Series & Parallel Resistor Calculator
Combine any number of resistors in series or parallel — and work backwards to find the stock pair that hits your target value.
Example: 2 resistors in parallel (10 kΩ, 10 kΩ) gives 5 kΩ.
Formula
series: R = R₁ + R₂ + … parallel: 1/R = 1/R₁ + 1/R₂ + …Worked example
2 resistors in parallel (10 kΩ, 10 kΩ) gives 5 kΩ.
1/R = 1/R₁ + 1/R₂ + …
1/R = 1/10 kΩ + 1/10 kΩ
1/R = 0.000200000 S
R = 1 / (sum of reciprocals)
R = 1 / 0.000200000
R = 5 kΩ
For exactly two, the shortcut R₁R₂/(R₁+R₂) gives the same answer.
Frequently asked questions
How do I calculate resistors in parallel?
Add the reciprocals and invert: 1/R = 1/R₁ + 1/R₂ + … For exactly two resistors the shortcut R₁R₂/(R₁+R₂) is easier. Two equal resistors in parallel always give half the value.
Why is the parallel result smaller than every resistor?
Because you are adding another path for current. More paths means less total opposition, no matter what you add — so the equivalent is always below the smallest resistor in the set.
Can I use two resistors to get a value I do not have?
Yes, and it is common practice. Two in series add, so 10k + 2.2k gives 12.2k. Two in parallel give something below both, so two 10k in parallel give 5k. Bear in mind that tolerances add too — two 5% resistors do not produce a 5% result in general.
Which resistor gets hot in a parallel combination?
The smallest one. Current splits inversely with resistance, so the lowest-value resistor takes the largest share and dissipates the most power. Size its wattage rating for its own share, not for an equal split.
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