🔗 Series & Parallel Resistor Calculator
Add any number of resistors and get the total resistance in series (they add) or parallel (reciprocals add).
Resistances (Ω)
Total resistance (series)
650 Ω
Rtotal = R1 + R2 + …
Series resistances add; parallel combine as 1/R_total = Σ(1/Rᵢ). 🔒 Computed in your browser.
How the series & parallel resistor calculator works
Resistances in series add up: R_total = R₁ + R₂ + …. In parallel, the reciprocals add: 1/R_total = 1/R₁ + 1/R₂ + …, so the total is always less than the smallest resistor. Add a row per resistor, pick series or parallel, and the tool computes the equivalent resistance.
Add as many resistors as your circuit has. For a mixed network, compute sub-groups and combine the results step by step.
Frequently asked questions
How do I calculate resistors in series?
Add them: R_total = R₁ + R₂ + R₃ + …. Three resistors of 100, 220 and 330 Ω in series total 650 Ω.
How do I calculate resistors in parallel?
Add the reciprocals: 1/R_total = 1/R₁ + 1/R₂ + …. The total is always smaller than the smallest resistor. The tool does the arithmetic for any number.
Why is parallel resistance smaller?
Parallel paths give current more ways to flow, lowering the overall resistance, always below the smallest branch.
How do I handle a mixed series-parallel circuit?
Break it into series and parallel sub-groups, compute each with this tool, then combine the equivalent values step by step.
Do the units matter?
Use the same unit for every resistor (all ohms, or all kilohms); the total comes out in that unit.
Worked example: two resistors in parallel
For 100 Ω and 300 Ω in parallel, 1/R = 1/100 + 1/300 = 4/300, so R = 75 Ω, less than the smaller 100 Ω resistor, as parallel resistance always is.
What is the shortcut for two parallel resistors?
For exactly two resistors, R = (R₁ × R₂)/(R₁ + R₂), the product over the sum. For 100 Ω and 300 Ω that is 30,000/400 = 75 Ω. For three or more, add the reciprocals.
How do current and voltage split in each arrangement?
In series the same current flows through every resistor and the voltages add up; in parallel every resistor sees the same voltage and the currents add. This follows directly from the series and parallel resistance rules.
What units does resistance use?
Ohms (Ω). Use the same unit for every resistor and the total comes out in that unit; kilohms (kΩ) and megohms (MΩ) are common shorthand for larger values.