Resistor Series/Parallel Calculator

This calculator gives the combined resistance of two resistors wired in series and in parallel, side by side, so you can see both options at once. It is also the quickest way to answer a parallel resistor question on its own.

Hobbyists and technicians use it to build a value they do not have in the parts drawer, to split power across two resistors, or to work out the equivalent resistance of a section of circuit before applying Ohm's law. Students use it to check homework on resistor networks.

The tool takes two resistors in ohms. For three or more, combine them two at a time — the result of one calculation becomes R1 for the next — or use the reciprocal formula below. The rules apply to any linear resistive element, including heater elements and speaker impedances at a fixed frequency.

These follow from Kirchhoff's laws: in series the same current flows through each resistor so the voltages add; in parallel the same voltage sits across each so the currents add. The two-resistor parallel form is often called "product over sum". For n equal resistors in parallel, R = value ÷ n.

Parallel pairs are a common way to reach values that are not in the E12 or E24 range.

• All in series: add them up.
• All in parallel: add the reciprocals and take the reciprocal of the sum. For 1 kΩ, 2.2 kΩ and 4.7 kΩ: 1/R = 0.001 + 0.000455 + 0.000213, so R ≈ 600 Ω.
• Mixed networks: collapse the innermost group first. Combine parallel branches into a single value, then add it to the series parts, and repeat until one resistor is left.
• Two in series feeding a load: that arrangement is a voltage divider, which the voltage divider calculator handles directly.

In series, the larger resistor dissipates more power because the same current flows through both (P = I²R). In parallel, the smaller resistor dissipates more because the same voltage sits across both (P = V²/R). Check each part's power separately rather than assuming the load splits evenly.

Tolerances do not cancel out. Two 5% resistors in series still give a combined value within 5%, although random errors tend to partly offset. When a combination sets an LED current, check the result with the LED resistor calculator.