Once the voltage is determined, the charge on the oil drop can be calculated.  The problem is that it is not known how many electrons were on each drop. 

 

Here Millikan had to rely on hundreds of measurements of charges on different oil drops so he could look at all the charges and determine what the smallest multiple that each one differed by. 

 

That smallest amount turned out to be 1.6 x 10-19 Coulombs, the charge on one electron!