Parallel Resistors

Calculate total resistance for two or more resistors in parallel. Enter resistor values in ohms and see the reciprocal formula and example.

Calculator inputs

  • Resistances (Ω, comma-separated)

Formula

1/R_total = 1/R1 + 1/R2 + ...

How this calculator works

Enter two or more positive resistor values separated by commas. The calculator adds their reciprocals and takes the reciprocal of that sum to find total parallel resistance.

Worked example

For 100 ohms, 200 ohms, and 300 ohms in parallel, 1/R = 1/100 + 1/200 + 1/300. The total resistance is about 54.55 ohms.

Common mistakes

  • Adding resistor values directly, which is the series formula rather than the parallel formula.
  • Including zero or negative resistance values.
  • Forgetting that total parallel resistance must be lower than the smallest branch resistance.

Frequently asked questions

How do you calculate resistors in parallel?

Add the reciprocal of each resistance, then take the reciprocal of the total: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 and so on.

What is the formula for two resistors in parallel?

For two resistors, the shortcut is Rtotal = (R1 x R2) / (R1 + R2).

Is parallel resistance always lower?

Yes. For positive resistor values, total parallel resistance is lower than the smallest individual resistor.

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