Enter any two of voltage, current, resistance and power and this free Ohm’s law calculator finds the other two, with the formula used. Two more modes combine resistors in series or parallel and turn an appliance’s wattage into units (kWh) and monthly electricity cost.
Fill in exactly two values.
Ohm’s law and power formulas
Worked examples
- A 1,000 W kettle on 230 V draws I = 1000 ÷ 230 = 4.35 A and has a hot resistance of 230² ÷ 1000 = 52.9 Ω, so a 6 A or larger fuse/breaker is needed.
- 100 Ω, 220 Ω and 470 Ω in series make 790 Ω; in parallel they make 60.0 Ω (always less than the smallest resistor).
- A 1,500 W air conditioner running 8 hours a day uses 12 kWh daily, about 360 units a month.
For AC circuits with motors or electronics, real power also depends on the power factor (P = V × I × PF), so measured current can be higher than this calculator shows for purely resistive loads.
Frequently asked questions
What is Ohm’s law?
Ohm’s law says the current through a conductor equals the voltage across it divided by its resistance: I = V / R. Combined with P = V × I, any two of voltage, current, resistance and power give the other two.
How do I convert watts to amps?
Divide watts by volts: I = P / V. A 2,000 W heater on 230 V draws about 8.7 A; on 120 V it draws about 16.7 A.
How do I calculate resistors in parallel?
Add the reciprocals of each resistance and take the reciprocal of the total: 1/R = 1/R₁ + 1/R₂ + … For two resistors, R = (R₁ × R₂) / (R₁ + R₂).
How do I calculate electricity units used?
Units (kWh) = watts × hours ÷ 1000. Multiply by your tariff per unit to estimate cost. Check your bill for the slab rate that applies.
Does it work for AC?
For resistive loads such as heaters, kettles and incandescent bulbs, yes. For motors and electronics, multiply by the power factor for real power.
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.