Series & Parallel Resistor Calculator
Total resistance of any set of resistors, in series and in parallel.
The formulas
series: R = R₁ + R₂ + ... parallel: 1/R = 1/R₁ + 1/R₂ + ...
Series resistance just adds. Parallel combines reciprocals, so the total is always LOWER than the smallest resistor in the group: 220, 470 and 1000 Ω in parallel make about 136 Ω.
Sanity checks worth memorizing
Two equal resistors in parallel halve; n equal ones divide by n. And for two resistors there's the product-over-sum shortcut: R = R₁R₂ / (R₁ + R₂). If your parallel answer comes out bigger than any input, a reciprocal got dropped somewhere.