Series & Parallel Resistor Calculator
Build a network of up to 8 resistors in series or parallel and find the equivalent resistance.
R1Ω
R2Ω
R3Ω
Equivalent resistance
7.9 kΩ
7900.00 Ω exact
Formula
Series: Req = R1 + R2 + … Parallel: 1/Req = 1/R1 + 1/R2 + …
Series adds; parallel always gives less than the smallest resistor.
Combining resistors
When you don't have the exact value you need, combine the ones you do. In series, resistances simply add. In parallel, the reciprocals add, so the total is always smaller than the smallest branch — the diagram above redraws as you switch modes.
Handy shortcuts
- Two equal resistors in parallel = half the value; N equal resistors = value ÷ N.
- Two in parallel: Req = (R1 × R2) / (R1 + R2) — the 'product over sum' rule.
- Series increases resistance and spreads power across more parts (useful for heat).
- Parallel lowers resistance and increases current-handling.
Where it's used
Hitting an odd resistor value from a standard kit, building precise dividers, sharing current across several resistors, and setting gain in op-amp circuits.
Frequently Asked Questions
How is series resistance calculated?+
Rtotal = R1 + R2 + R3 + … Total resistance is simply the sum of all resistors.
How is parallel resistance calculated?+
1/Rtotal = 1/R1 + 1/R2 + 1/R3 + … The equivalent resistance is always less than the smallest resistor in the group.