Ohm's Law

Well ... you can use Ohm's Law to calculate the voltage, if you know the current and the resistance. But this is not what Ohm's Law is normally used for. It's more often stated as "the current passing through a conductor is proportional to the voltage." If you double the voltage, you get double the current. The voltage is what produces the current – not the other way round. And the ratio between the voltage and the current is determined by the resistance of the conductor; the current is the voltage divided by the resistance.

You could use Ohm's Law to calculate either the current or the voltage, or the resistance, provided you know the other two values. But the principal use of the law is to find out what current results from passing a known voltage through a known resistor.