Resistor calculator
100 Ω and 220 Ω resistors in series give a total resistance of 320 Ω, in parallel only 68.75 Ω. Calculate the total resistance of a series or parallel connection and design a voltage divider.
Mode
Series connection: 320 Ω
Parallel connection: 68.75 Ω
How the calculator works
In a series connection resistances add up: R = R₁ + R₂ + …. In parallel the reciprocal sum applies: 1/R = 1/R₁ + 1/R₂ + …, so the result is always smaller than the smallest resistance – two equal resistors in parallel give half. Enter any number of values in Ω, kΩ or MΩ and see both results at once.
The voltage divider calculates Uout = Uin × R₂ / (R₁ + R₂) for an unloaded divider. If you connect a load to the output, its resistance combines in parallel with R₂ and the output voltage falls – which is why dividers are designed with resistances well below the load resistance.
Worked examples
| Input | Result |
|---|---|
| 100 Ω + 220 Ω in series | 320 Ω |
| 2× 100 Ω in parallel | 50 Ω |
| 1 kΩ ∥ 4.7 kΩ | ≈ 825 Ω |
| Divider: 12 V, R1 = 10 kΩ, R2 = 4.7 kΩ | Uout ≈ 3.84 V |
Frequently asked questions
What resistance do 100 Ω and 220 Ω give in series?
R = R₁ + R₂ = 100 + 220 = 320 Ω.
What resistance do 100 Ω and 220 Ω give in parallel?
1/R = 1/100 + 1/220, so R ≈ 68.75 Ω.
What output voltage does a divider with Uin = 12 V, R1 = 10 kΩ and R2 = 4.7 kΩ give?
Uout = Uin × R₂ / (R₁ + R₂) = 12 × 4.7 / 14.7 ≈ 3.84 V.
How do I remember the parallel formula?
For two resistors the form R = R₁ × R₂ / (R₁ + R₂) is handy. Two equal resistors in parallel always give half the value, three equal ones a third.
Why does a divider's voltage change under load?
The load resistance combines in parallel with the divider's lower resistor and reduces its effective value. Design the divider so its resistances are at least 10× smaller than the load resistance.
What power rating should my resistor have?
Calculate the dissipated power P = U² / R or P = I² × R and choose a resistor with at least twice that rating so it doesn't heat up unnecessarily.
What does resistor tolerance mean?
The allowed deviation from the nominal value, typically ±1% or ±5%. Manufactured values follow the E12 or E24 series – which is why shops stock 1 kΩ and 1.2 kΩ rather than '1,100 Ω'.
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Last updated: 16 July 2026