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
Preparing for the Texas Journeyman exam?
This calculator works through series and parallel circuit math, which is useful background for electrical calculations. If you're studying for the Texas Journeyman exam, the pack covers its broader calculation section under time limits.
The Texas Journeyman Electrician Exam Practice Pack (US$39) has 1,000 original practice questions, including 10 timed two-part mock exams. Each mock uses the published question counts and time limits of the Texas Journeyman exam: 59 questions in 130 minutes for NEC Knowledge, and 26 questions in 110 minutes for Calculations. The official exam requires 70% on each part. After you submit a mock on the site, you see your score by part and by topic, and every calculation question has a worked solution. A printable PDF with answers is included for offline study.
See the Journeyman practice pack →
Preparing for the Texas Master exam instead? See the Texas Master Electrician Exam Practice Pack (US$79), covering both NEC Knowledge and Calculations.
Original practice questions written by Tsai Electro. They are not official PSI or TDLR questions, and Tsai Electro is not affiliated with either.
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