How Do Metric Prefixes Work? SI Symbols and Powers of Ten

Metric prefixes tell you how much larger or smaller a unit is. Each prefix represents a fixed power of ten: kilo means 1000 times the unit, while milli means one-thousandth of it. The prefix changes the scale of the unit, not the kind of quantity being measured.

Read a measurement in three parts: the number, the prefix and the unit. In 7.2 kJ, the number is 7.2, k means kilo, and J means joule. Together they describe 7200 joules. Keeping those parts straight makes labels easier to compare and helps catch mistakes caused by a single capital letter.

The complete SI prefix table

There are 24 recognized SI prefixes, from quetta at 10³⁰ to quecto at 10⁻³⁰. The BIPM prefix table gives their official names, symbols and multipliers. Deca is the international English spelling; NIST uses deka in American English. Both use the symbol da.

Prefix	Symbol	Multiply the unit by
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 (deka)	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

An unprefixed unit has a multiplier of 10⁰, or 1. It is the reference point, not a twenty-fifth prefix. Positive exponents make larger units; negative exponents make smaller ones. For example, 10⁻² means 1 divided by 100, not a negative measurement.

Ronna, quetta, ronto and quecto were added in CGPM Resolution 3 in 2022. An older chart ending at yotta and yocto is therefore incomplete. You may rarely encounter the newest prefixes on everyday labels, but they follow the same rules as kilo and milli.

Why counting table rows can give the wrong answer

The table is ordered by size, but adjacent rows do not all differ by 1000. Kilo to hecto is a factor of 10, as is hecto to deca. Deca to deci spans a factor of 100 because the unprefixed reference point sits between them. Milli to micro spans a factor of 1000.

Use the exponents rather than memorizing a fixed jump per row. A difference of one in the exponent means a factor of 10; a difference of two means 100; a difference of three means 1000. This works even when a chart leaves out the less common prefixes.

Capital letters carry numerical meaning

Lowercase k is the prefix kilo. Uppercase K is the unit symbol for kelvin, so they cannot be exchanged. A label reading 4 kJ means four kilojoules. Changing it to 4 KJ does not produce the standard symbol for the same quantity.

Lowercase m means milli when attached before another unit symbol, while uppercase M means mega. Thus mJ means millijoule and MJ means megajoule. One megajoule equals one billion millijoules because the exponents differ by nine: 6 − (−3) = 9.

Position matters too. A lone m is the symbol for meter; in ms, the m is the milli prefix and s is second. The Greek letter μ is the prefix symbol for micro. Preserve it when copying a label rather than silently replacing it with m, which would change the multiplier by 1000.

Keep the prefix attached and the number separate

Write 18 ms, with a space after the number and no space between m and s. The written name is millisecond, joined into one word. Write 6 kJ rather than 6kJ, 6 k J or 6 k-J. These spacing rules make clear which characters belong together.

Unit symbols stay the same in the plural: 18 ms, not 18 mss. They do not need an abbreviation period, although a sentence ending in a unit symbol still takes normal sentence punctuation. NIST’s SI writing guide explains these conventions.

A prefix supplies a multiplier, not a complete unit. Writing “8 kilo” leaves the measured quantity unspecified. Use the full unit name or symbol, such as 8 kilograms or 8 kg. In a table heading, include the whole unit symbol so the values underneath retain their meaning.

Convert by subtracting the prefix exponents

For the same underlying unit, multiply the original number by 10 raised to the original prefix exponent minus the target prefix exponent. Treat an unprefixed unit as exponent zero. This changes the numerical value while preserving the quantity.

Suppose a timing label says 35 μs and a report needs milliseconds. Micro has exponent −6 and milli has exponent −3. The difference is −6 − (−3) = −3, so multiply 35 by 0.001. The equivalent entry is 0.035 ms. The larger unit needs a smaller number.

For another example, a recorded energy value of 2.8 MJ becomes 2800 kJ: 6 − 3 = 3, so multiply 2.8 by 1000.

Apply this simple rule to the unit as written. Squared or cubed units require the prefix factor to be squared or cubed too. Also check the quantity before converting: matching prefixes cannot turn a length into a mass or an energy into a time.

Why mass prefixes attach to gram

Kilogram is the SI base unit of mass, but its name already contains kilo. Form other decimal mass units by attaching a prefix to gram, symbol g. Milligram is mg, and megagram is Mg. Do not stack a second prefix onto kg.

For example, 0.004 kg is 4 g. Writing this as a prefixed “millikilogram” would combine prefixes and break the naming rule. The NIST rules for prefixes and kilogram specify this exception. More generally, use one prefix per unit name rather than inventing combinations.

A final check before copying a label

Check the unit, exact letter case, prefix multiplier and spacing. Then ask whether the converted number moves in the expected direction. Smaller units should require more of them to describe the same quantity.

Digital storage labels need an additional distinction between decimal and binary conventions. See how to convert MB to GB for that separate case.

If you want a physical reference alongside your notes, browse metric rulers on Amazon and check that the actual scale markings suit your work. The prefix table remains the reference for the multipliers, whatever tool you use.