Calculators
Series & Parallel Resistor Calculator
Combine resistors in series or parallel and calculate total current and power.
Find the equivalent resistance of two or more ideal resistors in a pure series or parallel network. Add a source voltage to estimate total circuit current and power.
Equivalent resistance
320 Ω
Total current at 5 V0.015625 A
Total ideal power0.078125 W
Series: Req = R1 + R2 + …
This calculator treats the network as ideal and purely series or purely parallel. Mixed resistor networks must be reduced section by section, and real resistor tolerance and power ratings still matter.
About This Tool
Resistors are commonly combined to set current, divide voltage, create loads, and obtain resistance values that are not available as one component. This calculator combines two or more positive resistor values in a pure series or pure parallel network. Each resistor can use Ω, kΩ, or MΩ independently. Enter an optional source voltage to also calculate the ideal total current and total power for the equivalent network. Everything is calculated locally in your browser.
How To Use It
- Choose whether all resistors are connected in series or all are connected in parallel.
- Enter at least two positive resistance values and select the unit for each resistor. Add more rows when needed.
- Enter a non-negative source voltage to calculate total current and ideal total power for the equivalent resistance.
- Check individual resistor power ratings and tolerances separately before using the result in a physical circuit.
Examples
100 Ω and 220 Ω in series
Series resistance adds directly, so 100 Ω + 220 Ω = 320 Ω.
100 Ω and 200 Ω in parallel
Using the reciprocal rule gives an equivalent resistance of about 66.667 Ω, which is lower than either branch resistance.
Three equal 1 kΩ resistors
Three 1 kΩ resistors equal 3 kΩ in series or about 333.333 Ω in parallel.
Useful Notes
Series resistance formula
For resistors in series, the same current flows through each component and the equivalent resistance is Req = R1 + R2 + … + Rn. Adding another positive series resistor always increases the total resistance.
Parallel resistance formula
For resistors in parallel, each branch has the same voltage and conductances add: 1/Req = 1/R1 + 1/R2 + … + 1/Rn. For positive resistances, the equivalent is always smaller than the smallest branch resistance.
Current and total power
After the network is reduced to Req, Ohm's law gives total current I = V/Req. The ideal total power is P = VI, equivalently V²/Req. These are network totals rather than the power dissipated by each individual resistor.
Mixed resistor networks
A circuit containing both series and parallel sections cannot generally be solved by selecting one mode for the whole network. Identify a reducible series or parallel section, replace it with its equivalent resistance, and repeat until the network is simplified.
Tolerance and power ratings
Real resistor values vary by their tolerance and change somewhat with temperature. Each resistor also has a maximum power rating. Equivalent resistance alone does not confirm that every component can safely dissipate the power it receives.
FAQ
Why is parallel resistance lower than the smallest resistor?
Parallel branches provide additional paths for current, so total conductance increases. Since resistance is the reciprocal of conductance, the equivalent resistance becomes smaller than any individual positive branch resistance.
Can I mix ohms and kilo-ohms?
Yes. Each resistor has its own Ω, kΩ, or MΩ selector and is converted to ohms before calculation.
Does this solve mixed series-parallel circuits automatically?
No. The tool combines a group that is entirely series or entirely parallel. Mixed networks should be reduced one valid section at a time.
Is total power the rating needed for every resistor?
No. The displayed power is the ideal power for the equivalent network. Individual resistor dissipation depends on the circuit arrangement and values, so each component must be checked separately.
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