Parallel Resistance Calculator
Enter resistor values to find the equivalent resistance of a parallel circuit. You can include any number of resistors.
What is parallel resistance?
In a parallel circuit, each resistor is connected across the same two nodes. That means every branch sees the same voltage, while current splits across the branches based on resistance. Lower resistance branches carry more current; higher resistance branches carry less.
The equivalent resistance of a parallel network is always lower than the smallest resistor in the group. This surprises many beginners, but it makes sense: every added branch creates another path for current, reducing total opposition to current flow.
Parallel resistance formula
The general formula is:
1 / Req = 1 / R1 + 1 / R2 + 1 / R3 + ... + 1 / Rn
Then invert the sum to get the equivalent resistance:
Req = 1 / (sum of conductances)
Conductance is just the reciprocal of resistance, measured in siemens (S). Many engineers find parallel calculations easier by summing conductance first.
Special case: two resistors in parallel
For two resistors, a shortcut is:
Req = (R1 × R2) / (R1 + R2)
This is handy for quick checks and mental math.
How to use this calculator
- Enter all resistor values in one field, separated by commas, spaces, or line breaks.
- Select your input unit (Ω, kΩ, or MΩ).
- Optionally enter source voltage to compute total current and branch currents.
- Press Calculate to view equivalent resistance and details.
Worked examples
Example 1: Three resistors
Suppose your resistors are 100 Ω, 220 Ω, and 330 Ω.
Compute conductance:
- 1/100 = 0.01 S
- 1/220 ≈ 0.004545 S
- 1/330 ≈ 0.003030 S
Total conductance ≈ 0.017576 S
Equivalent resistance: Req = 1 / 0.017576 ≈ 56.9 Ω
Example 2: Adding another branch
If you add a fourth 470 Ω branch in parallel, the total conductance increases again, so equivalent resistance drops further. This is why adding parallel loads can dramatically increase total current draw from a power supply.
Why this matters in real projects
Parallel resistor calculations show up everywhere in electronics:
- Designing LED arrays and current-sharing branches
- Finding sensor network loading effects
- Calculating load resistance seen by amplifiers
- Building resistor combinations when exact values are unavailable
- Estimating power supply current requirements
Common mistakes to avoid
- Mixing units: Don’t combine Ω and kΩ without conversion.
- Using series formula by accident: In parallel, resistances do not add directly.
- Forgetting positivity: Physical resistor values should be positive numbers.
- Ignoring tolerance: Real resistors have tolerance (1%, 5%, etc.), so real results vary.
Quick reference tips
- Equivalent resistance in parallel is always less than the smallest branch resistance.
- If all resistors are equal: Req = R / n.
- Doubling parallel branches roughly doubles total current at fixed voltage.
- Use Ohm’s law after finding Req: Itotal = V / Req.
Final thoughts
A good parallel resistance calculator removes arithmetic friction and lets you focus on circuit behavior. Use it as a design tool, but also use the formulas regularly so your intuition grows. Over time, you’ll estimate equivalent resistance and current distribution almost instantly.