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Parallel resistor calculator icon showing two parallel resistors

Parallel Resistor Calculator

Parallel Resistor Calculator

Combine any number of resistors in parallel — or work backwards to find the resistor you're missing.

Try an example

Enter at least two resistor values. Add a supply voltage to see how current and power split between branches.

History

#ModeResistorsResultVoltageTotal current

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How parallel resistors work

The rule in one line

1/Req = 1/R₁ + 1/R₂ + … + 1/Rₙ

Resistors in parallel all share the same voltage across them, but the current splits between the branches. Each branch is an extra path for current, so adding a resistor in parallel always makes it easier for current to flow — which is why the combined resistance is always smaller than the smallest resistor in the group.

Water analogy: one drain empties a sink at some rate; opening a second drain (even a small one) can only speed things up. It can never slow the sink down.

Why reciprocals? Meet conductance

The reciprocal of resistance is conductance (G = 1/R, measured in siemens, S) — "how easily current flows" instead of "how hard it is". Parallel paths simply add their conductances:

Gtotal = G₁ + G₂ + …

That's the whole trick: convert every resistor to conductance, add them up, then flip back. Req = 1 / Gtotal. This calculator shows each conversion in its worked steps.

Two handy shortcuts
  • Exactly two resistors — "product over sum": Req = (R₁ × R₂) / (R₁ + R₂). Example: 1 kΩ ∥ 2.2 kΩ = 2200000 / 3200 ≈ 687.5 Ω.
  • n equal resistors — just divide: Req = R / n. Four 100 Ω resistors in parallel make 25 Ω. This is also how you build a higher-wattage resistor from small ones, since each carries only 1/n of the power.
How current and power split

With a supply voltage V across the network, every branch sees the full V, so by Ohm's Law each branch carries Iᵢ = V / Rᵢ — the smallest resistor carries the most current. Branch power is Pᵢ = V² / Rᵢ.

The share of total current a branch takes equals its share of total conductance: a 100 Ω resistor in parallel with a 1 kΩ carries about 91% of the current. This calculator's donut chart shows exactly that split, and the branch table lists every current and power — useful for checking none of your real resistors exceeds its wattage rating.

A worked example

Combine 470 Ω, 1 kΩ, and 2.2 kΩ in parallel:

  1. Conductances: 1/470 = 2.128 mS,  1/1000 = 1 mS,  1/2200 = 0.455 mS.
  2. Add them: G = 2.128 + 1 + 0.455 = 3.582 mS.
  3. Flip back: Req = 1 / 0.003582 ≈ 279.2 Ω.
  4. Sanity check: 279.2 Ω is smaller than 470 Ω, the smallest branch ✓

Practical notes: real resistors have tolerance (±1% or ±5%), so your measured value will differ slightly; and if you need a specific equivalent, the "Find missing resistor" mode plus the E24 standard-value suggestion will get you to a part you can actually buy.

Combining resistors in parallel doesn’t average their values, it always produces something lower than the smallest individual resistor, this calculator finds the equivalent resistance for any number of resistors, or works backward to find a missing one.

How to use this calculator

  1. Choose your tab: Find equivalent R, or Find missing resistor.
  2. Enter your resistor values (add as many as needed).
  3. For the missing-resistor mode, enter your Target equivalent resistance.
  4. Optionally add a Supply voltage to see current and power split between branches.
  5. Read the equivalent resistance, plus per-branch current, power, and share.

What this calculator does

For resistors in parallel, the reciprocal of the equivalent resistance equals the sum of the reciprocals of each individual resistor. This calculator handles any number of parallel resistors, and can also work backward to find what value a missing resistor needs to be to hit a specific target equivalent resistance.

1/R_eq = 1/R1 + 1/R2 + 1/R3 + …

Why parallel resistance is always less than the smallest resistor

Adding more parallel paths gives current more routes to flow through, which always decreases the total resistance to current flow, no matter how large the additional resistor is, the equivalent resistance of a parallel combination is always less than the value of the smallest individual resistor in that combination.

Why the current and power split isn’t equal across branches

With a shared voltage across all parallel branches, a branch with lower resistance carries proportionally more current (and dissipates more power) than a branch with higher resistance, following Ohm’s Law within each branch individually, this calculator’s per-branch breakdown shows exactly how the total current and power distribute unevenly based on each branch’s resistance.

Frequently asked questions

Why is the equivalent resistance of parallel resistors always less than the smallest one?

Adding more parallel paths gives current more routes to flow, which always reduces total resistance to current flow, regardless of how large the added resistor’s value is, this is a fundamental property of parallel circuits.

Why does more current flow through the lower-resistance branch?

Since all parallel branches share the same voltage, Ohm’s Law (I = V/R) means a lower-resistance branch allows proportionally more current to flow through it than a higher-resistance branch under that same shared voltage.