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- Enter any two of voltage, current, resistance or power.
- Get the other two instantly.
- Shows the formulas used.
- Formats answers with the right units, such as mA, kΩ or mW.
How to use the Ohm’s law calculator
Choose which two values you know
For example voltage and resistance.
Enter them
Volts, amps, ohms or watts.
Read all four values
Voltage, current, resistance and power.
What is Ohm’s law?
Ohm’s law says the current through a conductor equals the voltage across it divided by its resistance. Together with the power formula, it links the four basic electrical quantities, so any two give you the other two.
Ohm’s law and power formulas
V = I × R
I = V ÷ R, R = V ÷ I
P = V × I = I² × R = V² ÷ R
V in volts, I in amps, R in ohms, P in watts.
Example 1: 12 V across 6 Ω
Find current and power.
- Current 2 A
- Power 24 W
Example 2: a 60 W bulb on 120 V
Find current and resistance.
- Current 500 mA
- Resistance 240 Ω
Example 3: 20 mA through 220 Ω
An LED resistor.
- Voltage across the resistor 4.4 V
- Power 88 mW
Where Ohm’s law is used
Electronics
Choose resistors for LEDs and circuits.
Home wiring
Estimate current draw of appliances.
Physics class
Solve circuit problems quickly.
Car and solar
Size wires and fuses for 12 V systems.
This calculator is for learning and planning. Work on mains electricity should be done by a qualified electrician.
Ohm’s law calculator FAQ
What is Ohm’s law formula?
V = I × R: voltage equals current times resistance.
How do I find current from voltage and resistance?
Divide voltage by resistance. 12 V ÷ 6 Ω = 2 A.
How do I find power?
Multiply voltage by current. 12 V × 2 A = 24 W.
How much current does a 60 W bulb draw on 120 V?
60 ÷ 120 = 0.5 A, or 500 mA.
Does Ohm’s law work for AC?
For purely resistive loads, yes. Motors and electronics with reactance need more complex formulas.
What units should I use?
Volts, amps, ohms and watts. For milliamps, enter 0.02 for 20 mA.