Prefix (Physics)

A prefix is a short notation attached to a unit name or symbol to indicate that the unit is multiplied by a specific power of ten. For example, "kilo" represents 1,000, so a kilogram is 1,000 grams. The International System of Units (SI) contains an official list of prefixes, each with a name and a symbol of one or two letters.

The purpose of prefixes is to avoid writing very long numbers with many zeros. Instead of writing 0.000 000 001 meter, one can write 1 nanometer. Similarly, instead of writing 2,000,000,000 hertz, one can write 2 gigahertz.

Increasing prefixes

Prefixes representing multiples greater than 1 include: deca (da, 10¹), hecto (h, 10²), kilo (k, 10³), mega (M, 10⁶), giga (G, 10⁹), tera (T, 10¹²), peta (P, 10¹⁵), exa (E, 10¹⁸), zetta (Z, 10²¹), and yotta (Y, 10²⁴). It is noted that from mega onward, symbols are written in capital letters, while kilo and smaller prefixes use lowercase letters.

Decreasing prefixes

On the other hand, there are prefixes representing fractions: 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), and yocto (y, 10⁻²⁴). The symbol for micro is the Greek letter µ (mu).

Extension of the list

In 2022, the General Conference on Weights and Measures (CGPM) added four new prefixes: ronna (R, 10²⁷) and quetta (Q, 10³⁰) for increasing multiples, and ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰) for decreasing multiples. The stated reason was the need in computing and science for names to express extremely large numbers.

Rules of use

Only one prefix is used per unit; they are not combined (for example, there is no "kilokilometer"). When a prefix is applied to a unit, the entire expression becomes a single unit, so (km)² equals one trillion square meters, not kilo times square meters. The kilogram is a special case: it is the base unit of mass, even though its name contains a prefix, which means that other prefixes are attached to the gram (for example, milligram).

Binary prefixes

In computer science, SI prefixes have sometimes been incorrectly used to represent powers of 2 (for example, kilobyte understood as 1,024 bytes). To distinguish between them, the IEC system introduced different prefixes such as kibi (Ki, 2¹⁰), mebi (Mi, 2²⁰), and gibi (Gi, 2³⁰).