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Resistor Calculator

Resistor Calculator is part of Toolspire and is ready as soon as this page loads. Series is R1 + R2. Parallel is R1 × R2 ÷ (R1 + R2). Two equal resistors in parallel halve. This is a pair, not a long ladder.

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What you can do here

Both arrangements

The headline is the series sum. The detail is the parallel value.

The half-value check

10 and 10 must say 20 and 5.

A pair only

A third resistor needs another pass: combine two, then combine that result with the third.

The sequence

How to use the Resistor Calculator

  1. 1

    Type ohms, not colour bands

    Brown black black is a different code. This page wants the ohm values.

  2. 2

    Read series on the headline

    Parallel is underneath, because it is the less obvious formula.

  3. 3

    Combine a third resistor in a second visit

    Use the result as one of the next pair.

Worth knowing

  • Resistor Calculator on Toolspire: series = R1 + R2. 10 + 10 = 20 ohms.
  • Parallel = R1 × R2 ÷ (R1 + R2). 100 ÷ 20 = 5 ohms.
  • Equal resistors in parallel are half of one of them.
  • A zero resistor in parallel shorts the pair. The parallel result is 0, and the detail says the combination is a short.

Series adds, parallel shares

Two resistors in series carry the same current and their voltages add, so the resistances add. In parallel they share the same voltage and the currents add, which is why the conductances add and the resistance is the product over the sum. Ten and ten become twenty in a chain and five side by side. A zero-ohm link in parallel is a wire across the other resistor, and the combination is zero.

FAQ

Questions about Resistor Calculator

Where does Resistor Calculator run?

Resistor Calculator runs in this tab on Toolspire. What you type or drop in is not sent to us and is not stored on a server.

What are 10 ohms and 10 ohms in series and in parallel?

20 ohms in series, 5 ohms in parallel.

Why is parallel smaller than either resistor?

The current gets a second path. The combination conducts better than one resistor alone.

What if one resistor is zero?

The parallel combination is a short, 0 ohms. The series combination is just the other resistor.

Can I do three resistors?

Combine two, then use that equivalent with the third. The page holds one pair.

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