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Look up zetta- in Wiktionary, the free dictionary.

Zetta- (symbol Z) is a prefix in the metric system denoting a factor of 10²¹ or 1000000000000000000000.

Added to the SI in 1991, it is evocative of the French numeral sept, meaning seven, because it is equal to 1000⁷.

A prefix of the same value, hepta, was informally introduced a few years before the promulgation of zetta. It was formed from the Greek ἑπτά, (hepta), also meaning seven. Hepta never received official sanction and is now obsolete.

Examples:

• The mass of Earth's atmosphere is approximately 5 zettagrams (Zg).
• The volume of seawater in the Earth's oceans is approximately 1.369 zettalitres (ZL).
• Multiplying the 10240 megabytes of storage each Gmail user has (as of April 2012) with the estimated 260 Million Gmail users, the total storage needed for Gmail is more than a zettabyte (not considering how google might compress the stored data).

Footnotes

1. ^ "Resolution 4 of the 19th CGPM (Conférence Générale des Poids et Mesures)". BIPM (Bureau International des Poids et Mesures). http://www.bipm.org/fr/CGPM/db/19/4/. Retrieved 2009-05-16. "Les noms zepto et zetta évoquent le chiffre sept (septième puissance de 10³) et la lettre « z » remplace la lettre « s » pour éviter le double emploi de la lettre « s » comme symbole. (The names zepto and zetta are evocative of the numeral sept (seventh power of 10³) and the letter 'z' is substituted for the letter 's' to avoid the duplicate use of the letter 's' as a symbol." 

External links

• BIPM website