Series resistor formula
When resistors are connected end to end in a single current path, their resistances add.
For example, 100 Ω, 220 Ω and 470 Ω in series give:
Parallel resistor formula
For resistors connected across the same two nodes, add the reciprocals of the individual resistances and then take the reciprocal of that result.
For 100 Ω, 220 Ω and 470 Ω in parallel, the equivalent resistance is approximately 59.98 Ω.
Two resistors in parallel
With exactly two resistors, the parallel formula can also be written as the product divided by the sum.
For example, 100 Ω and 200 Ω in parallel produce approximately 66.67 Ω.
Equal resistors in parallel
When every resistor has the same resistance, the parallel result can be found by dividing one resistor value by the number of identical resistors.
Two 1 kΩ resistors in parallel equal 500 Ω. Four 1 kΩ resistors in parallel equal 250 Ω.
Series vs. parallel resistance
Adding resistors in series increases total resistance. The series result is therefore greater than any individual resistor when all resistor values are positive.
Adding positive resistors in parallel creates additional current paths. The equivalent resistance is therefore lower than the smallest individual resistor in the parallel network.
Using Ω, kΩ and MΩ
The calculator accepts ohms, kilohms and megohms. Choose a default unit for values without a suffix, or add a suffix directly to an individual value.
For example, entering 100, 4.7k, 2M means 100 Ω, 4.7 kΩ and 2 MΩ.
One kilohm equals 1,000 ohms, and one megohm equals 1,000,000 ohms.
What is conductance?
Conductance describes how easily current can flow and is the reciprocal of resistance. Its SI unit is the siemens.
The calculator shows conductance for the selected equivalent resistance as an additional reference value.
Resistance vs. resistor color code
This calculator combines resistor values you already know. If you need to determine a resistor's value from its colored bands first, use the Resistor Color Code Calculator.
Resistance, voltage and current
Equivalent resistance alone does not calculate circuit current or voltage. For those relationships, use the Ohm's Law Calculator.
If you need to convert between current and power, you can also use the Amps to Watts Calculator or Watts to Amps Calculator.
This calculator uses ideal resistor values to find equivalent resistance. Real-world resistors also have tolerance, temperature coefficients, voltage limits and power ratings that are not included in the equivalent-resistance calculation.
Frequently asked questions
How do I calculate resistors in series?
Add all resistor values together. For example, 100 Ω + 220 Ω equals 320 Ω.
How do I calculate resistors in parallel?
Add the reciprocals of all resistor values and take the reciprocal of the total.
Why is parallel resistance lower?
Parallel branches provide additional paths for current, so the equivalent resistance is lower than the smallest positive resistor in the network.
What do two equal resistors in parallel equal?
Two identical resistors in parallel have half the resistance of one. Two 1 kΩ resistors therefore equal 500 Ω.
Can I mix ohms and kilohms?
Yes. Add k or M to an individual value to use kilohms or megohms while other entries use the selected default unit.
Using this calculator effectively
The Series & Parallel Resistor Calculator finds equivalent resistance for two or more known resistor values. It supports series and parallel connections, mixed Ω, kΩ and MΩ values, and reports both series and parallel totals for comparison.
Common uses
- Combining known resistor values to calculate equivalent resistance in a series circuit.
- Calculating the total resistance of multiple resistors connected in parallel.
- Comparing series and parallel arrangements or checking resistor-network calculations.
Tips for a better estimate
- Enter at least two positive resistor values and confirm the selected connection matches the actual circuit.
- Use k or M suffixes when an individual resistor uses kilohms or megohms instead of the selected default unit.
- Treat the result as ideal equivalent resistance because real components also have tolerance, temperature behavior and power limits.
Related calculations
Series & Parallel Capacitor Calculator · Resistor Color Code Calculator · Ohm's Law Calculator