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Series and Parallel Resistor Calculator

Calculate equivalent resistance for series and parallel circuits. Features a dynamic component list and standard E12/E24 series matching.

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About

Designing complex circuits often requires combining multiple resistors to achieve a precise target resistance or to distribute power dissipation. This calculator computes the equivalent resistance (Req) for both series and parallel configurations simultaneously. It allows for an unlimited number of resistor inputs, making it suitable for analyzing complex banks of components.

Beyond the raw calculation, the tool compares the result against standard E12 and E24 resistor series. In practical engineering, exact calculated values are rarely available off-the-shelf; knowing the closest standard value helps in feasibility checks and component selection.

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Formulas

Series Resistance:

Req = n∑i=1 Ri

Parallel Resistance:

Req = 1n∑i=1 1Ri

Reference Data

SeriesConfigurationFormulaTypical Application
SeriesChainedRtotal = R1 + R2...Voltage Dividers, Current Limiting
ParallelSide-by-sideRtotal = 1 / (1/R1 + 1/R2...)Current Dividers, Reducing Resistance
E12Standard10% ToleranceGeneral Purpose Electronics
E24Standard5% TolerancePrecision Circuits
PowerSeriesPtotal = ∑PnShared Heat Dissipation
PowerParallelPtotal = ∑PnShared Heat Dissipation
FailureSeriesOpen CircuitWhole branch fails
FailureParallelOpen CircuitResistance increases, circuit remains active

Frequently Asked Questions

In a parallel circuit, adding a new path (resistor) always provides an additional route for current to flow. This increases the total conductance, which is the inverse of resistance. Therefore, the total equivalent resistance must decrease.
These are EIA standard logarithmic sequences for resistor values. E12 has 12 values per decade (10, 12, 15, 18...) with 10% tolerance, and E24 has 24 values (10, 11, 12, 13...) with 5% tolerance. Engineers snap calculated values to these standards.
For this calculator, ensure all values are entered in the same unit scale (e.g., all in Ohms) for accuracy. Mixing 100 (Ohms) and 10 (kOhms) without conversion will yield incorrect results.
If you use N identical resistors in parallel, the effective resistance drops by a factor of N, but the power handling capability increases by a factor of N, assuming the resistors are rated identically.