Ohm's Law Calculator - Online Voltage Current Resistance Power

Ohm's Law is a fundamental principle in electrical engineering that describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. It states that the current flowing through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance. The law was named after German physicist Georg Simon Ohm, who first published it in 1827.

The three core Ohm's Law formulas are:

• V = I × R — Voltage equals Current multiplied by Resistance
• I = V ÷ R — Current equals Voltage divided by Resistance
• R = V ÷ I — Resistance equals Voltage divided by Current

Additionally, power (P) can be calculated using: P = V × I, P = I² × R, or P = V² ÷ R.

The Ohm's Law triangle is a visual memory aid. Place your finger over the value you want to find:

• Cover V → see I × R (multiply I and R to get V)
• Cover I → see V ÷ R (divide V by R to get I)
• Cover R → see V ÷ I (divide V by I to get R)

The horizontal line represents division, and the vertical line between I and R represents multiplication.

No, Ohm's Law primarily applies to ohmic conductors — materials where current is directly proportional to voltage at a constant temperature. Many common conductors (like copper wire and resistors) follow Ohm's Law under normal conditions. However, non-ohmic components like diodes, transistors, LEDs, and semiconductors do not follow Ohm's Law because their resistance changes with voltage, current, or temperature. The law also assumes the conductor's temperature remains constant.

When resistance approaches zero (as in superconductors), even a tiny voltage can theoretically produce an infinite current (I = V ÷ 0). In practical circuits, a zero-resistance condition is called a short circuit, which can cause dangerously high current, overheating, and damage to components. Real-world wires have very small but non-zero resistance. Our calculator will show an error if you try to divide by zero — always ensure resistance has a positive, non-zero value when using Ohm's Law calculations.

Power in an electrical circuit can be calculated using three derived formulas from Ohm's Law:

• P = V × I — Use when you know voltage and current
• P = I² × R — Use when you know current and resistance
• P = V² ÷ R — Use when you know voltage and resistance

All three formulas yield the same result. Power is measured in Watts (W) and represents the rate at which electrical energy is converted to heat, light, or mechanical energy.

Standard SI units are:

• Voltage: Volts (V) — also millivolts (mV = 0.001V), kilovolts (kV = 1000V)
• Current: Amperes (A) — also milliamps (mA = 0.001A), microamps (µA = 0.000001A)
• Resistance: Ohms (Ω) — also milliohms (mΩ = 0.001Ω), kilohms (kΩ = 1000Ω), megohms (MΩ = 1,000,000Ω)
• Power: Watts (W) — also milliwatts (mW = 0.001W), kilowatts (kW = 1000W)

Our calculator supports all these units — simply select your preferred unit from the dropdown next to each input field.

Ohm's Law is essential for:

• Choosing resistors: Calculate the right resistor value to limit current for LEDs and other components
• Circuit design: Determine voltage drops, current draw, and power dissipation
• Troubleshooting: Diagnose faulty components by measuring voltage and current
• Safety: Ensure components operate within their rated power limits
• Battery life: Calculate how long a battery will last under a given load

Understanding Ohm's Law is the foundation of all electronics and electrical engineering work.