Parallel Resistors Calculator
Calculate the equivalent resistance of up to 4 resistors connected in parallel.
Also calculates current through each resistor if voltage is known.
How Parallel Resistance Is Calculated
When resistors are connected in parallel, they share the same voltage across their terminals but split the current between them. The total resistance is always less than the smallest individual resistor.
Parallel Resistance Formula:
1/R_total = 1/R1 + 1/R2 + 1/R3 + ...
Or equivalently: R_total = 1 ÷ (1/R1 + 1/R2 + ...)
Special Case — Two Resistors:
R_total = (R1 × R2) / (R1 + R2)
This “product over sum” shortcut only works for exactly two resistors.
Worked Example: Three resistors in parallel: 100Ω, 220Ω, and 470Ω.
- 1/R = 1/100 + 1/220 + 1/470
- 1/R = 0.01000 + 0.00455 + 0.00213 = 0.01668
- R_total = 1 ÷ 0.01668 = 59.95Ω
Notice the result (≈60Ω) is less than the smallest resistor (100Ω).
Practical Reference Values:
- Two equal resistors in parallel = half the value of one (e.g., two 100Ω = 50Ω)
- Household wiring uses parallel circuits so each outlet gets full voltage
- Car headlights wired in parallel: one bulb dying doesn’t kill the other
- Standard resistor tolerance is ±5% (brown-black-red-gold) or ±1% (precision)
Current Distribution: Higher resistance = less current through that branch. A 100Ω and 1000Ω in parallel: the 100Ω branch carries 10× more current than the 1000Ω branch.