In physics and mathematics, peta- (symbol: P) is a prefix in the SI (system of units) denoting 10¹⁵, or 1,000,000,000,000,000. For example:

1 petametre = 10¹⁵ metres

1 petasecond = 10¹⁵ seconds

Adopted in 1975, it comes from the Greek πέντε, meaning five, because it is equal to 1000⁵. It is based on the model of tera- (from Greek τέρας = 'monster', but looking like tetra- from the Greek for "four" with a letter missing), and so peta-, coming from penta-, omits the third letter, n.

Examples:

• the mass–energy equivalence is 89.9 PJ/kg

• 1 light-year = 9.461 Pm

In computer science peta- can sometimes mean 1 125 899 906 842 624 (1024⁵ or 2⁵⁰), instead of 1 000 000 000 000 000, especially when used to prefix the byte, giving a petabyte. To resolve this ambiguity, the term pebibyte has been suggested to mean 2⁵⁰ bytes.

See also

• SI prefix

• Binary prefix