic

On this page