Ohm's Law Calculator: Volts, Amps, Ohms and Watts
Solve Ohm's law from any two of voltage, current, resistance and power, or convert amps, watts and kVA for DC, single-phase and three-phase circuits with a power factor.
Fill in exactly two values; the other two are calculated.
Results are ideal-circuit estimates: they ignore wire resistance, reactance and losses. In three-phase circuits the voltage is the line-to-line voltage.
About the Ohm's Law Calculator
The Ohm's Law Calculator works out the four basic quantities of a simple circuit: voltage, current, resistance and power. Type any two of them and the other two appear straight away, in the unit you pick for each field. A second mode, the power converter, turns amps into watts, watts into amps, kVA into amps, amps into kVA and kVA into kW for DC, single-phase AC and three-phase AC circuits, with a power factor for AC loads.
Results are ideal-circuit estimates. The calculator assumes a purely resistive load and perfect wires, so it ignores the resistance of the cable, inductive and capacitive reactance, and losses. Use it to size a resistor, check a fuse or estimate the current a heater draws, not to certify an installation.
How to use it
Ohm's law mode
- Choose Ohm's law.
- Type exactly two values, for example the voltage and the power of an appliance.
- Pick a unit for every field, including the empty ones: the answers are shown in those units. Voltage takes V, mV or kV, current A or mA, resistance Ω, kΩ or MΩ, and power W, mW or kW.
If one, three or four fields are filled, the calculator asks you to fill in exactly two. Every value must be greater than zero.
Power converter mode
Choose Power converter, pick a direction and the circuit type, then type the amount and the voltage. For AC circuits, type the power factor too: it starts at 1, which is a purely resistive load such as a kettle. DC has no power factor, so the field is hidden. In a three-phase circuit the voltage is the line-to-line voltage, such as 400 V or 480 V.
The formulas
Ohm's law and the power law tie the four quantities together:
The converter uses the power relations for each circuit, with PF the power factor:
Real power in kW is the apparent power in kVA times the power factor.
Worked examples
- A 1000 W heater on 230 V. The current is 1000 ÷ 230 = 4.34783 A and the resistance is 230² ÷ 1000 = 52.9 Ω.
- An LED string on 9 V drawing 20 mA. The resistance is 9 ÷ 0.02 = 450 Ω and the power is 9 × 0.02 = 0.18 W, or 180 mW.
- A 2 A load using 100 W. The voltage is 100 ÷ 2 = 50 V and the resistance is 100 ÷ 2² = 25 Ω.
- A 50 kVA three-phase supply at 400 V. The line current is 50,000 ÷ (√3 × 400) = 72.1688 A.
Precision
Values show six significant digits with grouped thousands. The calculation itself keeps full precision, so rounding never builds up between fields.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
The Work Calculator relates energy, force and distance, and the Scientific Notation Converter helps with very small and very large values such as microamps or megohms. To size heating and cooling loads in BTU, try the BTU Calculator.
Frequently asked questions
What is Ohm's law?
Ohm's law says the voltage across a resistor equals the current through it times its resistance, V = I × R. Combined with the power law P = V × I, any two of the four quantities give the other two.
Why do I have to fill in exactly two fields?
Two values fix a simple resistive circuit completely. With one value the answer is unknown, and with three or four the values could contradict each other, so the calculator asks you to clear the extras.
Which units can I use?
Voltage takes V, mV or kV, current takes A or mA, resistance takes Ω, kΩ or MΩ, and power takes W, mW or kW. Choose the unit of an empty field before reading its answer.
Why can a value not be zero?
A zero resistance or current would divide by zero or describe a short or open circuit, which has no meaningful answer. Every value must therefore be greater than zero.
Are the results exact for real circuits?
They are ideal-circuit estimates. Real wires add resistance, AC loads add reactance, and components heat up and change value, so measured figures differ slightly from the calculator's answer.
How does the power converter handle three-phase circuits?
It uses the line-to-line voltage and multiplies by the square root of three, so P = √3 × V × I × PF. Choose Three-phase AC and the voltage field is labelled line-to-line voltage.