How many milligrams are in a gram?

A thousand: milli- means a thousandth, so 1 g = 1000 mg. Likewise 1 mg = 1000 µg and 1 µg = 1000 ng — each step down the common lab prefixes is a factor of a thousand.

Is "u" the same as "µ"?

In practice yes. The correct symbol for micro is the Greek letter µ; "u" is an ASCII stand-in used when µ cannot be typed, so "ug" and "uL" mean µg and µL.

What does "mcg" mean?

Microgram. Medicine and pharmacy write mcg instead of µg to avoid confusing a handwritten µ with an m, which would be a thousand-fold dosing error. 1 mcg = 1 µg.

Why is the kilogram the base unit and not the gram?

History: the kilogram was defined by a physical prototype of convenient size when the metric system was created, and the SI kept it as the base unit of mass. It is the only base unit whose name carries a prefix, and prefixes are still attached to "gram", never to "kilogram".

What are the newest SI prefixes?

Ronna (R, 10²⁷), quetta (Q, 10³⁰), ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰), adopted in November 2022. Before them the largest and smallest were yotta (10²⁴) and yocto (10⁻²⁴), in use since 1991.

Free reference

Metric Prefixes

Every SI prefix from quecto (10⁻³⁰) to quetta (10³⁰) with its symbol and value, and a converter for jumping between them.

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All SI prefixes

The 24 prefixes adopted by the General Conference on Weights and Measures, the latest four in 2022.

Prefix	Symbol	Factor	Decimal	Name
quetta	Q	10³⁰	1,000,000,000,000,000,000,000,000,000,000	nonillion
ronna	R	10²⁷	1,000,000,000,000,000,000,000,000,000	octillion
yotta	Y	10²⁴	1,000,000,000,000,000,000,000,000	septillion
zetta	Z	10²¹	1,000,000,000,000,000,000,000	sextillion
exa	E	10¹⁸	1,000,000,000,000,000,000	quintillion
peta	P	10¹⁵	1,000,000,000,000,000	quadrillion
tera	T	10¹²	1,000,000,000,000	trillion
giga	G	10⁹	1,000,000,000	billion
mega	M	10⁶	1,000,000	million
kilo	k	10³	1,000	thousand
hecto	h	10²	100	hundred
deca	da	10¹	10	ten
(none)		10⁰	1	one
deci	d	10⁻¹	0.1	tenth
centi	c	10⁻²	0.01	hundredth
milli	m	10⁻³	0.001	thousandth
micro	µ	10⁻⁶	0.000001	millionth
nano	n	10⁻⁹	0.000000001	billionth
pico	p	10⁻¹²	0.000000000001	trillionth
femto	f	10⁻¹⁵	0.000000000000001	quadrillionth
atto	a	10⁻¹⁸	0.000000000000000001	quintillionth
zepto	z	10⁻²¹	0.000000000000000000001	sextillionth
yocto	y	10⁻²⁴	0.000000000000000000000001	septillionth
ronto	r	10⁻²⁷	0.000000000000000000000000001	octillionth
quecto	q	10⁻³⁰	0.000000000000000000000000000001	nonillionth

Highlighted rows are the prefixes you meet most in the lab.

How metric prefixes work

A prefix is a power of ten

A metric prefix scales a unit by a fixed power of ten: kilo- means × 1000, so a kilometer is 1000 meters, and milli- means × 0.001, so a milliliter is a thousandth of a liter. The unit itself does not change, only the factor in front of it, which is why the same prefixes work for grams, liters, moles and seconds alike.

Steps of a thousand

Above kilo and below milli the prefixes step by factors of 1000: kilo, mega, giga, tera upwards and milli, micro, nano, pico downwards. Converting between neighbors therefore moves the decimal point three places. The four exceptions — hecto, deca, deci and centi — step by ten and survive mainly in everyday units such as the centimeter and the hectare.

The lab’s favorites

Bench work lives between milli and femto. Volumes are pipetted in mL, µL and nL; concentrations are written in mM, µM, nM and pM; masses in mg, µg and ng. Keeping the thousand-fold steps in mind catches most dilution mistakes: 1 µL is 10⁻⁶ L, and 1 nM is a millionth of 1 mM.

The newest prefixes

In 2022 the General Conference on Weights and Measures added ronna (10²⁷), quetta (10³⁰), ronto (10⁻²⁷) and quecto (10⁻³⁰), the first new prefixes since 1991. They were driven by data storage, which was approaching a yottabyte, and by the masses of subatomic particles, which are more readable in quectograms than in long strings of zeros.

How this is calculated

A conversion multiplies the value by 10 raised to the difference of the two exponents, in one step: from kilo (10³) to milli (10⁻³) is × 10⁶. Doing it in a single multiplication keeps results such as 1 km = 1000 m exact instead of accumulating rounding from two separate scalings.

The prefixes, symbols and exponents follow the SI Brochure of the BIPM, 9th edition, as amended by the 2022 resolution that added the four newest prefixes. The decimal column is generated from the exponent, so it cannot disagree with the factor.