Electrons, like many other particles, have a charge. While some particles have a positive charge, electrons have a negative charge. The charge on an electron is equal to approximately -1.6 x 10−19 coulomb. The coulomb (commonly abbreviated C) is the SI unit of charge. One coulomb is defined as the electric charge carried by 1 ampere (amp) of current in 1 second. It is normal to ignore the negative nature of this charge when considering electricity.

If we have n particles with the same charge Qparticle, then the total charge Qtotal is given by:

Qtotal = n Qparticle

By a simple rearrangement:

${\displaystyle n={\frac {Q_{total}}{Q_{particle}}}}$