This resistor calculator finds the equivalent resistance of two resistors. Choose series or parallel, enter R1 and R2 in ohms, and read the combined value. Series adds the resistances. Parallel uses 1 ÷ (1÷R1 + 1÷R2).
Hobbyists and students use it while building voltage dividers and load networks. After you have equivalent R, solve current or voltage with the Ohms law calculator.
Series versus parallel
Series parts share the same current and stack resistance. Parallel parts share the same voltage and lower the equivalent resistance below either branch.
Worked example
R1 = 1000 Ω, R2 = 1000 Ω. Series equivalent = 2000 Ω. Parallel equivalent = 500 Ω.
| Mode | R1 | R2 | Equivalent |
|---|---|---|---|
| Series | 1000 Ω | 1000 Ω | 2000 Ω |
| Parallel | 1000 Ω | 1000 Ω | 500 Ω |
How to use the fields
- R1 and R2 must be positive ohm values on the same scale.
- Configuration selects series addition or parallel combination.
Power and ratings
Equivalent resistance does not tell you whether each part stays within its watt rating. Check branch currents and voltage drops before you trust a parallel pair on a hot load.
Building larger networks
For three or more resistors, reduce one pair at a time. Keep a sketch so you do not mix a series step with a parallel step out of order.
Common mistakes
- Adding resistors that are actually in parallel on the board
- Mixing kilohm and ohm entries without converting
- Ignoring that equal parallel resistors halve the value
- Forgetting contact resistance in low ohm shunts
Design tips
- Prefer E series values you can buy, then verify equivalent R.
- In dividers, confirm unloaded ratio before adding a load.
- Derate power so heat stays manageable.
If you need a nonstandard value, a series pair often gets closer than a single catalog part. Parallel pairs help when you need a lower ohm value and more power handling across two bodies.
Series strings for custom values
When a single preferred value is not in stock, two series resistors can approximate the target. Compute the equivalent, then check the power share. In series, the same current flows through both, so power splits with resistance.
Measure the finished string with a meter when tolerance is tight. Lead resistance and solder joints matter more at very low ohm targets.
Parallel pairs for current sharing
Parallel equals often share current well when the parts match. If values differ, the lower resistor carries more current. Confirm each branch stays inside its watt rating at the applied voltage.
Divider design reminders
A series pair across a supply creates a tap voltage based on the ratio. Load current at the tap changes the ratio. After you pick R1 and R2, estimate load effect or buffer the tap when accuracy matters.
Use the Ohms law calculator to translate equivalent R into current at your supply voltage before you buy parts.
Safety and ratings
Voltage rating and pulse withstand are separate from DC resistance. High voltage or surge environments need parts chosen for those stresses, not only for the ohm number this tool returns.
Tolerance stack
Two 5 percent resistors in series do not guarantee a 5 percent equivalent in the worst case. For precision networks, pick tighter tolerance parts or measure and select. This calculator assumes exact entered values.
Temperature coefficients also matter in precision circuits. If the board runs hot, expect ohm drift beyond what the series or parallel formula shows at room temperature.
Breadboard versus soldered
Breadboard contact resistance can swamp small ohm targets. Soldered joints are more stable for low value parallel shunts. Re measure after assembly when the design is critical.
Color code cross check
After you pick values, confirm the color bands or SMD codes match the ohms you typed. A calculator cannot catch a misread band that puts 10k where you wanted 1k.
Keep a spare of each value when prototypes iterate. Parallel and series experiments go faster when parts are on hand.
Grounded workflow
Sketch the network, mark series and parallel groups, reduce from the inside out, and only then enter the final pair into this tool. Skipping the sketch is the usual source of wrong equivalents.
Limitations
The model is DC ideal resistance only. Frequency, parasitic inductance, and temperature drift are outside the result. Measure critical circuits when tolerance stacks matter.