Electronics · Resistors
Resistors in series and parallel.
List the resistor values and get the total resistance if they are wired in series, and if they are wired in parallel.
Inputs
Result
Enter two or more resistor values.
Resistors in series and in parallel
In series the resistances add; in parallel you add the reciprocals and take the reciprocal of the sum. Series: R = R1 + R2 + R3 …. Parallel: 1/R = 1/R1 + 1/R2 + 1/R3 …. Parallel total is always lower than the smallest resistor.
Worked example. 100, 220 and 330 Ω. In series: 100 + 220 + 330 = 650 Ω. In parallel: 1 ÷ (1/100 + 1/220 + 1/330) = 56.9 Ω.
Two equal resistors. Two 10 kΩ resistors in parallel give 5000 Ω, exactly half. In series they give 20000 Ω. For two resistors in parallel the shortcut is R1 × R2 ÷ (R1 + R2).
Why combine them. It is the usual way to make a value you do not have in the drawer: put two standard values together and check the total here. Power divides among the parts too; with equal resistors in parallel each carries half the current. A 0 Ω entry (a wire link) makes the parallel total 0 Ω.
Frequently asked questions
How do you calculate resistors in parallel?
Add the reciprocals, then take the reciprocal of the sum: 1/R = 1/R1 + 1/R2 + …. For 100, 220 and 330 Ω that is 56.9 Ω.
How do you calculate resistors in series?
Just add them. 100 + 220 + 330 = 650 Ω.
What is two 10k resistors in parallel?
5 kΩ (5000 Ω): two equal resistors in parallel give half of one.
Is the parallel total always smaller?
Yes. The total of resistors in parallel is always less than the smallest single resistor.
How can I make a value I do not have?
Combine standard values in series or parallel and check the total here. Tolerances also combine, so the result is only as accurate as the parts.
Sources
Checked
Runs entirely in your browser. Nothing you type is sent or stored.
Scope and safety
Low-voltage hobby use only. These results are arithmetic from the formulas shown, not an engineering design or professional advice. Real parts vary with tolerance, temperature and age, so check the datasheet for every part. Never work on mains-powered circuits from a calculator result, and treat lithium and other rechargeable batteries with care.