How this calculation works
Parallel branches share voltage. Their conductances add, so the equivalent resistance is less than either positive branch resistance. A zero-ohm branch gives an ideal short circuit.
Equivalent resistance = R₁ × R₂ ÷ (R₁ + R₂)A worked example
Two 100 Ω resistors in parallel have an equivalent resistance of 50 Ω.
Good to know
Assumes the idealized model described above. Use consistent units and independently verify figures before a consequential decision.
Two equal branches halve the resistance
Two 100 Ω resistors in parallel produce 50 Ω, not 200 Ω. Both branches see the same voltage, so each provides a path for current. Their conductances add, reducing the resistance seen by the source.
For two positive resistances, the equivalent is below the smaller branch resistance. That gives a quick error check: an answer greater than both inputs would not fit a parallel arrangement.
Parallel is a connection pattern, not a visual arrangement
Resistors are in parallel when both ends connect to the same two circuit nodes. Two components drawn side by side are not necessarily connected that way. Check the electrical nodes rather than the drawing’s layout.
A zero-ohm ideal branch shorts the pair, so the model returns zero. Real leads, contacts and components have tolerances and nonideal behavior; the tool does not evaluate current or power limits.
A few questions, answered
What if both resistors are equal?
The equivalent resistance is half either resistor’s value.
Does it calculate series resistance?
No. Two resistors in series would add directly rather than use this parallel formula.