Add all resistances together. ​

in a series circuit, the total resistance is equal to the sum of all resistances.[1] The same current passes through each resistor, so each resistor does its job as you would expect. For example, a series circuit has a 2 Ω (ohm) resistor, a 5 Ω resistor, and a 7 Ω resistor. The total resistance of the circuit is 2 + 5 + 7 = 14 Ω.

Start with current and voltage instead. ​

If you don't know the individual resistance values, you can rely on Ohm's Law instead: V = IR, or voltage = current x resistance. The first step is to find the circuit's current and total voltage: The current of a series circuit is the same at all points on the circuit.[2] If you know the current at any point, you can use that value in this equation. The total voltage is equal to the voltage of the supply (the battery). It is not equal to the voltage across one component.[3]