Quick answer
In series, add the values: 100 Ω + 220 Ω = 320 Ω. In parallel, use 1/R = 1/R1 + 1/R2: 100 Ω and 220 Ω give 68.75 Ω.
How to use the Series & Parallel Resistor Calculator
- Enter the value of each resistor and choose its unit.
- Click + Add resistor for more resistors, or × to remove one.
- Read the series and parallel totals. They update as you type.
Example
Two resistors, 100 Ω and 220 Ω:
- Series:
100 + 220 = 320 Ω - Parallel:
1 ÷ (1/100 + 1/220) = 68.75 Ω
Need a value you don't have? Two 1 kΩ resistors in parallel make 500 Ω; in series they make 2 kΩ.
How it works
Series: the same current flows through every resistor, so the resistances simply add up:
Rtotal = R1 + R2 + … + Rn
Parallel: the current splits between the resistors, so the total resistance is always smaller than the smallest resistor:
1 ÷ Rtotal = 1/R1 + 1/R2 + … + 1/Rn
For exactly two resistors in parallel there's a shortcut: R = (R1 × R2) ÷ (R1 + R2). For n identical resistors in parallel, divide one resistor's value by n.
Handy combinations to make values you don't have
- Half a value: two equal resistors in parallel. Two 1 kΩ make 500 Ω.
- Double a value: two equal resistors in series. Two 4.7 kΩ make 9.4 kΩ.
- Fine-tune a value: put a large resistor in parallel with the main one. 1 kΩ in parallel with 47 kΩ gives about 979 Ω.
- More power handling: four ¼ W resistors (two in series, two of those in parallel) give the original value with about 1 W of capacity.
- Precision check: combinations add the tolerances too. For accurate results, measure the finished combination with a multimeter.
Frequently asked questions
Why is parallel resistance smaller than each resistor?
Each parallel resistor adds another path for the current to flow. More paths means more total current for the same voltage, which means lower total resistance.
How do I combine resistors to get a specific value?
Add resistors in series to increase resistance, or in parallel to reduce it. For example, 330 Ω in series with 150 Ω gives 480 Ω, close to the standard 470 Ω.
What happens to the power rating?
Resistors sharing power can handle more in total. Two identical ¼ W resistors in series or in parallel can together dissipate about ½ W.
Does this work for capacitors?
Capacitors work the other way round: in parallel their capacitances add, and in series they combine like parallel resistors.