The abcoulomb (abC) or electromagnetic unit of charge (emu) is the physical unit for electrical charge used in the cgs electromagnetic system of units. The SI system of units uses the coulomb (C) instead. The conversion is

1 abC = 10 C

In the electromagnetic cgs system, electrical current is a fundamental quantity defined via Ampere's law and takes the permeability as a dimensionless quantity (relative permeability) whose value in a vacuum is unity. As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system.

The definition of the abcoulomb follows from that of the abampere : given two parallel currents of one abampere separated by one centimetre, the force per distance of wire is 2 dyne/cm. The abcoulomb is the charge flowing in 1 second given a current of 1 abampere.