Technical utilities
Series & Parallel Resistor Calculator
Series resistances add directly. Parallel resistance is the reciprocal of the sum of each branch conductance.
Calculate equivalent resistance and basic branch values for a supported series or parallel resistor network.
Calculated result
Equivalent resistance
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
What this calculator returns
The result applies only to the selected topology, not an arbitrary mixed network.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Series: Req = ΣR. Parallel: 1/Req = Σ(1/R).
- V
- Voltage across the modeled component or network.
- I
- Current through the modeled component or network.
- R
- Resistance in ohms; subscripts identify individual resistors.
- P
- Power dissipated or delivered, in watts.
Each R is a positive resistance in ohms.
Worked example
| Input | Value |
|---|---|
| Topology | Parallel |
| Resistor values | See the example load list in the calculator |
Two 100 Ω resistors in parallel have an equivalent resistance of 50 Ω.
Reference table
| Solve for | Formula variants |
|---|---|
| Voltage | V = I × R; V = P / I; V = √(P × R) |
| Current | I = V / R; I = P / V; I = √(P / R) |
| Resistance | R = V / I; R = V² / P; R = P / I² |
| Power | P = V × I; P = I² × R; P = V² / R |
| Networks | Series: Req = ΣR; parallel: 1/Req = Σ(1/R) |
Frequently asked questions
Do parallel resistors always reduce equivalent resistance?
Yes. For positive resistor values, the parallel equivalent is lower than the smallest branch resistance.
Can I enter a mixed series-parallel circuit?
No. Each calculation treats the full list as either all series or all parallel.
Assumptions and limitations
- Ideal resistors.
- Only all-series or all-parallel lists are supported.
Method and sources
Review the electrical formulas, calculation methodology, and reference tables for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .