Resistors in parallel
Resistors in parallel share the same two connection points, so the current has more than one path. Each extra path lowers the total resistance:
1 / R = 1 / R1 + 1 / R2 + 1 / R3 + …
For just two resistors this simplifies to R = R1 × R2 / (R1 + R2), “product over sum”. With 10 kΩ and 4.7 kΩ: 47,000,000 / 14,700 = 3.197 kΩ. Two equal resistors give half the value, and n equal resistors give R / n.
The total is always smaller than the smallest resistor. The smallest resistor carries the most current: each one gets a share of Rtotal / Rn, shown in the results table.
Resistors in series
Resistors in series are connected end to end, so the same current flows through all of them and the values simply add: R = R1 + R2 + R3 + …. The voltage splits in proportion to each value, which is how a voltage divider works.
Making a value you don’t have
Standard resistors come in series like E24 (24 values per decade, ±5%). When you need a value in between, two resistors can get you there. The third panel tries every pair of E24 values and lists the five closest.
| You need | Parallel | Series |
|---|---|---|
| 5 kΩ | 10 kΩ ∥ 10 kΩ | 2 kΩ + 3 kΩ |
| 8 kΩ | 16 kΩ ∥ 16 kΩ | 3.3 kΩ + 4.7 kΩ |
| 25 Ω | 30 Ω ∥ 150 Ω | 12 Ω + 13 Ω |
| 50 Ω | 100 Ω ∥ 100 Ω | 20 Ω + 30 Ω |
| 6 kΩ | 12 kΩ ∥ 12 kΩ | 3 kΩ + 3 kΩ |
Frequently asked questions
Why is the parallel total smaller than each resistor?
Every resistor you add in parallel is another path for the current. More paths let more current flow at the same voltage, and more current at the same voltage means less resistance.
What power rating do the combined resistors have?
Equal resistors in parallel or in series share the power equally, so two 1/4 W resistors together handle 1/2 W. With unequal values, the one that takes the larger share heats up first: in parallel that is the smaller resistor, in series the larger one. Check each resistor’s own power with the Ohm’s law calculator.
Does combining resistors change the tolerance?
The tolerance of the combination is no worse than that of the parts. Two ±5% resistors give a total that is still within ±5%, and often closer, because their errors tend to partly cancel.
How do I enter the values?
Separate them with commas or spaces, for example 10k 4k7 2.2M or 10 kΩ, 4.7 kΩ. You can write 4.7k or 4k7, R47 for 0.47 Ω, and M for megohms. Up to 20 resistors at a time.