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
| Field | What to enter |
|---|---|
| Connection | Choose one: Series, Parallel |
| Source | Choose one: Constant voltage, Constant current |
| Source voltage | Units: V, mV, kV, µV, MV |
| Source current | Units: A, mA, µA, kA |
| Resistor R1 | Units: Ω, kΩ, MΩ, mΩ |
| Resistor R2 | Units: Ω, kΩ, MΩ, mΩ |
| Resistor R3 | Units: Ω, kΩ, MΩ, mΩ |
| Resistor R4 | Units: Ω, kΩ, MΩ, mΩ |
| Resistor R5 | Units: Ω, kΩ, MΩ, mΩ |
| Resistor R6 | Units: 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) |
|---|---|
| 6 | 0.1125 |
| 9 | 0.2531 |
| 12 | 0.45 |
| 15 | 0.7031 |
| 18 | 1.0125 |
| 24 | 1.8 |
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
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.
Frequently asked questions
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.