Add resistors in series or parallel, and reverse-solve the second resistor for a target combined value.
Reverse solve — find the 2nd resistor
About this tool
Series & Parallel Resistor Calculator is a free, in-browser tool that combines any number of resistors into a single equivalent value, and can also work backwards to find the resistor you need to hit a target. All math runs locally in your browser with nothing uploaded.
List your resistances separated by commas (a k or M suffix is accepted, or pick a base unit). In series the total is the simple sum, Rt = R1 + R2 + … . In parallel the reciprocals add, so Rt = 1 / (1/R1 + 1/R2 + …). The reverse solver takes a target total plus one known resistor and returns the second: in series R2 = Rt − R1 (so R1 must be smaller than the target), and in parallel R2 = 1 / (1/Rt − 1/R1), which only has a positive solution when R1 is larger than the target.
Use it to check a resistor network on a schematic, or to find a practical value to place alongside one you already have. Results are auto-scaled to Ω, kΩ or MΩ for readability and update live as you type.
Frequently asked questions
How are series and parallel totals calculated?
Series resistances add directly: Rt = R1 + R2 + … . Parallel resistances add as reciprocals: 1/Rt = 1/R1 + 1/R2 + …, so the equivalent is always smaller than the smallest resistor in the group.
How does the two-resistor reverse solver work?
Given a target total and one resistor, series returns R2 = Rt − R1 and parallel returns R2 = 1/(1/Rt − 1/R1). Parallel needs the known resistor to be larger than the target, and series needs it to be smaller, otherwise no positive resistor exists and the tool says so.
Can I mix units in the list?
Yes. Each entry may carry a k (×1000) or M (×1,000,000) suffix, and anything without a suffix uses the base unit you select. So 220, 1k, 2.2M are all valid in one list.
Is anything uploaded?
No. The resistor list and target are processed entirely in your browser; the tool computes offline and sends nothing to a server.