When you do not have the resistance you need on hand, you can connect two or three resistors you do have in series or parallel to get close to the value you want. It is also used to share heat among several small resistors where the power is large. List the resistor values separated by commas in the calculator above and choose series or parallel, and it works out the combined resistance and, if you enter a supply voltage, the voltage and current of each resistor.
Series connection
When resistors are connected in a single line, the combined resistance is simply the sum. The same current flows through all of them, and the voltage divides in proportion to resistance.
R_total = R1 + R2 + R3 + ... Voltage across each = supply voltage × (that resistor / R_total)
For example: 220 Ω + 100 Ω = 320 Ω, and 4.7 kΩ + 330 Ω = 5.03 kΩ.
Parallel connection
When resistors are connected side by side, the combined resistance is smaller than even the smallest one. The same voltage is applied across all of them, and the current divides in inverse proportion to resistance.
1 / R_total = 1/R1 + 1/R2 + 1/R3 + ... For two: R_total = (R1 × R2) / (R1 + R2) For n equal: R_total = R / n
For example: two 10 kΩ in parallel give 5 kΩ, four 100 Ω give 25 Ω, and 220 Ω with 330 Ω give 132 Ω.
Useful combinations
| Value to make | How to combine |
|---|---|
| 3 kΩ (not in the E12 series) | 1.5 kΩ + 1.5 kΩ in series |
| 2 kΩ | 1 kΩ + 1 kΩ in series |
| 5 kΩ | two 10 kΩ in parallel |
| 2.5 kΩ | four 10 kΩ in parallel |
| about 5.1 kΩ | 4.7 kΩ + 470 Ω in series (5.17 kΩ) |
| 50 Ω, rated 1/2 W | two 100 Ω 1/4 W in parallel |
Using n equal resistors in parallel makes the combined resistance 1/n, while the power rating becomes n times. It is useful when a high-power resistor is hard to find.
To read resistor values, continue with the resistor color code calculator, and for the application of dividing voltage with two in series, the voltage divider calculator.