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Power dissipation · P = V × I

Set the voltage and resistance. The tool calculates both the current and the power the resistor dissipates. Then choose a resistor rating and see whether it stays within its rating, based on the two-times derating rule.

5.0 V + 1.0 kΩ 25 mW
Voltage 5.0 V
Resistance 1.0 kΩ

Resistor rating

P = 5.0 V × 5.0 mA = 25 mW

Current  I = V / R
5.0 mA
Power  P = V × I
25 mW
Min. rating  (2× rule)
50 mW

Safe. The resistor dissipates 25 mW. Twice that is 50 mW, below the 0.25 W (250 mW) rating.