Parallel Resistor Calculator

How it is calculated

Enter two resistances R1 and R2 in ohms (Ω); both must be greater than zero. For two resistors in parallel the tool uses the product-over-sum form Rpar = (R1 × R2) ÷ (R1 + R2), which is the two-resistor case of 1/R = 1/R1 + 1/R2. For resistors in series it uses Rser = R1 + R2. Both results are shown in ohms with two decimals. The parallel value is always below the smaller of the two resistors, and the series value always above the larger one. Only two resistors are handled; for three or more in parallel the product-over-sum shortcut no longer works, but you can combine two at a time. The tool treats resistors as ideal, ignoring tolerance, temperature drift and wire resistance.

Common mistakes

• Adding resistances for a parallel circuit. Parallel combinations are always smaller than the smallest resistor; if your answer is bigger, you used the series formula.
• Mixing units, such as kΩ for one value and Ω for the other. Enter both in ohms: 4.7 kΩ is 4700.
• Using the tool for three or more resistors in one step. Combine two, then use that result as R1 with the next resistor.
• Forgetting resistor tolerance. Standard 5% parts can shift the combined value, so the real result may not be exactly what is shown.
• Entering 0 for a wire or short circuit. The tool requires positive values; a zero-ohm path gives zero parallel resistance in practice.