Series and Parallel Resistor Calculator

In short Resistors in series simply add, because the same current passes through each and their voltage drops accumulate. Resistors in parallel add as reciprocals — one over the total equals the sum of one over each — because the voltage across them is shared and the currents add instead. That gives one sanity check worth more than the formula itself: a parallel combination is ALWAYS smaller than the smallest resistor in it. Two 1 kΩ resistors in parallel give 500 Ω, and if a calculation ever produces something larger than the smallest member, the reciprocals have been added the wrong way round. Two equal resistors are the case to keep in your head: in parallel exactly half, in series exactly double. That catches most arithmetic slips without a calculator. The practical reason to do any of this is that the preferred series is lumpy. E24 values step by anywhere from 6.7% to 15.4%, so a value a design actually needs may simply not be manufactured — and combining two common resistors reaches it far more cheaply and quickly than sourcing a 1% E96 part. Tolerances combine as well, though less alarmingly than people expect: two ±5% resistors in series give a total still specified at ±5%, but because the two errors are independent the typical spread is narrower than the worst case.