Series and Parallel Resistors Calculator
Find the equivalent resistance of up to four resistors wired in series or in parallel.
How it works
Series: R = R₁ + R₂ + … Parallel: 1/R = 1/R₁ + 1/R₂ + …
Resistors in series add up directly, so the total is always larger than the biggest single resistor. In parallel the reciprocals add, so the equivalent resistance is always smaller than the smallest resistor because current has more paths to flow through. Leave a resistor at 0 to exclude it.
Worked example
Three resistors of 100, 200 and 300 Ω in series total 600 Ω. In parallel they combine to about 54.55 Ω.
Frequently asked questions
Why does parallel resistance drop?
Adding a parallel path gives current another route, so more current flows for the same voltage — which means less overall resistance. The equivalent is always below the smallest resistor.
How do I use fewer than four resistors?
Set the unused resistor fields to 0. Only values greater than zero are included in the calculation.
Can I mix series and parallel?
This tool handles a single group at a time. For mixed networks, reduce each series or parallel block separately and then combine the block results step by step.
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Educational estimate. Check the formula and units for your exact case.