Metric (SI) Prefixes Chart with Exact Prefix Converter

The four newest prefixes

In November 2022 the General Conference on Weights and Measures added four prefixes: ronna (R, 10 to the 27) and quetta (Q, 10 to the 30) for very large numbers, and ronto (r, 10 to the minus 27) and quecto (q, 10 to the minus 30) for very small ones. They are marked 2022 in the table. Older textbooks stop at yotta and yocto, so a table with only 20 prefixes is out of date. The reason for the change is data science, where quantities such as the mass of the Earth (about 6 ronnagrams) now need a prefix.

How to convert between prefixes

• Subtract the two powers of ten and move the decimal point by that many places. New value = old value times 10 to the power (old power minus new power).
• To convert 5 µg to mg: the powers are -6 and -3, the difference is -3, so 5 µg = 0.005 mg. To convert 2.5 GHz to kHz: 9 minus 3 is 6, so 2.5 GHz = 2 500 000 kHz.
• The converter above does the same thing with exact decimal arithmetic, so 1 Qm comes out as exactly 1 000 000 000 000 000 000 000 000 000 000 m instead of a rounded floating point number.

Common mistakes

• Case matters. Lower case m is milli and capital M is mega, a difference of 10 to the 9. Lower case p is pico and capital P is peta. Kilo is always lower case k, because capital K is the kelvin.
• The micro symbol is the Greek letter mu (µ). In plain text people write u, but the SI symbol is µ. Micro is 1000 times smaller than milli.
• The kilogram is the base unit of mass and already carries a prefix, so you never write a prefix in front of it. One millionth of a kilogram is 1 mg, not 1 µkg. And prefixes never stack: write nm, not mµm.

Decimal prefixes versus binary prefixes

SI prefixes are powers of 10, so 1 kB is exactly 1000 bytes and 1 GB is 1 000 000 000 bytes. Computer memory is built in powers of 2, which is why 1024 bytes was once also called a kilobyte. IEC 80000-13 settled the confusion with separate binary prefixes: kibi (Ki) is 2 to the 10 = 1024, mebi (Mi) is 2 to the 20, and gibi (Gi) is 2 to the 30. So 1 KiB is 1024 bytes and 1 kB is 1000 bytes. This page covers the decimal SI prefixes only.

Short scale and long scale names

The last column uses the short scale, where a billion is 10 to the 9 and a trillion is 10 to the 12. This is standard in the United States and in modern British English. Some countries use the long scale, where a billion is 10 to the 12. The SI names such as giga and tera never change, which is one reason scientists use them instead of the words. Values come from the BIPM SI Brochure, 9th edition, and agree with the NIST metric prefix list. Checked October 2026.