Prefixes for physical & chemical units
One system of names that stretches a single unit across sixty orders of magnitude — from the mass of the Sun to the mass of a proton. Here is the full set, for both the lab and the data centre.
Drag through every power of ten
Twenty-four decimal prefixes, mapped to things you can picture. Move the slider — or let it run.
A prefix scales a unit; two systems share the work
A unit prefix is a short symbol placed before a unit to multiply it by a fixed factor, so the same base unit can express enormous and tiny quantities without strings of zeros. The decimal SI prefixes (kept by the BIPM) are universal in physics, chemistry and engineering — every step is a power of ten, and IUPAC endorses them for chemical quantities. The binary IEC prefixes (ISO/IEC 80000-13) serve computing, where quantities scale in powers of two. The SI set was expanded from 20 to 24 prefixes in 2022 — its first change since 1991.
The 24 SI prefixes
Powers of ten, applicable to any SI unit — metre, gram, mole, second, joule, litre, pascal. Twelve multiples above the base unit, twelve submultiples below it.
Multiples · larger than the base unit
| Factor | Name | Sym | Typical use |
|---|
Submultiples · smaller than the base unit
| Factor | Name | Sym | Typical use |
|---|
Binary prefixes for computing
In storage, memory and data transmission, quantities scale in powers of two. The IEC prefixes remove the old ambiguity where “1 kilobyte” meant either 1 000 or 1 024 bytes. Each mirrors an SI name but is built on 2¹⁰ = 1 024 instead of 10³ = 1 000 — so the two drift apart at larger scales.
How far binary drifts from decimal
Percentage by which each binary prefix exceeds its decimal namesake.
IEC binary prefixes
| Factor | Name | Sym | ≈ SI |
|---|
Non-SI units still in use
Not everything in daily scientific, chemical and industrial use belongs to the SI. The BIPM keeps a short list of units accepted for use alongside it — kept because they are too embedded in practice to drop. Most convert to their SI equivalent by a factor that isn't a power of ten, which is precisely why they sit outside the coherent system.
Rules that keep prefixes unambiguous
Authoritative references
Every link points to a primary standards body — BIPM, NIST or IEC.
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Parts of this article were prepared with the assistance of AI and reviewed by Gallois Genootschap, which holds editorial responsibility for its publication.
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