The equation
Formula and assumptions
Series: Rₜ = R₁ + R₂ + … · Parallel: 1/Rₜ = 1/R₁ + 1/R₂ + …
All entered resistors must use the same unit. The result is returned in that unit.
Worked examples
See the method in practice
01
Series example
A 100 Ω and 220 Ω resistor in series give 320 Ω.
02
Parallel example
Two 100 Ω resistors in parallel give 50 Ω.
Questions
Frequently asked
Why is parallel resistance smaller?
Adding branches creates more paths for current, so total opposition falls below the smallest branch resistance.
Can I mix Ω and kΩ?
Convert every resistor to one common unit before calculating.
Sources & review
Equations and examples are checked against the references below. Results are educational and should be independently verified for safety-critical work.
OpenStax University Physics, DC CircuitsWritten by the STEM Hub editorial team · Reviewed August 11, 2026 · Review methodology