How the Ohm’s Law Calculator works

The Ohm’s Law Calculator solves voltage, current, resistance and electrical power from any two known positive values. It converts millivolts, volts, kilovolts, milliamps, amps, ohms, kilo-ohms, megaohms, milliwatts, watts and kilowatts to base units before calculating.

Ohm’s law describes the relationship between voltage, current and resistance. The National Institute of Standards and Technology identifies the ampere as the SI base unit of electric current, while derived electrical units include the volt, ohm and watt.

Ohm’s Law Calculator formulas

Voltage: V = I × R. Current: I = V ÷ R. Resistance: R = V ÷ I. Electrical power can be calculated with P = V × I, P = V² ÷ R or P = I² × R.

Supported value combinations

Voltage and current determine resistance and power. Voltage and resistance determine current and power. Voltage and power determine current and resistance. Current and resistance determine voltage and power. Current and power determine voltage and resistance. Resistance and power determine voltage and current.

Worked example

A 12-volt source driving 2 amperes through a load has a resistance of 6 ohms because R = 12 ÷ 2. Power is 24 watts because P = 12 × 2. These are the calculator’s default values.

Electrical unit conversions

One kilovolt equals 1,000 volts and one millivolt equals 0.001 volt. One milliamp equals 0.001 amp. One kilo-ohm equals 1,000 ohms and one megaohm equals 1,000,000 ohms. One kilowatt equals 1,000 watts and one milliwatt equals 0.001 watt.

Accuracy and practical limits

The calculator assumes an ideal direct-current circuit with consistent values. Real components can change with temperature, manufacturing tolerance, frequency and operating conditions. Alternating-current circuits with capacitance, inductance or phase differences require impedance and power-factor calculations.

Choosing values for circuit calculations

The Ohm’s Law Calculator can help compare component ratings before a prototype is assembled. Use nominal source voltage and the expected load current for a quick resistance and power estimate. Alternatively, enter a resistor value and its expected power to estimate voltage and current limits.

Always keep practical tolerances in mind. The Ohm’s Law Calculator returns mathematical values, while real resistors, batteries, wires and power supplies have tolerances, heat limits and maximum ratings. Select components with an appropriate safety margin and check the manufacturer’s data sheet. The Ohm’s Law Calculator should support, not replace, those specifications.

Frequently asked questions

What values can I enter?

Enter exactly two of voltage, current, resistance and power. The calculator selects the correct formula for that pair.

Can I use milliamps and kilo-ohms?

Yes. Select the appropriate unit beside each input; all calculations are converted internally to volts, amps, ohms and watts.

Why must values be positive?

This tool is designed for magnitude calculations. Zero or negative values would create undefined divisions or require circuit-direction conventions outside its scope.

Does Ohm’s law apply to AC circuits?

It applies to purely resistive AC loads using compatible RMS values. Reactive circuits require impedance and phase information.

Is electrical power always V times I?

For direct current, yes. For alternating current, real power may also depend on power factor.

Can this replace professional electrical design?

No. Use it for education and initial planning, then follow equipment specifications, safety standards and qualified professional advice.