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 = 0.010000 + 0.004545 + 0.002128 = 0.016673
- R_total = 1 ÷ 0.016673 = 59.98Ω
Notice the result (≈60Ω) is less than the smallest resistor (100Ω). Round those three conductances to three decimals before adding and you land on 59.95 instead, which is close enough for a breadboard and wrong enough to matter in a divider. The calculator above keeps full precision until the final division.
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.
Parallel Resistors for Power Handling
Splitting a load across parallel resistors is a common way to get more power handling than any single part offers. Two 200Ω half-watt resistors in parallel give you 100Ω rated at a full watt, because each one carries only half the current. The catch is that real resistors are never exactly matched. At ±5% tolerance one of a pair can sit at 190Ω against the other’s 210Ω, so it carries about 10% more current and runs measurably hotter. Reach for ±1% parts when the split actually matters, and derate anyway.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.