Exa is a decimal unit prefix in the metric system denoting 10^{18} or 1000000000000000000. It was added as an SI prefix to the International System of Units (SI) in 1975,^{[1]} and has the unit symbol E.
Exa comes from the ancient Greek ἕξ héx, used as a prefix ἑξά hexá, meaning six (like hexa), because it is equal to 1000^{6}.
Examples:
Prefix  Base 10  Decimal  English word  Adoption ^{[nb 1]}  

Name  Symbol  Short scale  Long scale  
quetta  Q  10^{30}  1000000000000000000000000000000  nonillion  quintillion  2022  
ronna  R  10^{27}  1000000000000000000000000000  octillion  quadrilliard  2022  
yotta  Y  10^{24}  1000000000000000000000000  septillion  quadrillion  1991  
zetta  Z  10^{21}  1000000000000000000000  sextillion  trilliard  1991  
exa  E  10^{18}  1000000000000000000  quintillion  trillion  1975  
peta  P  10^{15}  1000000000000000  quadrillion  billiard  1975  
tera  T  10^{12}  1000000000000  trillion  billion  1960  
giga  G  10^{9}  1000000000  billion  milliard  1960  
mega  M  10^{6}  1000000  million  1873  
kilo  k  10^{3}  1000  thousand  1795  
hecto  h  10^{2}  100  hundred  1795  
deca  da  10^{1}  10  ten  1795  
10^{0}  1  one  –  
deci  d  10^{−1}  0.1  tenth  1795  
centi  c  10^{−2}  0.01  hundredth  1795  
milli  m  10^{−3}  0.001  thousandth  1795  
micro  μ  10^{−6}  0.000001  millionth  1873  
nano  n  10^{−9}  0.000000001  billionth  milliardth  1960  
pico  p  10^{−12}  0.000000000001  trillionth  billionth  1960  
femto  f  10^{−15}  0.000000000000001  quadrillionth  billiardth  1964  
atto  a  10^{−18}  0.000000000000000001  quintillionth  trillionth  1964  
zepto  z  10^{−21}  0.000000000000000000001  sextillionth  trilliardth  1991  
yocto  y  10^{−24}  0.000000000000000000000001  septillionth  quadrillionth  1991  
ronto  r  10^{−27}  0.000000000000000000000000001  octillionth  quadrilliardth  2022  
quecto  q  10^{−30}  0.000000000000000000000000000001  nonillionth  quintillionth  2022  
