About this calculator
Ohm's law links three things in an electrical circuit: voltage (V), current (I) and resistance (R). With power (P) added, knowing any two of the four lets you work out the other two.
Fill in any two boxes below and leave the others empty. The calculator finds the rest and shows the formulas it used.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
12 V across 4 Ω
- Current
- 3 A
- Voltage
- 12 V
- Resistance
- 4 Ω
- Power
- 36 W
12 V and 4 Ω give a current of 3 A and a power of 36 W.
Show the working
- Ohm's law links voltage (V), current (I) and resistance (R): V = I × R. Power is P = V × I.
- Using the two values you gave, the formulas needed are I = V ÷ R and P = V² ÷ R.
- That gives current = 3 A and power = 36 W.
2 A of current and 100 W of power
- Voltage
- 50 V
- Current
- 2 A
- Resistance
- 25 Ω
- Power
- 100 W
2 A and 100 W give a voltage of 50 V and a resistance of 25 Ω.
Show the working
- Ohm's law links voltage (V), current (I) and resistance (R): V = I × R. Power is P = V × I.
- Using the two values you gave, the formulas needed are V = P ÷ I and R = P ÷ I².
- That gives voltage = 50 V and resistance = 25 Ω.
A 2,000 W kettle on 230 V
- Current
- 8.6957 A
- Voltage
- 230 V
- Resistance
- 26.45 Ω
- Power
- 2,000 W
230 V and 2,000 W give a current of 8.6957 A and a resistance of 26.45 Ω.
Show the working
- Ohm's law links voltage (V), current (I) and resistance (R): V = I × R. Power is P = V × I.
- Using the two values you gave, the formulas needed are I = P ÷ V and R = V² ÷ P.
- That gives current = 8.6957 A and resistance = 26.45 Ω.
10 V and 2 A
- Resistance
- 5 Ω
- Voltage
- 10 V
- Current
- 2 A
- Power
- 20 W
10 V and 2 A give a resistance of 5 Ω and a power of 20 W.
Show the working
- Ohm's law links voltage (V), current (I) and resistance (R): V = I × R. Power is P = V × I.
- Using the two values you gave, the formulas needed are R = V ÷ I and P = V × I.
- That gives resistance = 5 Ω and power = 20 W.
The formulas
Ohm's law: V = I × R, so I = V ÷ R and R = V ÷ I.
Power: P = V × I. Combined with Ohm's law this gives two more forms: P = I² × R and P = V² ÷ R.
| You know | Current I | Resistance R | Voltage V | Power P |
|---|---|---|---|---|
| V and I | · | V ÷ I | · | V × I |
| V and R | V ÷ R | · | · | V² ÷ R |
| V and P | P ÷ V | V² ÷ P | · | · |
| I and R | · | · | I × R | I² × R |
| I and P | · | P ÷ I² | P ÷ I | · |
| R and P | √(P ÷ R) | · | √(P × R) | · |
A real example: an electric kettle
A kettle rated at 2,000 W on a 230 V supply draws a current of I = P ÷ V = 2,000 ÷ 230 = 8.7 amps, and its heating element has a resistance of R = V² ÷ P = 230² ÷ 2,000 = 26.45 ohms. Try it in the examples above.
Units
Voltage is in volts (V), current in amps (A), resistance in ohms (Ω) and power in watts (W). For milliamps, divide by 1,000 first: 20 mA is 0.02 A. For kilo-ohms, multiply by 1,000: 2.2 kΩ is 2,200 Ω.
Limits, and a safety note
These formulas describe simple resistive circuits with steady (DC) current. Alternating current circuits with coils or capacitors involve impedance and power factor, which this calculator doesn't cover. It's for learning and for low-voltage projects; never work on mains electricity unless you're qualified to.
Frequently asked questions
What is Ohm's law?
The relationship V = I × R: the voltage across a resistor equals the current through it multiplied by its resistance.
How do I calculate current?
Divide the voltage by the resistance: I = V ÷ R. 12 V across 4 Ω gives 3 A. If you know the power instead, I = P ÷ V.
How do I calculate power in a circuit?
Multiply voltage by current: P = V × I. You can also use P = I² × R or P = V² ÷ R.
Why does it only take two values?
Any two of voltage, current, resistance and power fix the other two. Entering three would let them contradict each other, so the calculator asks you to clear one.
Does this work for AC circuits?
Only for simple resistive loads such as heaters and incandescent bulbs. Circuits with motors, coils or capacitors need impedance and power factor.
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.