Solve voltage, current, resistance and power — enter any two.
Calculated instantly in your browser.
How is Ohm's Law calculated?
V = I × R and P = V × I. From any two known quantities the remaining two can be derived, for example R = V ÷ I and P = I² × R. So 12 V across 2 A gives 6 Ω of resistance and 24 W of power. Make sure units are consistent: volts, amps, ohms and watts.
Understanding your result
Make sure units are consistent: volts, amps, ohms and watts.
Formula and method
V = I × R, P = V × I. From any two quantities the remaining two can be derived (for example R = V ÷ I and P = I² × R).
Assumptions and limitations
It applies Ohm's law and the power relationship to simple direct-current situations with a constant resistance. It assumes an ideal linear resistor and consistent SI units, and does not account for alternating-current effects such as impedance, reactance, phase or components whose resistance changes with temperature.
Worked example
12 V across 2 A gives 6 Ω of resistance and 24 W of power.
How to use this tool
- Enter any two known values.
- Leave the rest blank.
- Press Calculate.
Common mistakes to avoid
- Entering milliamps as amps (1 mA = 0.001 A).
About the Ohm's Law Calculator
Ohm's Law links voltage, current and resistance. Enter any two known values and this calculator finds the others, plus power.
Who should use this tool
Electronics hobbyists, students learning circuit basics, and technicians sizing components who know two of voltage, current and resistance and want the remaining quantities, plus power, worked out from Ohm's law without rearranging formulas by hand.
Benefits
- Finds the two unknowns from any two known values
- Also calculates power alongside voltage, current and resistance
- Saves rearranging Ohm's law and the power formula manually
- Runs free in the browser with nothing to install
Practical use cases
- Choosing a resistor to limit current through an LED
- Checking the power a component will dissipate
- Learning how voltage, current and resistance relate
- Sizing a supply for a known load and current
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Frequently asked questions
How many values do I need?
Exactly two. The calculator derives the remaining quantities from them.
How many values do I need to enter?
You need exactly two of the four quantities, voltage, current, resistance or power. From any two, the calculator derives the remaining two using V equals I times R and P equals V times I. With 12 volts across 2 amps, for example, it returns 6 ohms and 24 watts.
How is electrical power calculated from Ohm's law?
Power equals voltage times current, P equals V times I. Because voltage is current times resistance, you can also write power as current squared times resistance, or voltage squared divided by resistance. The tool uses whichever pair of known values you provide.