The SI unit of charge is Coulomb (C).
One coulomb of charge is that charge which when placed at rest in vacuum at a distance of one metre from an equal and similar stationary charge repels with a force of 9 × 109 N. One coulomb is evidently too big a unit to be used. In practice, in electrostatics, one uses smaller units like 1 mC or 1 µC. In CGS system, the unit of charge is stat coulomb or esu of charge.
