SI Metric Prefix Chart

An SI prefix is a single symbol that multiplies a unit by a fixed power of ten. Writing 2.4 GHz instead of 2 400 000 000 Hz does not change the quantity — it changes how many zeros you have to count. That is the whole job of the prefix system, and it is why the same twenty-four symbols work equally well for metres, grams, watts, and bytes.

The set spans sixty orders of magnitude, from quetta (10³⁰) down to quecto (10⁻³⁰). Twenty of them are whole powers of a thousand; the remaining four — hecto, deca, deci, centi — sit between kilo and milli and survive mainly in a handful of entrenched units like the centimetre, the hectopascal, and the decibel.

The 1795 French metric system launched with just six. Kilo, hecto, and deca came from Greek number words for the multiples; deci, centi, and milli from Latin for the fractions — a deliberate split that still lets you read the direction of a prefix off its etymology.

The 27th General Conference on Weights and Measures approved four new prefixes in November 2022 — the first expansion since 1991. The driver was data. Global storage and traffic figures were closing in on 10²⁷ bytes, and once a quantity outruns the largest prefix, people start inventing unofficial names. "Hellabyte" had already picked up a following.

The names were engineered rather than borrowed. Ronna (1000⁹) and quetta (1000¹⁰) echo the Greek words for nine and ten, matching the logic behind yotta (1000⁸, from októ). Their small counterparts ronto and quecto take the mirrored form. R, Q, r, and q were among the last unclaimed letters that would not collide with an existing unit symbol.

They are not decorative. Earth's mass is a tidy 5.97 ronnagrams, and an electron weighs about 0.91 rontograms — two quantities that previously required exponents in the middle of a sentence.

Early computing borrowed the SI prefixes and quietly redefined them: a kilobyte meant 2¹⁰ = 1024 bytes because powers of two are what memory addressing produces. The approximation held at small scales — 2.4% off at kilo — but the error compounds with every step, reaching 9.1% at tera and 20.9% at yotta.

IEC 60027-2 fixed the ambiguity in 1998 by introducing separate binary prefixes: kibi, mebi, gibi, tebi and up, formed from the first syllable of the SI prefix plus "bi" for binary. Under that scheme SI prefixes are always powers of ten and Ki/Mi/Gi are always powers of two.

Adoption is split, which is why a "1 TB" drive shows up as 931 GB in Windows. The manufacturer counted 10¹² bytes correctly; the operating system divided by 2⁴⁰ and labelled the result GB instead of GiB. macOS and Linux distributions have largely switched to decimal reporting, so the same drive reads as 1 TB there.

Scientific notation allows any exponent; engineering notation restricts it to multiples of three, so every value maps onto a prefix that actually exists. That is why component values, datasheets, and instrument displays show 4.7 kΩ and 220 nF rather than 4.7 × 10³ Ω and 2.2 × 10⁻⁷ F.

The converter above applies the same rule: it finds the prefix that puts the mantissa in the range 1 ≤ x < 1000 and marks that row as the natural way to write the quantity. Everything else in the table is the same number seen through a different exponent.