Use this electronics calculation for a first-pass component, converter, signal, motor, or sensor estimate when the stated operating conditions and units match the device data.

Rated power (W)

Power rating of the resistor.

Resistance (Ω)

Resistance of the resistor.

Compute the maximum safe continuous current in milliamps through a resistor from its power rating and resistance using I = sqrt(P/R). The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

Max safe current (mA)

Maximum current before exceeding the power rating.

• Resistor value, power, noise, and color-code calculations must account for tolerance, temperature coefficient, voltage coefficient, pulse rating, working voltage, package thermal resistance, ambient temperature, and mounting. Required steady-state power should include the manufacturer's derating curve.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Check the result against the current datasheet and worst-case operating corners, then verify the circuit or measurement with simulation and bench testing where failure matters.

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