An erg is the unit of energy and mechanical work in the centimetre-gram-second (CGS) system of units, symbol "erg". Its name is derived from the Greek ergon, meaning "work".

An erg is the amount of work done by a force of one dyne exerted for a distance of one centimeter. In the CGS base units, it is equal to one gram centimeter-squared per second-squared (g·cm2/s2). It is thus equal to 10−7 joules or 100 nanojoules (nJ) in SI units.

The impact energy of a small mosquito flying into a wall is equal to about 1 erg.

1 erg = 10−7 J = 100 nJ

1 erg = 624.15 GeV = 6.2415 ×1011 eV

1 erg = 1 dyne cm = 1 g·cm2/s2