Parallel Resistor Calculator
When resistors are connected in parallel, each is connected across the same two nodes so that the same voltage appears across every resistor. The total current entering the parallel combination is the sum of the currents through each branch. This means the parallel combination presents less resistance to the circuit than any single resistor alone. The governing formula is 1/Rt = 1/R1 + 1/R2 + 1/R3 + ..., giving Rt as the reciprocal of the sum of reciprocals. Parallel resistor combinations are used to obtain non-standard resistance values from standard E-series components, to share power dissipation across multiple resistors, and to increase the effective current capacity of a resistor network. This calculator supports up to six resistors in parallel. Leave unused fields blank or zero to exclude them from the calculation. Enter a supply voltage to also see the current drawn from the supply and the power delivered to the parallel combination. Results update as you type.
Parallel resistance formula
1/Rt = 1/R1 + 1/R2 + 1/R3 + ...
Two resistors shortcut: Rt = (R1 × R2) / (R1 + R2)
Worked example
R1 = 1,000 Ω, R2 = 1,000 Ω in parallel:
- 1/Rt = 1/1000 + 1/1000 = 0.002
- Rt = 1/0.002 = 500.00 Ω
- Note: equals half of one resistor, as expected for two equal values
Frequently asked questions
What is the formula for parallel resistors?
For resistors in parallel, the reciprocal of the total resistance equals the sum of the reciprocals of each individual resistance: 1/Rt = 1/R1 + 1/R2 + ... + 1/Rn. The total resistance is always less than the smallest individual resistor in the group.
Why is parallel resistance always less than the smallest resistor?
Each resistor added in parallel provides an additional current path. More paths mean the circuit opposes current less, so total resistance decreases. Even a very large resistor added in parallel slightly reduces total resistance.
What is the shortcut formula for two resistors in parallel?
For exactly two resistors, the total resistance equals the product divided by the sum: Rt = (R1 * R2) / (R1 + R2). This is sometimes called the product-over-sum formula. For two equal resistors, the result is always exactly half of one resistor value.
How does voltage behave in a parallel circuit?
In a parallel circuit, the voltage across each resistor is identical and equal to the supply voltage. This is in contrast to a series circuit where voltage is divided among resistors. Current, however, divides among the branches in inverse proportion to their resistance.
When would I use resistors in parallel?
Common uses include: achieving a non-standard resistance value by combining standard values, increasing current capacity when a single high-wattage resistor is unavailable, providing redundancy in sensor circuits, and creating specific current dividers. Parallel combinations also appear in load analysis and power distribution calculations.
Sources
- NIST: SI Units: Electric Current.
Reviewed by the CalculatorHub team, edited by James Graham. 14 June 2026.