SI Prefixes Explained: From Quecto to Quetta

All 24 prefixes

Prefix	Symbol	Factor	Short-scale name
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
deka	da	10¹	ten
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 table follows NIST's list. The number words in the last column are the ones NIST gives, the short scale used in American English, where a billion is 10⁹.

The four added in 2022

The 27th General Conference on Weights and Measures added ronna (R, 10²⁷), quetta (Q, 10³⁰), ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰). Its resolution cites the needs of data science in the near future to express quantities of digital information as a reason.

The rules for writing them

Case is meaning. NIST sets lowercase symbols for all submultiples, and uppercase for multiples except kilo (k), hecto (h) and deka (da). So mm is a millimetre and Mm a megametre, a factor of a billion apart.
No compound prefixes. NIST calls them unacceptable: 1 mg, not 1 μkg. Because the kilogram already carries a prefix, mass prefixes attach to the gram.
The symbol joins the unit directly: km, μs, GHz.

Using them

Moving between prefixes is moving the decimal point by the difference in exponents: from milli (10⁻³) to micro (10⁻⁶) is three places, so 2.5 mA is 2,500 μA. The scientific notation converter handles the powers of ten for numbers too large or small to count zeros, and the unit converter and length converter apply the prefixes to real units. Area and volume square and cube the factor: a square kilometre is 10⁶ square metres, not 10³, a trap the area units guide covers.

The exception everyone meets: bytes

In the SI, kilo is 1,000 without exception, so a kilobyte is 1,000 bytes and a gigabyte 10⁹. Computer memory comes in powers of two, though, and software long used kilo for 1,024. The binary prefixes kibi, mebi and gibi exist to keep the two apart; the data storage converter converts between both families, and the kilobyte vs kibibyte guide explains why a 1 TB drive shows 931 GB.

Frequently asked questions

What are SI prefixes?

Words and symbols placed in front of a unit to multiply it by a power of ten: kilo means 10³, so a kilometre is 1,000 metres; milli means 10⁻³, so a milligram is a thousandth of a gram. NIST lists 24 of them, from quecto at 10⁻³⁰ to quetta at 10³⁰.

What are the newest SI prefixes?

Ronna (R, 10²⁷), quetta (Q, 10³⁰), ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰), adopted by the 27th General Conference on Weights and Measures in 2022. The resolution names the expected needs of data science, which is approaching the yottabyte range, as a reason.

Why is kilo a lowercase k?

By rule, the symbols for submultiples are lowercase, and the symbols for multiples are uppercase with three exceptions: kilo (k), hecto (h) and deka (da). So 5 km is correct, and a capital K means kelvin, not kilo.

Can I combine two prefixes?

No. NIST is explicit that compound prefixes are unacceptable: write 1 mg, not 1 μkg. Because the kilogram already carries a prefix, NIST forms the mass prefixes on the gram instead.

Is a kilobyte 1,000 or 1,024 bytes?

In the SI, kilo always means 1,000, so a kilobyte is 1,000 bytes. The 1,024-byte unit has its own binary prefix, the kibibyte (KiB). The kilobyte vs kibibyte guide explains why drives and operating systems disagree.

Sources

Written by Tobias. Updated October 22, 2026.