Resistor Power Dissipation

Enter any two values to compute watts instantly. Add a resistor wattage rating to see headroom and risk.

How to Use

  1. Enter any two of: Voltage across the resistor, Current through it, or Resistance.
  2. Optionally set a resistor wattage rating (e.g., 0.25W, 0.5W, 1W) and a safety factor.
  3. Open “Show Work” to see the formula used (V²/R, I²R, or V×I) in base units.
  4. Use Copy/Share from the Tool Menu when you want to export or restore a setup.
Thermal Lab View
Visual feedback: wattage, percent of rating, and heat intensity.
P
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Rating
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% Used
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Headroom
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Risk: —
Resistor Power dissipation drives heat intensity P: — Rating: — Tip: enter any two of V, I, R. Add a rating to see pass/fail headroom.
Inputs & Settings
Enter any two values. Use Solve For + locks for guided solving.
Across the resistor (not necessarily supply voltage)
If you know current, power can be found via I²R
Power can be found via V²/R or I²R
If power is known, solve for V/I/R (with one more input)
Optional. Used to compute % used and headroom.
Example: 2× means you want rating ≥ 2× dissipation.
Show Work (step-by-step)
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Work is shown in base units (V, A, Ω, W) for clarity and consistency.

Power Dissipation Formulas

Quick answer: resistor power is energy converted to heat: P in watts.

Use whichever pair you know (V & R, I & R, or V & I).

  • Power from voltage + resistance: P = V² ÷ R
  • Power from current + resistance: P = I² × R
  • Power from voltage + current: P = V × I
Where V = volts, I = amps, R = ohms, P = watts.

FAQ

What wattage resistor should I choose?

A common rule is to pick a rating comfortably above dissipation (often 2× or more), depending on ambient temperature, airflow, and enclosure conditions.

Why can the same resistor run hotter in a box?

Ratings are typically specified under certain conditions. Less airflow and higher ambient temperature reduce how much heat a resistor can shed.

Is the resistor seeing the full supply voltage?

Not always. Use the voltage drop across the resistor itself (for series circuits, that may be supply minus other drops).

What happens if I exceed the rating?

Overheating can shift resistance, damage surrounding parts/PCB, and cause failure. This tool flags risk based on dissipation vs rating and your safety factor.

Tool Info

Last updated:

Updates may include UI improvements, unit support, and calculation edge-case handling.