Resistor Calculator
Combining Resistors Changes the Math, Not Just the Number
Wiring resistors in series adds their values directly, but wiring the same resistors in parallel gives a total that's always smaller than the smallest individual value — a detail that trips up anyone doing it by hand for the first time. This calculator handles both arrangements for any number of resistors you enter.
The Formulas
Parallel: 1/Rtotal = 1/R1 + 1/R2 + ... + 1/Rn
In series, the same current flows through every resistor, so their opposition to that current simply stacks. In parallel, each resistor offers the current an independent path, so adding more paths always makes it easier for current to flow overall — hence the lower combined resistance.
A Worked Example
Three resistors — 100 Ω, 220 Ω, and 330 Ω — combined two ways:
| Configuration | Total Resistance |
|---|---|
| Series (100 + 220 + 330) | 650 Ω |
| Parallel | 56.90 Ω |
The parallel total is lower than even the smallest resistor (100 Ω) in the group.
Where This Calculation Matters
- Prototyping and repair — combining resistors you already have on hand to reach a value that isn't sold as a single standard part.
- LED and sensor circuits — fine-tuning current-limiting resistance by adding a second resistor in series or parallel with the first.
- Load banks and test equipment — building a specific resistive load from standard-value components.
- Circuit analysis — simplifying a network of resistors down to a single equivalent value before analyzing the rest of the circuit.
How to Use This Calculator
- Select Series Resistance or Parallel Resistance.
- Enter the resistor values as a comma-separated list in ohms, for example
100, 220, 330. - Select Calculate to see the combined resistance along with the full substitution.
Related Calculations
Once you have a combined resistance, the Ohm's Law Calculator finds the resulting current or voltage, and the Wattage Calculator finds the power that resistance will dissipate.