If you have more than one resistor in a circuit it is often useful to be able to calculate the total resistance of the combination. The current flowing through each resistor will be the same as they are connected in series, but the energy used per coulomb (for instance, pd) will depend on the value of each resistance. So: But V = IR, so: Cancel the I's: In parallel: Here the potential difference across all three routes will be the same, but the current through each route depends on its resistance. So: Where V is the potential difference across each of the resistors. Cancel the V's to get: