Ohm's law formula
Ohm's law links the voltage across a component to the current through it and its resistance. It is the most fundamental relationship in electronics, letting you find any one of voltage, current, or resistance when you know the other two.
What each symbol means
| Symbol | Meaning | Units |
|---|---|---|
| V | Voltage across the component | volts (V) |
| I | Current through it | amperes (A) |
| R | Resistance | ohms (Ω) |
Rearranged forms
Worked example
A 12 V supply drives a 4 Ω resistor. Find the current.
- Start from V = I R and solve for current: I = V / R.
- Substitute V = 12 V and R = 4 Ω.
- Divide: I = 12 / 4.
Use volts, amperes, and ohms together. Power follows directly: P = V I = I² R = V² / R, in watts. Ohm's law applies to resistive components; for capacitors and inductors in AC circuits, use impedance instead.
Solving for V, I, or R?
Enter any two values and the Ohm Law Calculator returns the third, plus the power dissipated.
How Ohm's law works
Voltage is the push, current is the flow, and resistance is what opposes that flow. The law says current is directly proportional to voltage and inversely proportional to resistance: raise the voltage and more current flows; raise the resistance and less flows. It holds for ordinary resistors across a wide range of conditions.
Where it is used
Ohm's law is behind almost every electronic calculation — sizing resistors, choosing wire, working out LED current limiting, checking power dissipation, and diagnosing faults. Combined with the power equations, it lets you design and troubleshoot circuits with just a few numbers.
FAQ
What is Ohm's law?
V = I R: the voltage across a resistor equals the current through it times its resistance. Rearranged, I = V / R and R = V / I.
How do I calculate power from Ohm's law?
Power in watts is P = V I. Using the law you can also write P = I² R or P = V² / R, whichever values you have.
Does Ohm's law work for AC?
For purely resistive loads, yes. For capacitors and inductors you replace resistance with impedance, which depends on frequency.