atto-

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This article describes the SI prefix. For other meanings, see Atto.
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atto- (symbol a) is an SI prefix to an unit and means that it is 10−18 times this unit. Examples are one attosecond or one attometre (U.S. spelling attometer).

Adopted in 1964, it comes from the Danish atten, meaning eighteen.

SI prefixes
1000n 10n Prefix Symbol Short scale Long scale Decimal equivalent in SI writing style
10008 1024 yotta- Y Septillion Quadrillion 1 000 000 000 000 000 000 000 000
10007 1021 zetta- Z Sextillion Trilliard (thousand trillion) 1 000 000 000 000 000 000 000
10006 1018 exa- E Quintillion Trillion 1 000 000 000 000 000 000
10005 1015 peta- P Quadrillion Billiard (thousand billion) 1 000 000 000 000 000
10004 1012 tera- T Trillion Billion 1 000 000 000 000
10003 109 giga- G Billion Milliard (thousand million) 1 000 000 000
10002 106 mega- M Million 1 000 000
10001 103 kilo- k Thousand 1 000
10002/3 102 hecto- h Hundred 100
10001/3 101 deca- da Ten 10
10000 100 (none) (none) One 1
1000−1/3 10−1 deci- d Tenth 0.1
1000−2/3 10−2 centi- c Hundredth 0.01
1000−1 10−3 milli- m Thousandth 0.001
1000−2 10−6 micro- µ Millionth 0.000 001
1000−3 10−9 nano- n Billionth Milliardth 0.000 000 001
1000−4 10−12 pico- p Trillionth Billionth 0.000 000 000 001
1000−5 10−15 femto- f Quadrillionth Billiardth 0.000 000 000 000 001
1000−6 10−18 atto- a Quintillionth Trillionth 0.000 000 000 000 000 001
1000−7 10−21 zepto- z Sextillionth Trilliardth 0.000 000 000 000 000 000 001
1000−8 10−24 yocto- y Septillionth Quadrillionth 0.000 000 000 000 000 000 000 001