What Is Ohm's Law?
Ohm's Law describes the relationship between voltage, current and resistance in an electrical circuit: V = I × R. Voltage (V, in volts) is the electrical "pressure," current (I, in amps) is the flow of charge, and resistance (R, in ohms) opposes that flow. This Ohm's Law calculator solves for whichever value is missing when you enter the other two, and it also calculates the power used in watts.
Ohm's Law Formulas
Example: a 12 V battery connected to a 4 Ω resistor drives a current of 12 ÷ 4 = 3 A, and the resistor dissipates 12 × 3 = 36 W of power.
- Voltage: V = I × R
- Current: I = V ÷ R
- Resistance: R = V ÷ I
- Power: P = V × I = I² × R = V² ÷ R
Where Ohm's Law Is Used
Electricians, engineers, hobbyists and students use Ohm's Law every day. Common tasks include choosing the right resistor for an LED, sizing wires and fuses, troubleshooting circuits and checking whether a component can handle the power it will dissipate.
Frequently Asked Questions
How do I pick a resistor for an LED?
Subtract the LED's forward voltage from the supply voltage, then divide by the desired current in amps. For a 5 V supply, a 2 V LED and 20 mA: R = (5 − 2) ÷ 0.02 = 150 Ω.
Does Ohm's Law work for AC circuits?
It applies directly to purely resistive AC loads such as heaters and incandescent bulbs. Circuits with motors, coils or capacitors involve impedance, so a more advanced form of the law is needed.
What is the Ohm's Law triangle?
It is a memory aid with V on top and I and R on the bottom. Cover the value you want: covering V leaves I × R, covering I leaves V ÷ R, and covering R leaves V ÷ I.