Fill in any two of Voltage, Current and Resistance to solve for the rest, plus Power.
Fill in exactly 2 of the 3 fields — the 3rd (and Power) is solved for.
Ohm's Law (V = I × R) has three variables, and any two of them are enough to solve for the third — the calculator needs exactly two filled fields to determine which one to solve for and compute Power from the result.
Solving for current requires dividing by resistance, and solving for resistance requires dividing by current — if the field you're solving around would require dividing by zero, the calculator shows an error instead of an undefined result.
Power can be derived three equivalent ways once V, I and R are all known: P = V × I, P = I² × R, or P = V² / R. The "All Ohm's Law Forms" section shows all of them calculated from your result so you can see they agree.