Engineering
Parallel Resistor Calculator
Calculate the equivalent resistance of two parallel branches, total source current, total circuit power, and current through the first resistor. The page explains the reciprocal rule, Kirchhoff current check, limiting behavior, resistor loading, and the difference between an ideal calculation and a buildable circuit.
Equivalent resistance-
Total circuit current-
Total power-
Current through R1-
Decision view
Parallel branch circuit and current split
Parallel branch circuit and current splitBoth resistors share the entered voltage; branch currents recombine into the calculated total current and equivalent resistance.
Exact scenario comparisonResistance R2 (ohms) changes while all other entered assumptions remain constant.
| Resistance R2 (ohms) | Equivalent resistance | Total circuit current | Total power | Current through R1 |
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How to use Parallel Resistor Calculator
- Enter positive resistance values for both branches and the voltage that appears across their common nodes.
- Confirm that the calculated equivalent resistance is lower than the smaller individual resistance.
- Compare branch currents and total power with component current, power, tolerance, and temperature ratings.
Calculator guide
Understanding Parallel Resistor Calculator
Two resistors connected across the same pair of nodes share voltage while dividing current. This calculator keeps the branch behavior visible instead of presenting equivalent resistance as an isolated formula.
Calculation method
How the calculation works
For two parallel resistors, divide their product by their sum, then apply Ohm's law to the equivalent and each branch. For two branches, Req = R1R2/(R1+R2). Apply the entered voltage to each branch, calculate I1 = V/R1 and I2 = V/R2, then verify Itotal = I1 + I2 and Ptotal = VItotal.
Circuit audit
Three checks that catch most parallel-network mistakes
The reciprocal calculation can be verified without repeating the same algebra.
If any check fails, recheck topology, units, and whether the entered voltage is actually across both branches.
Worked situations
Practical examples
- With 100 ohms and 220 ohms in parallel, equivalent resistance is 68.75 ohms.
- At 12 V, the 100-ohm branch carries 0.12 A and the 220-ohm branch carries about 0.0545 A.
- The two branch currents sum to about 0.1745 A, matching 12 V divided by 68.75 ohms.
Better inputs
Useful tips
- Use the voltage across the branches, not necessarily the source nameplate voltage after wiring losses.
- Calculate each resistor's dissipation with V squared divided by R and allow practical thermal margin.
- For more than two branches, add conductances 1/R rather than repeatedly averaging resistance values.
Before relying on the result
Limitations and common mistakes
- The model assumes ideal, constant, purely resistive components and zero wiring or source impedance.
- Tolerance, temperature coefficient, transient energy, parasitic inductance and capacitance, and failure modes are excluded.
- A short circuit or resistance approaching zero requires current limiting and cannot be treated as an ordinary resistor branch.
Reference
Key terms
- Parallel branch
- A component path connected across the same two electrical nodes as another path.
- Equivalent resistance
- Single resistance drawing the same total current at the same applied voltage.
- Conductance
- Reciprocal of resistance; parallel conductances add directly.
- Branch current
- Current flowing through one individual parallel path.
Important note
Calculated from the entered values using the displayed engineering relationship. Confirm design values, load cases, safety factors, standards, and field conditions with a qualified professional.
Frequently asked questions
Why is equivalent resistance smaller than both resistors?
Adding a parallel path increases total conductance, so the network draws more current at the same voltage.
Do parallel resistors split voltage?
No. Ideal parallel branches share the same voltage; current divides according to resistance.
Which branch dissipates more power?
At the same voltage, the lower-resistance branch dissipates more power because P = V squared/R.
Can I use this for AC impedance?
Only for purely resistive branches. Complex impedance requires magnitude and phase calculations.