Parallel Resistor Calculator
Equivalent parallel resistance with branch currents and power.
What it does
Resistors in parallel all see the same voltage. Their equivalent resistance is always lower than the smallest one. The optional target finds the second resistor to put in parallel with the first to reach a value you need.
Where it's used
Making odd values from standard parts, sharing power across several resistors, and working out total load on a supply.
How to use it
- Type the values separated by commas.
- Enter the voltage to see each branch current and power.
- Optional: enter a target to find R2 so that R1 ∥ R2 equals it (R1 is your first value).
The formula explained
- 1/R_eq
- 1/R1 + 1/R2 + … + 1/Rn
- Two resistors
- R_eq = R1·R2 / (R1 + R2)
- I_n
- V / R_n, the current in branch n
Worked example
Two equal 1 kΩ resistors in parallel across 5 V.
Inputs:
- Resistor values (comma separated): 1k, 1k Ω
- Applied voltage: 5 V
The calculator gives:
- Equivalent resistance: 500 Ω
- Total conductance: 2 mS
- Total current: 10 mA
Tips and common mistakes
- N equal resistors in parallel give R / N.
- The smallest resistor carries the most current — check its power rating first.
- Adding any resistor in parallel always lowers the total.
Questions people ask
Is there a quick formula for two resistors?
Yes — product over sum: R = R1 × R2 / (R1 + R2). For 1 kΩ and 2.2 kΩ that is 2 200 000 / 3 200 = 687.5 Ω.
What resistor do I add in parallel to reach a value I need?
R2 = R1 × R_target / (R1 − R_target). To bring 1 kΩ down to 750 Ω you need 3 kΩ in parallel. Type your resistor and the target and the calculator finds R2 and the nearest standard value.
What happens if one parallel resistor fails open?
The others keep working at the same voltage and current, but the total resistance rises because one path has gone. In a circuit that relies on the combined value — a current-sense resistor, say — the reading will be wrong.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.