Frequently Asked Questions
What is the formula for resistors in parallel?
How do I calculate parallel resistance for just two resistors?
Why is the total resistance always less than the smallest resistor?
What is the difference between resistors in parallel and resistors in series?
Formula and Method for the Parallel Resistor Calculator
When resistors are connected in parallel, each one provides an independent path for current between the same pair of nodes, so every branch shares the identical voltage while sharing out the total current. Because conductance — the reciprocal of resistance, 1/R — is additive for parallel branches, the standard method is to sum each resistor's conductance and then invert that sum: 1/Req = 1/R1 + 1/R2 + ... + 1/Rn. This calculator sums the reciprocals of up to four resistor values, then reports the equivalent resistance and conductance, plus — if you supply a voltage — the total current and power the combination draws.
How the calculation works
Enter Resistor 1 and Resistor 2 (both required), and optionally Resistor 3 and Resistor 4 for larger networks, then choose the unit — ohms (Ω), kilohms (kΩ), or megohms (MΩ) — that applies to all of them. The calculator converts every value to base ohms, adds their reciprocals to get total conductance G = ΣGi, and inverts that sum to get the equivalent resistance Req = 1/G. If you also enter a supply voltage, it applies Ohm's law to find the total current drawn from the source, I = V / Req, and the total power dissipated across the network, P = V × I = V² / Req.
Common mistakes
- Adding resistors directly: R1 + R2 + R3 gives the series total, not the parallel total — parallel resistance requires summing reciprocals, then inverting.
- Forgetting Req must be smaller than every branch: if your answer is larger than the smallest resistor you entered, recheck the formula or look for a unit mismatch.
- Mixing units: keep every resistor in the same unit (or let the unit selector convert them) before combining — a 1 Ω resistor and a 1 kΩ resistor are not simply "1 and 1" in the formula.
- Misusing the two-resistor shortcut: Req = R1R2/(R1+R2) only works for exactly two resistors; use the full reciprocal-sum formula for three or more.
Real-world applications
- Combining standard resistor values to hit a non-standard target resistance that isn't sold off-the-shelf.
- Analyzing parallel branches in power distribution, LED strings, or battery pack balancing resistors.
- Determining the equivalent load resistance a power supply sees when multiple devices share the same two terminals.
- Estimating current sharing and power dissipation across parallel components before selecting wattage ratings.