Resistors in parallel share the same voltage. Their branch currents add, so the equivalent resistance is lower than the smallest individual resistance. A calculator can speed the arithmetic, but the designer must still check tolerance, power, voltage and what happens if one branch opens or shorts.
General parallel resistance formula
For resistors R1 through Rn connected across the same two nodes:
1 / REQ = 1 / R1 + 1 / R2 + … + 1 / Rn
After adding the reciprocals, take the reciprocal of the result. Use the same resistance unit for every term. The answer will be in that unit.
As a quick error check, the equivalent resistance must be lower than the smallest branch resistance. If it is not, recheck units and arithmetic.
Two-resistor shortcut
For two resistors:
REQ = (R1 × R2) / (R1 + R2)
Example: 1 kΩ in parallel with 2 kΩ gives:
(1 × 2) / (1 + 2) = 0.667 kΩ, or approximately 667 Ω.
This shortcut is only for two branches. For more branches, use the reciprocal formula or combine two at a time.
Equal-value resistor shortcut
When n equal resistors of value R are in parallel:
REQ = R / n
Four 1 kΩ resistors in parallel equal 250 Ω. If the total current divides equally, each resistor dissipates one quarter of the total resistor-bank power, subject to tolerance and temperature differences.
Calculate branch current and power
Each branch sees the full applied voltage. For branch i:
Ii = V / Ri
Pi = V² / Ri
Total current is the sum of branch currents. Total power is the sum of branch powers and also equals V² divided by the equivalent resistance.
Check the voltage and power rating of every resistor, not only the combined value. Apply the manufacturer's temperature derating and consider PCB heating and airflow.
Include tolerance and temperature coefficient
Nominal arithmetic does not give the worst-case equivalent resistance. For a conservative bound, calculate once with each resistor at the low end of its tolerance and again at the high end. Correlation and ratio specifications may require a more detailed analysis for matched networks.
Temperature coefficient changes each branch with temperature. Unequal self-heating can also shift current sharing. Precision current sharing may need matched resistors, a resistor network or active control.
Why use parallel resistors
- Create a resistance value not available as one standard part.
- Share power across several components.
- Reduce effective inductance in some layouts.
- Adjust an existing feedback or bias resistance.
- Build a load bank from available values.
Parallel parts use more board area and introduce additional solder joints. They may not improve reliability if the circuit depends on every branch remaining connected.
Failure and layout checks
If one branch opens, the equivalent resistance increases and the remaining branches carry more current. If one branch shorts, the equivalent resistance approaches zero. Determine whether either condition creates an unsafe voltage, current or temperature.
Route the branches so each sees the intended voltage and current path. High-current resistor banks can share unevenly when copper resistance or cooling differs. Use symmetric connections where current sharing matters and measure temperature on production-intent hardware.
Practical calculation workflow
- Convert all values to one unit.
- Calculate nominal equivalent resistance.
- Verify the result is below the smallest branch value.
- Calculate current and power for each branch.
- Repeat with tolerance and temperature extremes.
- Check resistor voltage, pulse and thermal ratings.
- Review open- and short-circuit failure effects.
For sourcing, provide resistance, tolerance, TCR, power, package and qualification requirements. See our passive component sourcing page or submit the bill of materials through our contact page.
