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Series and parallel resistor calculator

Calculate equivalent resistance for 2 to 10 resistors connected in series or parallel. Includes a live circuit diagram, mixed units, tolerance limits and DC voltage, current and power for each resistor.

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Add your resistors, choose a connection and watch the circuit update. Both equivalent resistances are compared live; supply voltage is optional.

Your resistors

R1
R2
R3
Try an example

Try an example

Leave blank for resistance only. DC voltage, ideal source.

Connection

One path: the current is shared and voltage divides between resistors.

Your circuit

Terminals A and B bound the network. Matching colours identify the same resistor. Scroll larger diagrams.

Equivalent resistance

—

Tolerance range: —

Extreme bounds with all resistors simultaneously at their lower or upper tolerance limit; not a statistical interval.

Total current

—

Total power

—

Same components, two connections

Series—
Parallel—

Individual resistor results

Each bar shows the share of total voltage across that resistor.

Individual resistor results
ResistorValueVoltageCurrentPower%

Understand the calculation

    Ohmic resistors under DC conditions. Voltages, currents and powers use nominal values. Wire resistance and source internal resistance are ignored. Thermal sizing and mixed networks are not modelled.

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    How does this tool work?

    The same resistors produce different results depending on how they are connected. This calculator compares an entirely series network with an entirely parallel network using your component list. Enter two to ten resistances in mΩ, Ω, kΩ or MΩ: the diagram and results follow your changes. Use it to check an exercise, plan a simple electronic circuit or compare ways to combine components you already have.

    In series, charge passes through every resistor in turn, so resistance values add up. The equivalent resistance is greater than any individual resistance. With a fixed supply voltage, every component carries the same current and the voltage drop is proportional to resistance. A resistor with twice the resistance receives twice the voltage drop.

    In parallel, all resistors connect across the same terminals A and B. Their conductances, the reciprocals of resistance, add up. The equivalent resistance is lower than the smallest individual resistance. Every branch receives the same voltage; the smaller the resistance, the greater its current. For just two resistors the formula simplifies to R1 × R2 / (R1 + R2).

    Supply voltage is optional. When entered, the table displays voltage drop, current and dissipated power for each component. The bars show the voltage split in series or the current split in parallel, rather than a power-rating assessment. Export the diagram as an SVG to preserve the values and enlarge the drawing without losing sharpness.

    How to use this tool

    1. Enter your components

      Choose each resistor’s unit and tolerance. You can mix 470 Ω and 1 kΩ without converting by hand. Remove unused rows or add up to ten components.

    2. Compare connections

      Switch between series and parallel. The circuit and table follow your selection, while both equivalent resistances remain visible for comparison.

    3. Check electrical requirements

      Enter the supply voltage if known. Compare each calculated power with the component datasheet, allowing for temperature conditions and an appropriate design margin.

    Understand the calculation

    Series

    Req = R1 + R2 + … + Rn

    One path: the current is shared and voltage divides between resistors.

    Parallel

    1/Req = 1/R1 + 1/R2 + … + 1/Rn

    Several branches: the voltage is shared and branch currents add up.

    Total current

    I = U / Req

    Total power

    P = U × I

    Ohmic resistors under DC conditions. Voltages, currents and powers use nominal values. Wire resistance and source internal resistance are ignored. Thermal sizing and mixed networks are not modelled.

    Use cases

    Combine parts already in stock

    Compare equivalent values made from available resistors before ordering an additional component.

    Understand a circuit exercise

    Connect the diagram to current and voltage laws, then check each component’s values in the results table.

    Prepare a power dissipation check

    With a known supply voltage, identify the components dissipating most power before checking their thermal limits.

    Examples

    Two identical resistors

    At 12 V, total current is 60 mA and 240 mA respectively. Wiring changes the current drawn from the source.

    Input

    100 Ω and 100 Ω

    Result

    200 Ω in series; 50 Ω in parallel

    Mixed units

    The calculation first uses 1,000 Ω and 500 Ω. Rounding affects the display, not intermediate calculations.

    Input

    1 kΩ and 500 Ω

    Result

    1.5 kΩ in series; approximately 333.333 Ω in parallel

    Tips and best practices

    • A ±5% tolerance on 100 Ω allows an actual resistance from 95 Ω to 105 Ω. The range calculation uses the individual limits and assumes no cancellation between components.
    • Zero ohms would represent an ideal wire and can make the current undefined. All resistances must be strictly positive; empty rows are treated as incomplete entries.
    • For a mixed network, reduce simple groups one at a time and enter each equivalent as a component in your next calculation. The tool does not solve bridge circuits or arbitrary circuit diagrams.

    Sources and references

    Frequently asked questions

    Why does resistance decrease in parallel?

    Each branch provides another path for current. At the same voltage the total current increases, so equivalent resistance decreases. For n identical resistors of resistance R, the result is R/n.

    Is the tolerance range a statistical confidence interval?

    No. It shows theoretical extremes with every resistor simultaneously at its lower or upper bound. No probability distribution or temperature drift is modelled.

    Can I choose a power rating from these results?

    Use the powers as an initial nominal DC check. Allowable power also depends on packaging, ambient temperature, cooling and the manufacturer’s derating requirements. The calculation does not validate those conditions.

    What does the calculation link preserve?

    It stores the connection, optional voltage, resistor values, units and tolerances in the URL fragment. Opening the link restores the calculation; fragment parameters are not sent to the server.

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