Series and parallel resistor calculator

Formula

How it works

In series, the same current flows through both resistors, so their resistances simply add. In parallel, the voltage across both is the same but the current splits, which combines the resistances as a reciprocal sum — always less than the smaller resistor alone.

Worked example

The same calculation with the default values:

• Connection Series
• Resistor R1 100 Ω
• Resistor R2 220 Ω

1. R = 100 + 220 = 320

Result: Equivalent resistance: 320 Ω

Common mistakes

• Applying the series formula to resistors that are actually wired in parallel, or vice versa - the two configurations have opposite effects (series always increases total resistance, parallel always decreases it below the smallest individual resistor).
• Forgetting that for more than two parallel resistors, the reciprocal formula (1/R = 1/R₁ + 1/R₂ + ...) is needed - this calculator's two-resistor shortcut doesn't directly extend to three or more by simple pairing without care.
• Mixing resistor tolerance into the ideal calculation - the result assumes exact resistor values; real resistors (see the resistor color code calculator) carry a tolerance band around their nominal value.