Ohm's Law Calculator

The Formula

Ohm's LawThe relationship between voltage (V), current (I), and resistance (R): V = I × R. The fundamental law of electrical circuits. defines the fundamental relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. The power formula P = VI tells you how much energy a component dissipates.

Variable Definitions

How to Use This Calculator

1. 1

   Select what you want to solve for: voltage, current, resistance, or power.

2. 2

   Enter the two known values.

3. 3

   View the calculated value plus power dissipation.

4. 4

   Use the power result to check if a component is within its rated power tolerance.

Quick Reference

Common Applications

• Electrical troubleshooting — verify that components are operating within their rated voltage, current, and power specifications to diagnose failures
• Circuit design — calculate the correct resistor value for LED current limiting, voltage dividers, and sensor biasing networks
• Home electrical planning — determine whether a circuit can safely handle additional loads before installing new outlets, lights, or appliances
• Audio system setup — match amplifier output to speaker impedance (4Ω, 8Ω) to avoid overloading the amp or underpowering speakers
• Automotive electrical work — calculate fuse sizes, wire gauges, and load capacities for aftermarket accessories like lights, amplifiers, and winches

Ohm's Law triangle — Voltage = Current x Resistance

Understanding the Concept

Ohm's LawThe relationship between voltage (V), current (I), and resistance (R): V = I × R. The fundamental law of electrical circuits. (V = IR) is the foundational relationship in electronics and electrical engineering. It describes how voltage, current, and resistance interact in a linear resistive circuit. Understanding this relationship is essential for circuit design, troubleshooting, and safety. Power (P = VI) tells you how much energy a component consumes or dissipates as heat — critical for selecting appropriate wire gauges, fuses, and components. Real-world example: a 120V circuit powering a 60W light bulb draws I = P ÷ V = 60 ÷ 120 = 0.5A. The filament resistance is R = V ÷ I = 120 ÷ 0.5 = 240 Ω. If you replace the bulb with a 100W bulb, the current jumps to 0.83A and the resistance drops to 144 Ω. This is why using a higher-wattage bulb than the fixture is rated for creates a fire risk — the wire and socket may not handle the increased current. Another example: a 12V automotive circuit with a 3A fuse can safely power devices up to 36W (12V × 3A). Exceeding this causes the fuse to blow, which is designed to protect the wiring from overheating. In audio systems, speakers have impedance ratings (typically 4Ω or 8Ω). An amplifier rated for 100W into 8Ω delivers 200W into 4Ω, which can overheat the amplifier if it is not designed for 4Ω loads. The relationship V = IR explains why brownouts (reduced supply voltage) cause motors to draw higher current — the motor tries to maintain power output, and lower voltage forces higher current draw through the same winding resistance, potentially causing overheating.

Frequently Asked Questions

Sources & References