Electrical power
Formulas, limitations and content checked according to our Editorial method.
Calculate the power dissipated by any combination of voltage, current and resistance. Enter two values, get power and the third value.
P = V × I
P = V² / R
P = I² × R
Calculation
Enter any 2 values between V, I, R. Power calculated automatically.
Resistor size chart
| Cut | Max. power | When to use it |
|---|---|---|
| 1/8W | 125mW | Logic signals, pull-ups, currents < 10 mA on 5V |
| 1/4W | 250mW | The most common, LED, low current voltage dividers |
| 1/2W | 500mW | Medium currents, 12V power supplies |
| 1W | 1000mW | Shunt resistors, power circuits |
| 2–5W | 2–5W | Power loads, filaments, heat sinks |
See also
Frequently asked questions
How do I calculate power dissipated by a resistor?
P = V²/R = I²×R. With V=5V, R=100Ω: P = 250mW.
When should I use a 0.5W instead of 0.25W resistor?
When calculated power exceeds 0.25W. Rule: use resistor with at least 2× power rating.