Add two or more resistor values and get the total resistance for a series or parallel combination, with the parallel working shown.
Resistors in series simply add together, so the total is the sum of every value. Resistors in parallel combine by reciprocals: one divided by the total equals the sum of one divided by each resistor, which is why a parallel total is always lower than the smallest individual resistor. Each row takes its own unit — ohms, kilohms or megohms — and values are converted to ohms before the calculation. At least two resistor rows with values above zero are required.
Why is a parallel total lower than the smallest resistor? Parallel paths give current more than one route, so the combined resistance falls. Mathematically the reciprocal of the total is the sum of the reciprocals, which makes the total smaller than any single branch.
How are series resistors calculated? Series resistors are added directly: total resistance equals R1 + R2 + R3 and so on, with all values first converted to ohms.