For study estimates only, not engineering design
This series & parallel resistor calculator adds up any list of resistors in series (Rtotal = R1 + R2 + …) or combines them in parallel (1/Rtotal = 1/R1 + 1/R2 + …). Enter two or more resistor values and get the total instantly.
Want the underlying concepts first? Read our full Series vs Parallel Circuits guide – this tool is the fast, compute-it-now companion, not a replacement for that explanation.
Series & Parallel Resistor Calculator
Choose series or parallel, list your resistor values, and press Calculate.
Result
- Total resistance:
- Number of resistors:
On the Texas Journeyman exam
Where this calculation shows up
On the PSI Texas Journeyman exam this is part of the Calculations part: Electrical Theory. That part has 26 questions in 110 minutes, and you need 70% on it on its own.
Try an exam-style calculation (our own numbers): Two 10-ohm resistors in parallel are wired in series with a 5-ohm resistor, across 120 V. What is the total current?
- A. 4.8 A
- B. 8 A
- C. 12 A
- D. 24 A
Show the answer
C. 12 A. Two equal 10-ohm resistors in parallel make 5 ohms. Add the 5-ohm series resistor: 10 ohms total. I = 120 / 10 = 12.0 A.
Try 70 free Journeyman questions How the exam tests calculations
Exam format from the PSI candidate bulletin, read October 3, 2026.
How the formulas work
- Series: Rtotal = R1 + R2 + R3 + … – resistances simply add up. The total is always at least as large as the biggest single resistor.
- Parallel: 1 ÷ Rtotal = 1÷R1 + 1÷R2 + 1÷R3 + … – the total is always smaller than the smallest single resistor.
For the concepts behind why current/voltage behave this way, see our full Series vs Parallel Circuits guide.
Worked examples
- Series, three resistors: 10Ω + 20Ω + 30Ω in series → enter Mode=Series, values 10, 20, 30 → Total = 60Ω.
- Parallel, two equal resistors: two 100Ω resistors in parallel → enter Mode=Parallel, values 100, 100 → Total = 50Ω (always half of one, for two equal resistors).
- Parallel, three different resistors: 10Ω, 20Ω, 30Ω in parallel → Total ≈ 5.45Ω – notice it’s smaller than the smallest single resistor (10Ω), which is a good sanity check on any parallel answer.
What this tool does not do
This is pure resistor-combination math and carries no jurisdiction dependency. It does not model reactive components (inductors/capacitors – see our Impedance guide for that), and it does not make any wire-size or ampacity claim.
Next step
If circuit fundamentals like this are part of your own study plan toward an electrician career, a judged, personal read on your specific background can save months of guessing.
FAQ
Q1. What if I have more than a handful of resistors?
A. Use “+ Add resistor” as many times as you need – the formula works identically for any number of resistors in one mode (all series, or all parallel).
Q2. Can I mix series and parallel in one calculation?
A. Not in a single pass here – this tool solves one mode (all-series or all-parallel) at a time. For a mixed network, group your resistors into series or parallel sub-groups first, calculate each sub-group’s total here, then combine those sub-totals in a second pass.
Q3. Why is my parallel total smaller than every individual resistor?
A. That’s expected and correct – adding a parallel path always gives current more ways to flow, so the combined resistance always drops below the smallest individual branch.
Further reading
Back to the Electrical Calculator Toolbox
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