🔌 Parallel Resistor Calculator

Enter multiple resistor values and instantly get the equivalent resistance for a parallel connection (1/Rt = Σ1/Ri) or a series connection (Rt = ΣRi).

Equivalent Resistance

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Parallel & Series Equivalent Resistance Formulas

When multiple resistors are connected in a circuit, the equivalent (total) resistance is calculated differently depending on the wiring.

Parallel: 1/R_t = 1/R₁ + 1/R₂ + ... + 1/R_n
Series: R_t = R₁ + R₂ + ... + R_n

A parallel equivalent resistance is always smaller than the smallest individual resistor, while a series equivalent resistance is always larger than the largest individual resistor.

Why Does Parallel Resistance Get Smaller?

Connecting resistors in parallel adds more paths for current to flow, so the overall resistance decreases. For example, two 10Ω resistors in parallel give 1/R_t = 1/10 + 1/10 = 2/10, so R_t = 5Ω. In general, N identical resistors in parallel give an equivalent resistance of R/N.

Practical Use: Building a Target Resistance

When standard resistors (E12/E24 series) don't offer the exact value you need, combine multiple resistors in series or parallel to approximate it. For example, if 330Ω isn't available, you could use 220Ω + 100Ω in series (320Ω), or two 680Ω resistors in parallel (340Ω). Check individual values with the resistor color code calculator, then verify the combined result here.

Frequently asked questions

What's the difference between parallel and series connections?

In series, resistors are connected in a single line so current flows through only one path and the resistances simply add up. In parallel, resistors are connected side by side so current splits across multiple paths, and the equivalent resistance is always smaller than any individual resistor.

How many resistor values can I enter?

It starts with 2 rows, and you can add up to 10 using the "+ Add resistor" button. Empty rows are excluded from the calculation.

What happens with several identical resistors in parallel?

N identical resistors in parallel produce an equivalent resistance equal to the individual value divided by N. Example: three 10Ω resistors in parallel = 10/3 ≈ 3.33Ω.

What if I enter 0 or a negative value?

Resistance must physically be greater than 0, so entering 0 or a negative value shows an error message and the calculation is not performed.