Quick brief

What to know before you calculate

A short read on the assumptions, trade-offs and definitions that shape the answer.

  • Ohm's law is V = I × R. Rearranged, I = V / R and R = V / I.
  • Electrical power is P = V × I. Using Ohm's law, power can also be calculated from current and resistance or voltage and resistance.
  • Keep volts, amps, ohms and watts consistent before calculating.

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Calculate voltage, current, resistance and power

Enter any two known electrical values to calculate the remaining Ohm's law and power values.

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Ohm's law formula: voltage, current and resistance

The core equation is V = I × R, where V is voltage in volts, I is current in amps and R is resistance in ohms. Rearranging the same relationship gives I = V / R when calculating current, and R = V / I when calculating resistance.

Power, voltage, current and resistance formulas

Electrical power is P = V × I, where power is measured in watts. Combining the power equation with Ohm's law also gives P = I²R and P = V² / R. If voltage and power are known, current can be calculated as I = P / V. If current and power are known, voltage can be calculated as V = P / I.

Worked Ohm's law example

If a simple circuit has 12 volts across a resistance of 6 ohms, current equals 12 divided by 6, giving 2 amps. Power then equals 12 volts multiplied by 2 amps, giving 24 watts. The same result can be checked with current squared multiplied by resistance: 2 squared multiplied by 6 also gives 24 watts.

Keep volts, amps and ohms consistent

Small unit mistakes can change the answer by a large factor. Milliamps must be converted to amps, kilohms to ohms and kilowatts to watts when the formula expects the base units. A unit conversion calculator can help before using the electrical equations.

Know the limits

These formulas describe a simple DC-style relationship. AC circuits, reactive components, heat, component tolerance, wiring, fuses and electrical safety rules can all change the practical answer. Use the formulas for calculation and learning, not as a substitute for qualified electrical advice.

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Try the numbers yourself

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Ohm's LawCalculate voltage, current, resistance or power from any two known electrical values.EnergyCalculate kinetic, electrical or heat energy using common formulas, with joule and kilojoule results where relevant.Unit ConversionConvert common length, weight, temperature and volume units with instant results.