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.

Volts across (V)

Voltage dropped across the resistor.

Current (A)

Current through the resistor.

Safety factor

Derating multiplier between 1 and 5.

Compute the required resistor power rating from volts times current and recommend a derated rating using a safety factor between 1 and 5. 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]

Required power (W)

Dissipated power the resistor must handle.

Recommended rating (W)

Recommended component rating after derating.

• 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.

• Motor Power Factor At Load

  Calculate balanced three-phase motor power factor as real input power divided by √3 times line voltage and line current; the result is a magnitude.

• Current From Power Rating

  Compute the maximum safe continuous current in milliamps through a resistor from its power rating and resistance using I = sqrt(P/R).

• Ac Motor Reactive Power

  Calculate positive reactive-power magnitude for a balanced three-phase motor from line voltage, line current, and power factor.