SI prefixes let you write very large and very small quantities compactly by attaching a single letter to a unit. This reference lists every official prefix alongside its symbol, power of ten, and full numeric multiplier, and includes a converter to rescale a value from one prefix to another.
How it works
Each prefix represents a fixed power of ten. From kilo (10³) upward and milli (10⁻³) downward the steps are powers of one thousand, which is why they map cleanly onto engineering notation — notation where the exponent is always a multiple of three. The four small exceptions near unity (deca 10¹, hecto 10², deci 10⁻¹, centi 10⁻²) step by ten instead.
To convert a value between two prefixes the tool applies the difference of their exponents:
result = value × 10^(sourceExponent − targetExponent)So 2.5 giga in mega is 2.5 × 10^(9 − 6) = 2500 mega. No unit information is
needed because both endpoints share the same base unit.
Complete SI prefix table
| Prefix | Symbol | Power | Multiplier | Engineering tier |
|---|---|---|---|---|
| quetta | Q | 10³⁰ | 1 000 000 000 000 000 000 000 000 000 000 | — |
| ronna | R | 10²⁷ | 1 000 000 000 000 000 000 000 000 000 | — |
| yotta | Y | 10²⁴ | 1 000 000 000 000 000 000 000 000 | exa × 1M |
| zetta | Z | 10²¹ | 1 000 000 000 000 000 000 000 | — |
| exa | E | 10¹⁸ | 1 000 000 000 000 000 000 | peta × 1000 |
| peta | P | 10¹⁵ | 1 000 000 000 000 000 | tera × 1000 |
| tera | T | 10¹² | 1 000 000 000 000 | giga × 1000 |
| giga | G | 10⁹ | 1 000 000 000 | mega × 1000 |
| mega | M | 10⁶ | 1 000 000 | kilo × 1000 |
| kilo | k | 10³ | 1 000 | base × 1000 |
| hecto | h | 10² | 100 | non-engineering |
| deca | da | 10¹ | 10 | non-engineering |
| — | — | 10⁰ | 1 | base unit |
| deci | d | 10⁻¹ | 0.1 | non-engineering |
| centi | c | 10⁻² | 0.01 | non-engineering |
| milli | m | 10⁻³ | 0.001 | base ÷ 1000 |
| micro | μ | 10⁻⁶ | 0.000 001 | milli ÷ 1000 |
| nano | n | 10⁻⁹ | 0.000 000 001 | micro ÷ 1000 |
| pico | p | 10⁻¹² | 0.000 000 000 001 | nano ÷ 1000 |
| femto | f | 10⁻¹⁵ | — | pico ÷ 1000 |
| atto | a | 10⁻¹⁸ | — | — |
| zepto | z | 10⁻²¹ | — | — |
| yocto | y | 10⁻²⁴ | — | — |
| ronto | r | 10⁻²⁷ | — | — |
| quecto | q | 10⁻³⁰ | — | — |
The 2022 CGPM additions — ronna, quetta, ronto, and quecto — were driven by the need to describe data storage at planetary and civilisation scale (one quettabyte = 10³⁰ bytes) and particle physics measurements at sub-yocto scale.
Common traps and tips
- Capitalisation matters.
M(mega, 10⁶) andm(milli, 10⁻³) differ by a factor of 10⁹.G(giga) andg(gram, not a prefix) are different things. Always preserve the exact case from the original spec sheet or data sheet. - Prefixes do not stack. There is no such thing as a millikilometre. Write it as a metre. Similarly, a microfarad is μF, never mF (which would be millifarad).
- Engineering notation vs. scientific notation. Engineering notation restricts the exponent to multiples of three (matching prefix steps), so 47,000 Ω is written 47 kΩ or 47 × 10³ Ω, not 4.7 × 10⁴ Ω. This is why component values in electronics always land on a prefix boundary.
- The non-engineering prefixes (deci, centi, hecto, deca) are common in everyday measurements (centimetres, decibels, hectares, decalitres) but are avoided in most engineering disciplines because they break the ×1000 step rule that makes prefix conversion predictable.