Resistor Calculator
Calculate series and parallel resistance, and voltage divider outputs.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Resistor Calculator does
Resistors in series add their values because the same current passes through each. In parallel the reciprocals add, so the total is always less than the smallest resistor — adding a parallel path always gives current somewhere else to go.
Formula
Series: R = R₁ + R₂ + R₃ …Parallel: 1 ÷ R = 1 ÷ R₁ + 1 ÷ R₂ + 1 ÷ R₃ …Voltage divider: V_out = V_in × R₂ ÷ (R₁ + R₂)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Configuration | one of 3 options | Yes | — |
| Resistor 1 | Ω | Yes | Accepts more than 0. |
| Resistor 2 | Ω | Yes | Accepts more than 0. |
| Resistor 3 | Ω | Optional | Accepts 0 or more. Shown Configuration is Series, or Configuration is Parallel. |
| Resistor 4 | Ω | Optional | Accepts 0 or more. Shown Configuration is Series, or Configuration is Parallel. |
| Supply voltage | V | Optional | Accepts 0 or more. |
How to use it
- Choose Configuration.
- Enter Resistor 1 and Resistor 2.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Supply voltage.
- Select Calculate.
Worked example
A 100 Ω and 220 Ω resistor in parallel.
- Mode
- Parallel
- R1
- 100
- R2
- 220
1/R = 1/100 + 1/220 = 0.014545, so R = 68.75 Ω — lower than either resistor.
Frequently asked questions
Why is parallel resistance lower than the smallest resistor?
Each parallel path offers current another route. More routes means less total opposition, so the combined resistance always falls below the smallest branch.
Why does my voltage divider output sag under load?
The load sits in parallel with the lower resistor, reducing the effective resistance and the output. Use lower divider values or a buffer amplifier.
Method and sources
Method. Series and parallel combination — R = ΣR in series, and the reciprocal sum in parallel — with the nearest preferred values from the E-series.
Applies to. None — a circuit relationship
Assumptions
- Ideal resistors at their nominal value, ignoring tolerance and temperature coefficient.
Limitations
- Real resistors carry a tolerance, commonly 1% or 5%, so a combination lands within a band rather than on the calculated figure.
- Power dissipation is not checked here, and a resistor operated beyond its rating fails — sometimes open, sometimes not.
Sources
- Standard circuit theory and the IEC preferred-value (E-series) system — Established electrical engineering fundamentals. The combination rules and the preferred values suggested.