Calculating your combined resistance is straightforward once you know the Parallel Resistor formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Parallel Resistor Calculator.
What is Parallel Resistor?
The Parallel Resistor calculation tells you your combined resistance from a few simple inputs. The figure you are solving for here is the combined resistance.
The Parallel Resistor formula
The core formula is:
Combined resistance = Resistor R1 × Resistor R2 ÷ (Resistor R1 + Resistor R2)Here is what each input means:
- Resistor R1 — a value measured in Ω. Example: 100 Ω.
- Resistor R2 — a value measured in Ω. Example: 200 Ω.
How to calculate it step by step
- Write down the resistor r1 (for example, 100 Ω).
- Write down the resistor r2 (for example, 200 Ω).
- Apply the formula above to get your combined resistance.
- Double-check the result with the Parallel Resistor Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Resistor R1 | 100 Ω |
| Resistor R2 | 200 Ω |
| Combined resistance | 66.667 |
With resistor r1 of 100 Ω and resistor r2 of 200 Ω, the combined resistance works out to 66.667.
Example 2
With resistor r1 of 200 Ω and resistor r2 of 200 Ω, the combined resistance works out to 100.000.
| Result | Value |
|---|---|
| Combined resistance | 100.000 |
Example 3
With resistor r1 of 50 Ω and resistor r2 of 200 Ω, the combined resistance works out to 40.000.
| Result | Value |
|---|---|
| Combined resistance | 40.000 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Parallel Resistor Calculator does it instantly, for free, with the formula and a worked example built in.
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