Series and Parallel Resistor Calculator finds equivalent resistance, circuit current, total power and individual resistor behavior for practical resistor networks. Use series, parallel or voltage-divider mode without repeating manual calculations.
Calculate a resistor network
Enter resistor values separated by commas. Choose Ω, kΩ or MΩ once for the complete list.
| Resistor | Voltage drop | Power |
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Series and Parallel Resistor Calculator formulas
In series, the same current passes through every component and resistances add directly. In parallel, each branch has the same voltage and conductances add.
Parallel: 1 / Req = 1/R₁ + 1/R₂ + … + 1/Rₙ
Current: I = V / Req
Power: P = V × I = I²R
A voltage divider is a series network where the output after a selected resistor equals the source voltage multiplied by that resistor’s share of total resistance. This simple rule assumes the output is not significantly loaded.
Series and Parallel Resistor Calculator examples
Series example
Resistors of 100 Ω, 220 Ω and 330 Ω in series equal 650 Ω. Across 12 V, current is approximately 18.46 mA and total power is about 0.222 W.
Parallel example
The same three resistors in parallel equal approximately 56.90 Ω. Across 12 V, total current is approximately 210.90 mA. Branch current is highest through the smallest resistance.
For a quick accuracy check, remember two limits: a series result must be larger than the largest entered resistor, while a parallel result must be smaller than the smallest entered resistor. If either condition fails, review the values and selected unit. The calculator keeps full precision internally and rounds only the displayed result.
Resistor tolerance also matters. A nominal 1 kΩ component with ±5% tolerance may measure from 950 Ω to 1,050 Ω. In networks, individual tolerances can combine and shift current, voltage division and power. For precision applications, evaluate worst-case limits and use measured component values when available.
Selecting resistor values and power ratings
Equivalent resistance is only one part of circuit design. Check the voltage across and power dissipated by each resistor. Choose a standard power rating above the calculated value rather than operating continuously at the exact limit. Temperature, tolerance, pulse loads, ventilation and enclosure conditions can reduce practical capacity.
Use the Series and Parallel Resistor Calculator for ideal DC planning. Reactive components, semiconductor behavior, source resistance and temperature coefficients require a more complete circuit model. Consult component datasheets and qualified engineering guidance for safety-critical designs. For measurement terminology, see the NIST guide to electric current and SI units.
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Frequently asked questions
Does order matter for series resistors?
No. Changing the physical order does not change the ideal equivalent resistance.
Can parallel resistance exceed the smallest resistor?
No. Ideal parallel equivalent resistance is always lower than the smallest branch resistance.
How many resistor values can I enter?
This calculator accepts between two and twenty positive resistor values.
What happens if a parallel branch is open?
An open branch has effectively infinite resistance and contributes no branch current.
Why can a real voltage divider output differ?
A connected load appears in parallel with part of the divider and changes the effective resistance and output voltage.
Series and parallel resistor example
Three resistors of 100 Ω, 220 Ω, and 330 Ω produce 650 Ω in series and approximately 56.90 Ω in parallel. With a 12 V source, the series current is approximately 18.46 mA. Always compare calculated resistor power with the component rating and include a suitable safety margin.