Resistor Calculators
Parallel Resistor Calculator
Add as many parallel branches as you need, get the equivalent resistance, and optionally see the current flowing in each one.
Parallel Resistors
Two or more branches across the same two nodes.
How Parallel Resistance Combines
Resistors are in parallel when both ends of each one connect to the same pair of nodes. Every branch therefore sees the same voltage, and the total current is the sum of the branch currents. Because conductance adds while resistance does not, the combination is expressed through reciprocals.
Two useful consequences follow. The equivalent resistance is always smaller than the smallest branch, because adding another path can only make it easier for current to flow. And n identical resistors of value R in parallel give exactly R ÷ n, which is why four 100 Ω resistors combine to 25 Ω.
How to Use the Parallel Resistor Calculator
Two resistor rows are shown by default. Enter a value in each and choose its unit from the selector beside it. Use Add resistor to create another branch, up to twelve, and the bin icon beside any row to remove it. The last two rows cannot be deleted, because a parallel combination needs at least two branches.
The supply voltage field is optional. Leave it blank if you only want the equivalent resistance. Fill it in and the calculator adds the total current drawn, the total power, and a table showing the current and power in every individual branch.
Select Calculate to solve. Any row left empty or containing a negative number is flagged inline, so it is clear which entry needs attention rather than the whole panel simply refusing to produce a result.
Worked Example: Three Resistors in Parallel
Two 100 Ω resistors in parallel:
Add a 12 V supply and the total current becomes 12 V ÷ 50 Ω = 240 mA, split evenly at 120 mA per branch, with each resistor dissipating 1.44 W.
Now make the branches unequal: 100 Ω and 10 Ω. The equivalent falls to 9.09 Ω, dominated by the smaller resistor. At 12 V the 10 Ω branch carries 1.2 A while the 100 Ω branch carries only 120 mA, so the small resistor handles more than 90% of the current and nearly all of the heat. This is the essential behaviour of parallel networks: the lowest-resistance path takes the largest share.
Reading Your Parallel Network Results
The equivalent resistance is the single value that could replace the whole network without any other part of the circuit noticing. Use it directly in Ohms Law calculations for the rest of the circuit.
When you supply a voltage, the branch table becomes the more useful output. It shows how the current divides, which is what determines whether any individual resistor is being overworked. It is easy to build a network whose equivalent resistance looks reasonable while one branch quietly dissipates several watts.
A branch entered as 0 Ω is handled explicitly. Because a zero-resistance path is a short circuit across the network, the equivalent resistance becomes 0 Ω and the calculator says so rather than dividing by zero or reporting infinity.
Where Parallel Resistors Appear
The most frequent practical use is reaching a value you do not have. Two 100 Ω resistors in parallel give 50 Ω, and three 1 kΩ resistors give 333 Ω, which often solves a problem faster than ordering a single part.
Parallel connection also shares heat. A single 10 Ω resistor dissipating 4 W needs a large part, but four 40 Ω resistors in parallel give the same 10 Ω while each handles only 1 W, spreading the heat across four packages that are easier to cool.
The same arithmetic explains household wiring. Outlets on a circuit are in parallel, so each appliance sees the full supply voltage and the currents add. Plugging in more devices lowers the combined resistance and raises the total current, which is precisely what the breaker is watching.
Sensing and measurement rely on it too. A current shunt is a very low value resistor placed in the path so the small voltage across it can be measured; putting several in parallel lowers the value further while spreading the heat. Pull-up resistors on a shared bus behave the same way, and a designer working out the effective pull-up on a line with several devices attached is solving exactly this equation.
Limits of the Parallel Resistance Model
- The calculation is ideal. Connection resistance, track resistance and contact resistance all add small series terms that matter most when the branch values are low.
- Tolerance propagates. Combining several 5% resistors gives an equivalent whose spread depends on all of them, so measure the result if the value is critical.
- Power sharing depends on the individual values being what you think they are. Two nominally equal resistors at opposite ends of their tolerance bands do not share current equally.
- The model is resistive only. Real components have parasitic inductance and capacitance that matter at high frequencies.
Questions
Parallel Resistor FAQs
Why is the total always less than the smallest resistor?
Because each extra branch gives current another route. Adding a path can only increase the total conductance, and higher conductance means lower resistance.
How many resistors can I add?
Up to twelve branches. That covers almost every practical network, and beyond it the arithmetic is better handled by grouping sections.
What happens if I enter zero for one resistor?
The calculator reports an equivalent resistance of 0 Ω and warns that the branch is a short circuit across the network, rather than attempting a division by zero.
Do parallel resistors need to be the same value?
No. Any mix works. Unequal branches simply carry unequal currents, with the smallest resistance taking the largest share.
Take the Equivalent Value to the Ohms Law Wheel
Open the full Ohms Law Wheel calculator to solve voltage, current, resistance and power from any two known values.