# Resistor Wattage Calculator

URL: https://speedycalculator.com/calculators/physics/resistor-wattage/
Last updated: 2026-10-30

> The resistor wattage calculator finds the power dissipated in each of up to six resistors connected in series or parallel and driven by a constant voltage or a constant current source, to choose the right power rating.

## Inputs

- Connection
- Source
- Source voltage (V, mV, kV, µV, MV)
- Source current (A, mA, µA, kA)
- Resistor R1 (Ω, kΩ, MΩ, mΩ)
- Resistor R2 (Ω, kΩ, MΩ, mΩ)
- Resistor R3 (Ω, kΩ, MΩ, mΩ)
- Resistor R4 (Ω, kΩ, MΩ, mΩ)
- Resistor R5 (Ω, kΩ, MΩ, mΩ)
- Resistor R6 (Ω, kΩ, MΩ, mΩ)

## Results

- Equivalent resistance
- Total power
- Power in R1
- Power in R2
- Power in R3
- Power in R4
- Power in R5
- Power in R6
- Suggested rating for the hottest resistor

## Formula

```
Series: I_k = I, P_k = I²R_k · Parallel: V_k = V, P_k = V²/R_k · With a current source in parallel: V = I·R_eq
```

## Explanation

Resistors come in power ratings – ⅛ W, ¼ W, 1 W and more. Exceed the rating and they overheat, drift or burn out. Choose a rating at least double the dissipated power.

### How to use the Resistor Wattage Calculator

- **Connection**
- **Source**
- **Source voltage**
- **Source current**
- **Resistor R1**
- **Resistor R2**
- **Resistor R3**
- **Resistor R4**
- **Resistor R5**
- **Resistor R6**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Choose the connection and source type, enter the source value and the resistances (0 = unused).

### Inputs explained

FieldWhat to enterConnectionChoose one: Series, ParallelSourceChoose one: Constant voltage, Constant currentSource voltageUnits: V, mV, kV, µV, MVSource currentUnits: A, mA, µA, kAResistor R1Units: Ω, kΩ, MΩ, mΩResistor R2Units: Ω, kΩ, MΩ, mΩResistor R3Units: Ω, kΩ, MΩ, mΩResistor R4Units: Ω, kΩ, MΩ, mΩResistor R5Units: Ω, kΩ, MΩ, mΩResistor R6Units: Ω, kΩ, MΩ, mΩ

### What each result means

- **Equivalent resistance** = [Req]
- **Total power** = [V]×[I]

### Example values

How the main result changes when **Source voltage** changes and everything else stays at the example values above:

Source voltage (V)Total power (W)60.112590.2531120.45150.7031181.0125241.8

## Worked example

Inputs — Connection: Series; Source: Constant voltage; Source voltage: 12 V; Resistor R1: 100 Ω; Resistor R2: 220 Ω; Resistor R3: 0 Ω; Resistor R4: 0 Ω; Resistor R5: 0 Ω; Resistor R6: 0 Ω.

Results — Equivalent resistance = 320 Ω; **Total power = 0.45 W**; Power in R1 = 0.14063 W; Power in R2 = 0.30938 W; Power in R3 = 0 W; Power in R4 = 0 W; Power in R5 = 0 W; Power in R6 = 0 W; Suggested rating for the hottest resistor = 1 W.

## Key terms

- **Power rating**: Maximum continuous dissipation.

## FAQ

**What rating should I choose?**
At least twice the calculated power.

**Why do resistors fail?**
Overheating changes their value or burns them open.

**Does ambient temperature matter?**
Yes – ratings are derated above about 70 °C.

**Which resistor dissipates most in parallel?**
The smallest one.


## Sources

- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)