Metric Prefixes from Yocto to Yotta: The Complete Guide

What Are Metric (SI) Prefixes?

A metric prefix is a name-and-symbol combination that, when placed in front of an SI unit name or symbol, multiplies that unit by a fixed power of ten. Instead of writing 0.000000001 meters, you write 1 nanometer (1 nm). Instead of 1,000,000,000,000 bytes, you write 1 terabyte (1 TB — using the decimal prefix; binary is separate and covered below).

Prefixes are maintained by the International Bureau of Weights and Measures (BIPM) and defined in the SI Brochure. They attach to any SI base or derived unit: kilogram, megawatt, microsecond, nanotesla, femtofarad. The only exception is the kilogram itself, which already carries the prefix "kilo-" as its base name; prefixes attach to "gram," not "kilogram" (e.g., milligram, not microkilogram).

Prefixes reduce the number of significant zeros in a value, eliminate ambiguity, and provide a universal vocabulary shared across all SI-using countries. They are not units themselves — they are multipliers.

The Full Table: 24 Prefixes (10⁻²⁴ to 10²⁴)

The following table lists all 24 SI prefixes as defined after the 2022 update (discussed in the next section). Prefixes are listed from largest to smallest.

Each step between adjacent prefixes (except hecto/deka and deci/centi) is a factor of 10³ = 1,000. The table spans 60 orders of magnitude in total (30 above and 30 below the base unit) after the 2022 additions.

2022 Update: Ronna, Quetta, Ronto, Quecto

At the 27th General Conference on Weights and Measures (CGPM) in November 2022, four new prefixes were officially adopted:

The motivation was data. Global data storage volumes are expected to reach the ronnabyte (10²⁷ bytes) range within decades, and the previous largest prefix, yotta (10²⁴), was becoming insufficient. The Earth's mass is approximately 5.972 × 10²⁴ kg — just under 6 yottagrams. The mass of the observable universe is roughly 10⁵³ kg, far beyond any current prefix.

The letters R and Q were chosen specifically because they are not already used as SI unit symbols, avoiding ambiguity. The naming pattern (-nna/-etto for large; -nto/-ecto for small) follows the existing zetta/zepto and yotta/yocto pattern.

Common Prefixes in Daily Life (k, M, G, T)

Four prefixes dominate everyday use in consumer technology and commerce:

Kilo- (k, 10³): 1 kilometer = 1,000 meters. 1 kilogram = 1,000 grams. 1 kilowatt = 1,000 watts. Speed limits in metric countries are stated in km/h. Body weight is expressed in kg.

Mega- (M, 10⁶): 1 megapixel = 1,000,000 pixels. 1 megawatt = 1,000,000 watts — the output of a small wind turbine. Radio frequencies are stated in megahertz: FM radio is roughly 87.5–108 MHz.

Giga- (G, 10⁹): CPU and GPU clock speeds are stated in gigahertz (GHz). A 3.5 GHz processor completes 3,500,000,000 clock cycles per second. Mobile data allowances are quoted in gigabytes (GB).

Tera- (T, 10¹²): Hard drive capacity is advertised in terabytes (TB). 1 TB = 10¹² bytes in decimal convention. Note: operating systems often display this as approximately 931 GiB because they use binary gigabytes (2³⁰ bytes); see the binary prefixes section.

Small Prefixes in Science (µ, n, p, f, a)

Sub-millimeter science relies on these five prefixes:

Micro- (µ, 10⁻⁶): The micron (micrometer, µm) is the standard unit of biological cell size — a red blood cell is about 6–8 µm in diameter. Electronic transistors shrank from the micrometer scale in the 1970s to nanometers today.

Nano- (n, 10⁻⁹): Modern CPU transistors are measured in nanometers. Intel's 3 nm process node (commercial in 2023) has gates with critical dimensions of approximately 2.5–6 nm depending on feature type. Visible light wavelengths span roughly 380–700 nm.

Pico- (p, 10⁻¹²): Capacitance in RF circuits is stated in picofarads (pF). 1 pF = 10⁻¹² farads. Light travels approximately 0.3 mm (300 µm) in one picosecond.

Femto- (f, 10⁻¹⁵): The femtometer (fm) is the natural length scale of atomic nuclei. The proton charge radius is approximately 0.84 fm. Ultrafast laser pulses have durations in the range of tens to hundreds of femtoseconds.

Atto- (a, 10⁻¹⁸): Attosecond (as) pulses of light are now produced in laboratories; the 2023 Nobel Prize in Physics was awarded for attosecond photonics. 1 as = 10⁻¹⁸ s. The elementary charge is 1.602176634 × 10⁻¹⁹ C = 0.1602 attoampere-seconds.

Binary Prefixes: Ki, Mi, Gi (IEC Standard)

Metric prefixes are defined as powers of ten. Computers, however, address memory in powers of two. For decades, the industry used metric prefix names (kilo, mega, giga) to mean powers of two (2¹⁰, 2²⁰, 2³⁰), causing systematic confusion:

The IEC 80000-13 standard (1998, revised 2008) introduced binary prefixes: kibi (Ki), mebi (Mi), gibi (Gi), tebi (Ti), pebi (Pi), exbi (Ei), zebi (Zi), yobi (Yi). Each is 2¹⁰ = 1,024 times the previous.

In practice: hard drive manufacturers use decimal gigabytes (10⁹ bytes), which is why a "1 TB" drive shows as ~931 GB in Windows. RAM is manufactured in binary multiples and is correctly stated in GiB, though often labeled as GB. Linux and macOS have moved toward using GiB notation in system tools.

See the data storage category for a complete breakdown of bytes, kibibytes, and all related units.

Memory Aids for Prefix Order

A common mnemonic for the large prefixes (kilo through yotta) is:

"King Henry Died By Drinking Cold Milk, Eating Zucchini, Yodeling" → kilo, hecto, deka, base, deci, centi, milli, micro, nano, pico, femto, atto, zepto, yocto

For the six most-used large prefixes above kilo: "Most People Need Good Ethernet Today" → mega, peta, nano (wrong direction) — this one doesn't work neatly. The reliable approach is to remember that each step up is ×1,000: kilo → mega → giga → tera → peta → exa → zetta → yotta → ronna → quetta.

A numeric anchor: if you know 1 GHz = 10⁹ Hz and 1 nm = 10⁻⁹ m, you can count multiples of 10³ in either direction.

Quick Conversion: Shifting the Decimal

To convert between prefixes, determine the exponent difference and shift the decimal point:

• 1 km = 10³ m = 10⁶ mm = 10⁹ µm = 10¹² nm
• 1 GHz = 10³ MHz = 10⁶ kHz = 10⁹ Hz
• 1 µA = 10⁻³ mA = 10⁻⁶ A = 10³ nA

The rule: multiply by 10ⁿ to go from a larger prefix to a smaller one (the number gets bigger), divide by 10ⁿ to go from smaller to larger (the number gets smaller).

Converting 450 nm to µm: nano is 10⁻⁹, micro is 10⁻⁶. The difference is 10³, and nano is smaller, so the numeric value decreases when moving to µm: 450 nm ÷ 10³ = 0.45 µm.

For hands-on practice with any combination of metric prefixes and units, use the length converter or any other category converter on UnitFYI.