🔌 Ohm’s Law Calculator
Solve V = I × R and P = V × I from any two values, or convert between watts, amps and volts for DC and AC circuits.
Quick answer: Ohm’s law: V = I × R, and power P = V × I. With 12 V across a 6 Ω resistor, the current is 2 A and the power is 24 W.
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Ohm’s Law Calculator inputs
Result
Current and Power
2 A · 24 W
From voltage 12 V and resistance 6 Ω
| Voltage (given) | 12 V |
| Current | 2 A |
| Resistance (given) | 6 Ω |
| Power | 24 W |
Ohm’s law formula wheel
| Find | From V & I | From V & R | From V & P | From I & R | From I & P | From R & P |
|---|---|---|---|---|---|---|
| Voltage V | – | – | – | I × R | P ÷ I | √(P × R) |
| Current I | – | V ÷ R | P ÷ V | – | – | √(P ÷ R) |
| Resistance R | V ÷ I | – | V² ÷ P | – | P ÷ I² | – |
| Power P | V × I | V² ÷ R | – | I² × R | – | – |
Watts, amps and volts by circuit type
| Circuit | Watts | Amps | Volts |
|---|---|---|---|
| DC | V × I | W ÷ V | W ÷ I |
| AC single-phase | V × I × PF | W ÷ (V × PF) | W ÷ (I × PF) |
| AC three-phase | √3 × V × I × PF | W ÷ (√3 × V × PF) | W ÷ (√3 × I × PF) |
How to use the Ohm’s law calculator
In Ohm’s law mode, fill in any two of voltage, current, resistance and power and leave the other two blank. In watts ↔ amps ↔ volts mode, choose DC, single-phase or three-phase AC, pick what to find and enter the other two values (plus power factor for AC).
The formulas
V = I × R · I = V ÷ R · R = V ÷ I · P = V × I = I² × R = V² ÷ R
AC single-phase: P = V × I × PF · Three-phase: P = √3 × V(line) × I × PF
Worked examples
Ohm’s law: 12 V across a 6 Ω resistor gives I = 12 ÷ 6 = 2 A and P = 12 × 2 = 24 W.
Watts to amps (AC): a 1,500 W load on 120 V with a power factor of 0.9 draws 1,500 ÷ (120 × 0.9) = 13.89 A. On a 400 V three-phase supply with PF 0.85, a 15 kW motor draws 15,000 ÷ (1.732 × 400 × 0.85) ≈ 25.5 A.
Common values
| Appliance (resistive) | Watts | Amps at 120 V | Amps at 230 V |
|---|---|---|---|
| LED bulb | 10 W | 0.08 A | 0.04 A |
| Laptop charger | 65 W | 0.54 A | 0.28 A |
| Microwave | 1,000 W | 8.3 A | 4.3 A |
| Kettle / space heater | 1,500 W | 12.5 A | 6.5 A |
| Electric oven | 3,000 W | 25 A | 13 A |
Safety tips
- Size wires and breakers for the current with a safety margin – typically no more than 80% of a breaker’s rating for continuous loads.
- Mains electricity is dangerous: leave fixed wiring to a qualified electrician.
Frequently asked questions
What is Ohm’s law?
Ohm’s law states that the current through a conductor is proportional to the voltage across it: V = I × R, where V is volts, I is amps and R is ohms. Combined with P = V × I it links all four electrical quantities.
How do I convert watts to amps?
For DC, divide watts by volts: 1,500 W ÷ 120 V = 12.5 A. For single-phase AC also divide by the power factor, and for three-phase divide by √3 × volts × power factor.
How many amps is 1,500 watts at 230 volts?
About 6.5 A for a resistive load such as a kettle or heater (1,500 ÷ 230). At 120 V the same appliance draws 12.5 A.
What is power factor?
Power factor (0–1) is the ratio of real power (W) to apparent power (VA) in an AC circuit. Resistive loads have a PF of 1; motors and some electronics are typically 0.8–0.95.
Does Ohm’s law work for AC?
For purely resistive loads, yes. With motors, capacitors or inductors you need impedance instead of resistance and must include the power factor when converting between watts and amps.