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Ohm's Law Power Calculator

Find power and resistance from voltage and current.

How it is calculated

This tool takes two known values, voltage V in volts and current I in amperes, and returns P = V × I in watts and R = V ÷ I in ohms. Those are the power and Ohm's law relations for a DC circuit or a purely resistive load. The current must not be zero, since the resistance would be undefined, and no result is shown for it. Negative values are not rejected, so a negative current gives negative power and resistance, which is not physically meaningful for a resistor. Only voltage and current are inputs here; it does not solve from other pairs, such as power and resistance. Results are rounded to two decimals.

Worked example

A 12 V device draws 2.5 A. Power: 12 × 2.5 = 30.00 W. Resistance: 12 ÷ 2.5 = 4.80 Ω. Check using the other power formula: P = I² × R = 2.5² × 4.8 = 6.25 × 4.8 = 30 W, the same answer. A power supply feeding this device should be rated well above 30 W, because supplies are not meant to run at their limit.

Try this example →

Common mistakes

  • Entering current in milliamps. A 500 mA load must be typed as 0.5, otherwise the power is a thousand times too high.
  • Applying the DC formula to an AC load without the power factor. For motors and transformers, V × I is apparent power, not real power.
  • Treating the resistance as a property of a lamp or LED at all voltages. The value shown applies at that operating point; it changes with temperature.
  • Entering the supply voltage when the voltage across the component is smaller, for example after a series resistor.
  • Confusing watts with watt-hours. Power is the rate; energy is power multiplied by time.

About this calculator

This calculator combines Ohm's law (V = I·R) with the power formula (P = V·I) to find electrical power and resistance from the voltage and current you enter — the same relationships that give P = I²R and P = V²/R. These equations describe how voltage, current, resistance, and power relate in any DC circuit, from a single resistor to a full electrical load.

Electrical and electronics students working through circuit-analysis homework, hobbyists building Arduino or Raspberry Pi projects who need to size a resistor or check a component's power rating, and electricians verifying that a device won't exceed a circuit's wattage capacity use this.

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Comments (1)

  • Editorial team

    The power shown is dissipated only in the part across which you measured the voltage. For an LED on a 5 V supply with a 2 V drop at 20 mA, the series resistor sees 3 V, so enter 3 and 0.02: 150 Ω and 0.06 W. A common 0.25 W resistor then has comfortable headroom. Which part are you sizing, and what voltage actually sits across it?

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