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Parallel Resistance Calculator

Find the combined resistance of parallel resistors.

How it is calculated

For resistors in parallel the reciprocals add up: 1/Rt = 1/R1 + 1/R2 + 1/R3, so Rt = 1 ÷ (1/R1 + 1/R2 + 1/R3). All values are in ohms (Ω). R1 and R2 are required and must be above zero. R3 is optional: if you leave it empty, or enter zero or a negative number, it is simply left out and only two resistors are combined. This means the tool does not treat a 0 Ω entry in the third field as a short circuit. The total is rounded to two decimals. The combined resistance is always lower than the smallest single resistor, because every extra branch gives the current one more path. For exactly two resistors the formula can be written R1×R2 ÷ (R1+R2).

Worked example

Three resistors of 100 Ω, 220 Ω and 330 Ω are connected in parallel. Reciprocals: 1/100 = 0.010000, 1/220 ≈ 0.004545, 1/330 ≈ 0.003030. Their sum is about 0.017576. Taking the reciprocal gives 1 ÷ 0.017576 ≈ 56.90 Ω. That is below the smallest resistor (100 Ω), as expected. With only the first two resistors the same method gives 68.75 Ω.

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Common mistakes

  • Adding the resistances as in a series circuit (100 + 220 + 330 = 650 Ω). In parallel the total must be smaller than the smallest resistor.
  • Mixing units, such as one value in kΩ and another in Ω. Convert everything to the same unit first; the result is shown as Ω.
  • Leaving the third field at 0 to represent a short circuit. The tool ignores it, so the answer will not drop to 0 Ω.
  • Entering 0 for R1 or R2, which gives no result. A real short circuit makes the equivalent resistance near zero and has to be handled separately.
  • Using the formula for ideal resistors when tolerances matter. A 5% resistor can change the real total by a few percent.

About this calculator

When resistors are connected in parallel, the same voltage appears across each of them but the current splits between the branches, so the total (equivalent) resistance is always less than the smallest individual resistor. The relationship is 1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃ + ... — the reciprocal of the total resistance equals the sum of the reciprocals of each resistor. For just two resistors this simplifies to the handy shortcut Rₜ = (R₁×R₂)/(R₁+R₂).

This calculator finds the combined resistance of two or three resistors wired in parallel. Electronics hobbyists, electrical engineers and technicians use parallel resistance calculations constantly when designing circuits — for setting a specific load resistance, combining standard resistor values to hit a non-standard target, calculating current-sharing in power distribution, or analyzing how adding a component to an existing circuit branch changes total impedance.

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Comments (1)

  • Editorial team

    A useful shortcut for choosing a value: identical resistors in parallel give the value divided by the count, so three 300 ohm parts make 100 ohms. The total also sits just under the smallest resistor, so a very low value in one branch dominates the result. Standard resistor series rarely hit an exact target, which is why two or three get combined in parallel. What target value are you trying to reach?

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