Ohm's Law Calculator
Enter any two of voltage, current, resistance and power to get the other two
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Enter any two of voltage, current, resistance and power, and the other two are calculated with Ohm's law and the power formulas, with the working shown. It also turns power and time into energy in Wh and kWh.
How to use
- On "V, I, R, P", type values in two of "Voltage", "Current", "Resistance" and "Power".
- Choose the unit next to each box, or type a prefix such as 4k7 or 20m.
- The other two values and the "Working" appear at once, and the "Formula wheel" highlights the formulas used. "Copy results" copies them.
- For energy, open "Energy (Wh, kWh)" and enter the power and the time.
How it works
- Every formula comes from two relations, V = I × R and P = V × I. That is why P = I² × R = V² ÷ R as well.
- Energy = power × time, with 1 Wh = 3,600 J and 1 kWh = 1,000 Wh. With a "Supply voltage (optional)", the ideal current and charge drawn (mAh) are shown too.
- For learning and low-voltage electronics. It is not for choosing wires, breakers or anything in household wiring.
Questions and answers
- What is the formula for Ohm's law?
- Voltage = current × resistance (V = I × R). With 12 V across a 4 Ω resistor, the current is 12 ÷ 4 = 3 A.
- How do I calculate power?
- Power = voltage × current. At 12 V and 2 A it is 24 W. If you know the resistance, I² × R or V² ÷ R also work.
- How do I convert watts to amps?
- Current = power ÷ voltage. A 60 W load on 12 V draws 5 A. Without the voltage, the current cannot be found from watts alone.
- How do I calculate kWh?
- Multiply the power in kW by the hours of use. A 60 W device used for 5 hours uses 300 Wh, which is 0.3 kWh.