Parallel Resistance Calculator

A parallel resistance calculator returns the combined resistance of up to four resistors wired side by side. It adds the reciprocals of each resistor and takes the reciprocal of that sum. Two resistors, 100 Ω and 200 Ω, give a total of 66.67 Ω, which is lower than either one.

Last reviewed September 2026Method & sources Report an error

Total Resistance

66.667 Ω

Breakdown
Total Resistance	66.667 Ω
Resistors Used	2

How to Calculate Parallel Resistance

Resistors in parallel share the same voltage across each branch, but current splits between them. The more paths current has, the lower the combined resistance. The reciprocal of total resistance equals the sum of the reciprocals of each resistor: `1 ÷ Rtotal = 1/R1 + 1/R2 + 1/R3 + ...`. Any resistor left at 0 is treated as unused rather than a short circuit, so this works for two, three, or four resistors in parallel.

Example: Two resistors, 100 Ω and 200 Ω, wired in parallel

Two resistors, 100 Ω and 200 Ω, wired in parallel: 1. Reciprocal sum: 1/100 + 1/200 = 0.01 + 0.005 = 0.015 2. Total resistance: 1 ÷ 0.015 = 66.67 Ω The combined resistance (66.67 Ω) is lower than either individual resistor. This is true of any parallel combination.

What Is Parallel Resistance?

A parallel resistance calculator finds the combined resistance of two or more resistors wired side-by-side (in parallel) rather than end-to-end (in series). It is useful for basic circuit design, checking a wiring plan, or understanding why adding more parallel paths (like additional light fixtures on the same branch) lowers a circuit's overall resistance.

Parallel Resistance: Pros and Cons

Pros

Uses the exact reciprocal-sum formula, not an approximation
Handles two, three, or four resistors at once by leaving unused slots at 0
Total resistance always comes out lower than the smallest individual resistor, a useful sanity check on the result

Cons

Assumes ideal, purely resistive components. Real-world resistor tolerances (often ±5% or ±1%) mean an actual measured value will vary slightly from this calculation
For resistors wired end-to-end (in series) instead, the total is simply the sum of the resistances (R1 + R2 + ...), not this reciprocal formula
Handles up to four resistors. For more, the same reciprocal-sum pattern extends to any number of parallel branches

Parallel Resistance: What a Pro Checks

Expect the actual measured combined resistance to differ slightly from this calculated value. Real resistor tolerances, often marked as ±5% or ±1% on the part, mean true resistance varies from its nominal number.
Wiring resistors end-to-end instead of side-by-side? Use the simple sum (R1 + R2 + ...) instead of this reciprocal formula, since series and parallel resistance combine differently.
Need more than four resistors in parallel? Add every additional resistor's reciprocal into the sum before taking the final reciprocal. The same formula extends to any number of branches.

Frequently Asked Questions

How do you calculate parallel resistance?

Take the reciprocal of the sum of each resistor's reciprocal: 1 ÷ Rtotal = 1/R1 + 1/R2 + ... For two 100 Ω and 200 Ω resistors, that's 1 ÷ (0.01 + 0.005) = 66.67 Ω.

Why is total resistance lower than any individual resistor in parallel?

Because parallel resistors give current more than one path to flow through. More paths means less overall opposition to current, so the combined resistance is always lower than the smallest single resistor in the group.

What is the difference between series and parallel resistance?

In series (end-to-end), resistances simply add: Rtotal = R1 + R2 + .... In parallel (side-by-side), resistances combine by the reciprocal-sum formula, which always produces a total lower than the smallest resistor.

What happens with two equal resistors in parallel?

The total is exactly half of one resistor's value. Two 100 Ω resistors in parallel give 50 Ω. More generally, N identical resistors of value R in parallel give R ÷ N.

Can I calculate more than four resistors in parallel?

This calculator handles up to four at once. The same formula (1 ÷ Rtotal = sum of 1/R for every resistor) extends to any number of parallel branches. Add each additional resistor's reciprocal to the sum before taking the final reciprocal.

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