The amount of current flowing through a conductor is proportional to the voltage applied across the conductor. This is true of all conductors with the exception of super-conductors. Mathematically this is stated as:

V = k ⋅ I


  • V is the voltage (in volts) drop across the conductor
  • I is the current (in amps) flowing through the conductor
  • k is the constant of proportionality

In this case, the constant of proportionality is replaced by the resistance (in ohms) of the conductor. The equation becomes:

V = I ⋅ R

Given two of the three values, it is possible to calculate the unknown value.

This equation can be rearranged to allow the calculation of the current:

I = V / R

or the resistance:

R = V / I

See Driving LEDs with Current Limiting Resistors for an example of using Ohm’s Law.

Further Reading