Resistor Calculator
Combine two resistors in series or parallel.
Results update as you type.
How it works
Combining resistors is one of the most common tasks in circuit design, and this resistor calculator finds the equivalent resistance of two resistors wired either in series or in parallel. In series the resistors sit end to end so their values add, while in parallel they sit side by side so the combined resistance drops below the smaller of the two. Selecting the configuration gives the single resistance the pair behaves as. This matters because real circuits rarely use a single ideal resistor. Engineers combine standard values to hit a target resistance that no single stock part provides, set bias currents, build attenuators, and share load between components. Knowing the equivalent value lets you predict currents and voltages elsewhere in the circuit, so a resistor calculator saves both breadboard time and arithmetic mistakes. The formulas work from two rules: series resistors carry the same current and their voltages add, so resistance sums; parallel resistors share the same voltage and their currents add, so their reciprocals sum, which for a pair reduces to product over sum. Electronics students, technicians, and hardware designers use a resistor calculator to reach precise resistances and to analyse networks quickly and reliably.
series: R = R1 + R2 · parallel: R = (R1 × R2) ÷ (R1 + R2)
Resistors in series carry the same current, so their voltage drops add and the equivalent resistance is simply the sum R1 plus R2. Resistors in parallel share the same voltage, so their currents add; this makes the reciprocals of resistance add, which for two resistors simplifies to the product over the sum. The parallel result is always smaller than either resistor because adding a second path lets more total current flow.
Source:Standard electrical engineering (series/parallel resistance)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter resistor r1, resistor r2 and configuration.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Equivalent resistance updates instantly as you type — no button to press.
Frequently asked questions
How do resistors add in series?
Resistors in series are connected end to end so the same current flows through each one. Their voltage drops add up, which means the equivalent resistance is simply the sum of the individual values. Two resistors of 100 and 200 ohms in series give 300 ohms. Series connection always increases total resistance, and it is used to build up to values that single resistors do not provide.
How do resistors combine in parallel?
Parallel resistors are connected across the same two nodes, sharing the same voltage while their currents add. The reciprocals of the resistances add, so for two resistors the equivalent equals their product divided by their sum. The result is always smaller than the smallest resistor, because offering current more than one path lowers the overall opposition to flow through the combination.
Why is parallel resistance smaller than either resistor?
Adding a resistor in parallel gives current an extra route, so more total current flows for the same voltage, which by Ohm's law means less overall resistance. No matter how large the second resistor is, it can only add current and never remove any, so the combined resistance must fall below the smaller of the two. Identical parallel resistors halve the resistance.
Can I use this for more than two resistors?
This calculator handles two resistors at a time, but you can chain it for larger networks. Combine the first two, then treat their equivalent as one resistor and combine it with the third, and so on. Work outward from the innermost series or parallel groups. For complex networks that are neither purely series nor parallel, you may need mesh or nodal analysis instead.
Why combine resistors instead of buying one value?
Standard resistors come in fixed value series like E12 or E24, so the exact resistance a design needs often falls between available parts. Combining two stock resistors in series or parallel reaches intermediate values precisely and cheaply. It also lets designers share power dissipation across two parts, improve tolerance by averaging, or make use of resistors already on hand rather than ordering a special value.