How do you calculate equivalent resistance for series and parallel resistor combinations?

For series resistors, equivalent resistance is the sum: Req = R1 + R2 + R3... (current is same, voltages add). For parallel resistors, the reciprocal of equivalent resistance equals sum of reciprocals: 1/Req = 1/R1 + 1/R2 + 1/R3... (voltage is same, currents add). For two parallel resistors, the formula simplifies to Req = (R1 x R2)/(R1 + R2).