Ohm's Law Calculator
Enter any two of voltage, current, resistance and power, and the calculator returns the other two with their units. That covers watts to amps and amps to watts at a known voltage, and the resistance of a load from its wattage.
How to use the Ohm's Law Calculator
- Enter your numbers in the Ohm's Law Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
Ohm's Law and the power equation
V = I × R · · · P = V × I = I² × R = V² / R · · · I = P / V · · · R = V² / P
V is voltage (volts), I is current (amps), R is resistance (ohms), P is power (watts). The power equations follow from Ohm's Law by substitution. Pick the two values you know and the calculator rearranges these equations for the other two. From power and resistance it takes square roots, V = √(P × R) and I = √(P / R). Zero works too: 0 A through 6 Ω gives 0 V. When a formula would divide by zero, that result shows as no finite value, and 5 V across 0 Ω is refused, since both of its results divide by zero. Resistance and power can't be negative. A negative voltage or current carries its sign into the results, so -12 V across 4 Ω gives -3 A and 36 W.
Worked example
A 12 V battery connected through a 4 Ω resistor: I = V/R = 12/4 = 3 A. Power dissipated = V × I = 12 × 3 = 36 W. Or equivalently I²R = 9 × 4 = 36 W.
Frequently asked questions
How do I convert watts to amps?
Divide the watts by the volts. A 1,500 W space heater on a 120 V outlet draws 1,500 / 120 = 12.5 A. Pick Watts + volts above, enter 1500 and 120, and the calculator shows 12.5000 A and the heater's resistance, 9.6000 Ω. Amps to watts is the reverse. Pick Volts + amps, enter 120 and 12.5, and you get 1500.0000 W.
Does Ohm's Law apply to AC circuits?
For purely resistive AC circuits, yes (using RMS values). For circuits with inductors or capacitors, impedance Z replaces R: V = I × Z, and Z is complex (resistance + reactance).
What is the difference between voltage and current?
Voltage is electrical potential difference — the "pressure" pushing current. Current is the flow of charge — the actual amount of electrons moving. Voltage causes current to flow through resistance.
Why does power equal I²R?
Combining P = V×I and V = I×R: P = (I×R) × I = I²R. The squared term means doubling current quadruples power dissipated (and heat generated).