Series and Parallel Resistance Calculator — Total Resistance Online

How to use the Series and Parallel Resistance Calculator

Enter the first resistor in “Resistor 1 (R1) in Ohms” and the second in “Resistor 2 (R2) in Ohms”. Click “Calculate Resistance” and the tool shows both results: the series total and the parallel total, each rounded to two decimal places. A small bar chart underneath compares the two totals visually. If you leave a field empty, the tool asks you to enter both resistors — it needs both values to calculate either total.

Worked example: two 100 ohm resistors

Enter 100 for R1 and 100 for R2. The series total is 200.00 Ohms, because series resistance simply adds: 100 + 100 = 200. The parallel total is 50.00 Ohms, because the parallel formula is 1 ÷ (1/100 + 1/100) = 1 ÷ 0.02 = 50.

Try an uneven pair to see the pattern: 100 and 200 ohms give a series total of 300.00 Ohms and a parallel total of 66.67 Ohms — 1 ÷ (1/100 + 1/200) = 1 ÷ 0.015 = 66.67. Notice the parallel total, 66.67, is below the smaller resistor (100). That is always true for parallel resistors, and it is the fact students most often get wrong on first meeting it.

Where it helps

Electronics students use it to check homework on equivalent resistance, the standard first chapter of circuit theory. Hobbyists use it to make a value they do not own: two 100 ohm resistors in parallel give 50 ohms, and two in series give 200, so one pair of resistors covers three values. Anyone designing a simple voltage divider or a load circuit can test combinations quickly before soldering anything. And the bar chart makes the series-versus-parallel contrast obvious in a way a pair of numbers alone does not, which is why teachers reach for this kind of visual when the topic is introduced.

Honest limitations

This calculator handles exactly two resistors. Real circuits often have three or more, and those need repeated application of the same formulas — combine two, then combine the result with the third. It also assumes ideal resistors: no tolerance, no heating effects, and direct current only. The chart is a simple comparison of the two totals, not a circuit diagram, so it will not show you how to wire anything. If you need to solve for a single unknown from voltage and current first, start with the Ohm’s law calculator, then bring the resistance values here.

Series and Parallel Resistance Calculator — frequently asked questions

Why is parallel resistance lower than either resistor?

Because parallel wiring gives the current two paths instead of one, so more current flows overall — and more current at the same voltage means lower effective resistance. The total always ends up below the smallest resistor in the pair.

Can I combine more than two resistors?

Not directly. Combine two resistors first, then combine that result with the third using the same formulas, repeating as needed.

What if I need a resistor value I do not have?

Combine what you own. Two equal resistors in series double the value, and two in parallel halve it — so a pair of 100 ohm resistors gives you 50, 100 or 200 ohms.

Which wiring gives more resistance, series or parallel?

Series, always. Series adds the values, while parallel always comes out below the smallest resistor in the pair.