Parallel Resistor Calculator
Find the equivalent resistance of N resistors in parallel. Two-resistor shortcut, find the missing R, and per-branch current and power.
Parallel Resistance Examples
| Resistors | Equivalent resistance | Formula result |
|---|---|---|
| 100 Ohm + 100 Ohm | 50 Ohm | 1/R = 1/100 + 1/100 |
| 100 Ohm + 200 Ohm | 66.67 Ohm | 1/R = 1/100 + 1/200 |
| 1 kOhm + 2 kOhm + 2 kOhm | 500 Ohm | 1/R = 1/1000 + 1/2000 + 1/2000 |
| 220 Ohm + 220 Ohm | 110 Ohm | 1/R = 1/220 + 1/220 |
| 330 Ohm + 470 Ohm | 193.88 Ohm | 1/R = 1/330 + 1/470 |
Frequently Asked Questions about the Parallel Resistor Calculator
What is the formula for resistors in parallel?
Add the reciprocals: 1 / R_total = 1 / R1 + 1 / R2 + ... + 1 / R_n. Then invert to get R_total. The equivalent resistance is always smaller than the smallest resistor in the network.
Is there a shortcut for two resistors in parallel?
Yes. For exactly two resistors the product-over-sum form is faster by hand: R_total = (R1 x R2) / (R1 + R2). It gives the same answer as the reciprocal formula but skips one division.
How do I find a missing resistor when I know the target R_total?
Solve 1 / R_missing = 1 / R_total minus 1 / R_known, then invert. R_total must be smaller than R_known, otherwise no positive resistor can hit the target.
How is the current split between parallel branches?
Each branch sees the same voltage, so its current is V / R_branch. The ratio i_branch / i_total equals R_total / R_branch. Smaller resistors carry more current.
What if one resistor is much larger than the others?
A very large resistor in parallel contributes almost no conductance, so R_total stays close to the smallest resistor. For example, 1 ohm in parallel with 1 megohm is about 0.999999 ohms.
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