Parallel & Series Resistor Calculator

Real-world use cases

• Size an LED current-limiting network from parts on hand

  You worked out you need about 165 Ω to drive an LED, but the drawer only holds common E24 values. Drop a target into the reverse helper and it suggests two standard resistors in parallel that land within a percent or two, so you solder what you have instead of waiting on a mail order.

• Verify a series string before powering up

  Building a high-value bias chain from several resistors in series? Paste the whole string, confirm the total matches your schematic, and catch a swapped 47 kΩ for a 4.7 kΩ before it changes the operating point of the circuit.

• Teach the reciprocal rule with live numbers

  In a lab class the parallel formula clicks faster when students change a value and watch the total move. Share a URL pre-loaded with three 100 Ω resistors, have them add a fourth, and the 33.3 Ω total dropping to 25 Ω makes the 1/R sum concrete.

• Hit a precise value you cannot buy

  Need 1.5 kΩ but only stock 1 kΩ and a bag of 4.7 kΩ? The reverse helper finds that a standard pair in parallel gets you close enough for a non-critical divider, saving a special order for a single odd resistor.

Common pitfalls

• Adding resistors in parallel instead of taking the reciprocal sum. Parallel resistance is not R1 + R2; that is the series rule. The parallel total is always smaller than the smallest resistor, so if your answer is larger than every part you used the wrong formula.

• Forgetting the suffix and entering 4.7 when you meant 4.7k. A bare 4.7 is read as 4.7 ohms, a thousand times smaller than the 4700 ohms you intended. Always include k or M, or use the 4k7 shorthand, so the value matches the band code on the part.

• Treating the two-resistor shortcut R = R1R2 / (R1 + R2) as if it works for three or more resistors. It only applies to exactly two in parallel. For three or more, fall back to the full reciprocal sum, which this tool does automatically for any list length.

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