Parallel Resistance Calculator

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).

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.