Power Dissipation Calculator — what rating do I actually need?

Component dissipation and the derated part rating you should actually specify.

Example: Dissipating 250 mW average at 25 °C means specifying at least a 0.5 W part.

Inputs

Known values

Resistance(Ω)

Current(A)

Voltage across component(V)

Used in voltage-and-current mode.

Duty cycle

1 means always on. 0.1 means conducting 10% of the time.

Ambient temperature(°C)

Inside an enclosure, well above room temperature.

Safety margin

2× is standard practice.

Result

Specify at least

0.5 W

Average power: 250 mW
Peak power: 250 mW
Required rating: 500 mW
Derating at temperature: 100%
Voltage across: 5 V

Effective rating of a standard part at this temperature

Nameplate	At 25 °C	Enough here?
0.0625 W	62.5 mW	no
0.1 W	100 mW	no
0.125 W	125 mW	no
0.25 W	250 mW	no
0.5 W	500 mW	no
1 W	1 W	yes
2 W	2 W	yes
3 W	3 W	yes
5 W	5 W	yes

Formula

P = I²R or V×I    P_avg = P_peak × duty    rating ≥ P_avg / derating × margin

Worked example

Dissipating 250 mW average at 25 °C means specifying at least a 0.5 W part.

P = I² × R
(50 mA)² × 100 Ω
250 mW
P_avg = P_peak × duty
250 mW × 1
250 mW
rating ≥ P_avg / derating × margin
250 mW / 1 × 2
500 mW

Frequently asked questions

What power rating resistor do I need?

At least twice the dissipated power, and more if it runs hot. Compute P = I²R, apply your duty cycle, divide by the temperature derating factor, then double it. A resistor dissipating 0.2 W at room temperature wants a 0.5 W part.

Why derate for temperature?

Because the rating assumes the part can shed heat into cooler surroundings. Resistor ratings are quoted at 70 °C ambient and fall linearly to zero at 155 °C. Inside a warm enclosure a 1 W resistor may only be good for half a watt.

Can I use average power for a pulsed load?

Usually, but only if the pulses are much shorter than the part's thermal time constant — a few seconds for a small resistor. Slower pulses let it heat towards the peak between them, and then you must size for the peak instead.

Is power the only rating I need to check?

No. Resistors also have a maximum working voltage, and small SMD parts are often limited to 50–200 V regardless of power. A high-value resistor in a high-voltage divider can be well within its power rating and still arc over.

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