Yocto

Yocto- (symbol y) is a prefix in the metric system denoting a factor of 10−24 or 0.000000000000000000000001.

It was adopted in 1991 by the General Conference on Weights and Measures. It comes from the Greek οκτώ, meaning "eight", because it is equal to 1000−8. As of 2012, yocto is the smallest official SI prefix.

It can be used to state a subatomic particle's mass, for example:

Rest mass of electron: 0.000911 yg
Rest mass of proton: 1.6726 yg
Rest mass of neutron: 1.6749 yg
Atomic mass unit: 1.6605 yg
Metric prefixes
Prefix Symbol 1000m 10n Decimal Short scale Long scale Since
yotta Y 10008 1024 1000000000000000000000000 septillion quadrillion 1991
zetta Z 10007 1021 1000000000000000000000 sextillion trilliard 1991
exa E 10006 1018 1000000000000000000 quintillion trillion 1975
peta P 10005 1015 1000000000000000 quadrillion billiard 1975
tera T 10004 1012 1000000000000 trillion billion 1960
giga G 10003 109 1000000000 billion milliard 1960
mega M 10002 106 1000000 million 1960
kilo k 10001 103 1000 thousand 1795
hecto h 10002/3 102 100 hundred 1795
deca da 10001/3 101 10 ten 1795
10000 100 1 one –
deci d 1000−1/3 10−1 0.1 tenth 1795
centi c 1000−2/3 10−2 0.01 hundredth 1795
milli m 1000−1 10−3 0.001 thousandth 1795
micro μ 1000−2 10−6 0.000001 millionth 1960
nano n 1000−3 10−9 0.000000001 billionth milliardth 1960
pico p 1000−4 10−12 0.000000000001 trillionth billionth 1960
femto f 1000−5 10−15 0.000000000000001 quadrillionth billiardth 1964
atto a 1000−6 10−18 0.000000000000000001 quintillionth trillionth 1964
zepto z 1000−7 10−21 0.000000000000000000001 sextillionth trilliardth 1991
yocto y 1000−8 10−24 0.000000000000000000000001 septillionth quadrillionth 1991