Ohm's Law Explained: V = IR Formula, Examples, and Calculator

By Jorge Sanchez · April 20, 2026 · 5 min read

Bottom line: Ohm's Law: Voltage (V) = Current (I) × Resistance (R). If a circuit has 12V and 4Ω resistance, the current is 3A.

Ohm's Law is the foundation of electrical engineering. It describes the relationship between voltage, current, and resistance in a circuit — and lets you calculate any one if you know the other two.

The Three Forms of Ohm's Law

Find Voltage

V = I × R

Find Current

I = V ÷ R

Find Resistance

R = V ÷ I

The Variables Explained

Symbol	Quantity	Unit	What It Means
V	Voltage	Volts (V)	Electrical pressure pushing current through the circuit
I	Current	Amperes (A)	Rate of electron flow (how much electricity is moving)
R	Resistance	Ohms (Ω)	Opposition to current flow

Worked Examples

Example 1: Find Current

A 9V battery is connected to a 3Ω resistor. What is the current?

I = V ÷ R = 9 ÷ 3 = 3 A

Example 2: Find Voltage

A circuit draws 2A of current through a 50Ω resistor. What is the voltage?

V = I × R = 2 × 50 = 100 V

Example 3: Find Resistance

A 120V circuit carries 5A of current. What is the resistance?

R = V ÷ I = 120 ÷ 5 = 24 Ω

Power Formula (Extension of Ohm's Law)

Combine Ohm's Law with the power formula P = V × I to get:

P = V × I

P = I² × R

P = V² ÷ R

Example: A 60W light bulb on a 120V circuit draws I = P ÷ V = 60 ÷ 120 = 0.5 A and has resistance R = V ÷ I = 120 ÷ 0.5 = 240 Ω.

Real-World Applications

Fuse sizing: Calculate current draw to choose the right fuse
Speaker wiring: Match speaker impedance to amplifier output
LED circuits: Calculate resistor value to limit current
Home electrical: Understand why high-wattage appliances need dedicated circuits
Battery life: Estimate how long a battery will power a device

Ohm's Law Calculator

Enter any two values (V, I, or R) to solve for the third. Also calculates power.

Use the Ohm's Law Calculator →

Jorge Sanchez · Live Event Production Specialist · CalQpro