How to use the series & parallel resistance calculator
The formulas
Series: R = R1 + R2 + R3 + …
Parallel: 1/R = 1/R1 + 1/R2 + 1/R3 + …
How to use it
Pick Series or Parallel, enter at least two resistor values in ohms, and leave the optional boxes blank for any slots you do not need. Press Calculate.
Intuition worth keeping
Series always makes the total bigger than the largest resistor — current has one longer path. Parallel always makes it smaller than the smallest resistor — current gains extra paths. If a result surprises you, that pair of checks usually finds the mistake.
Example
Two 10 Ω resistors: in series they make 20 Ω; in parallel they make exactly 5 Ω. Both pairs are the values loaded when the page opens — try each tab.
Frequently asked questions
How do series resistors combine?
They simply add: R total = R1 + R2 + R3… The same current flows through each, and the voltages across them add up to the supply voltage.
How do parallel resistors combine?
1/R total = 1/R1 + 1/R2 + 1/R3… For exactly two resistors there is a shortcut: product over sum, R1 × R2 ÷ (R1 + R2). Two equal resistors in parallel give exactly half of one.
What happens if one parallel resistor is 0 ohms?
A perfect 0 Ω path short-circuits the rest — the total is 0 Ω and all current takes that path. The calculator reports 0 for that case.
Is this only for resistors?
The same rules describe anything purely resistive: heating elements in a heater, lamp filaments (approximately), and speaker loads are wired series/parallel with the same maths.
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