Series Resistors Equivalent Resistance
The combination of series resistances is called a string. A string could have two or more resistors. To get the total or equivalent resistance (Req) of a series string, just add the individual resistances of all the resistors.
Parallel Resistors Equivalent Resistance
In a parallel connection, all resistors have the same voltage across them. So to calculate the equivalent resistance, you can simply divide the applied voltage by the total current. But even if there’s no voltage source, resistors can still be connected in parallel and to get the equivalent resistance (Req), just invert the sum of the inverses of all resistances.