Ohm's Law Calculator

How to Use the Ohm's Law Calculator

The Ohm's Law Calculator solves for any electrical quantity when you provide two known values. Ohm's Law (V = I × R) and the power equation (P = V × I) are the foundational relationships in electrical engineering and electronics. Enter any two of voltage, current, resistance, or power to find the remaining values.

The Four Quantities

Voltage (V): Measured in volts (V). The electrical potential difference that drives current through a circuit. Household outlets provide 120V (US) or 230V (Europe).

Current (I): Measured in amperes/amps (A). The flow rate of electrical charge. A typical phone charger draws about 1-2A.

Resistance (R): Measured in ohms (Ω). The opposition to current flow. A standard resistor might be 1kΩ (1,000 ohms).

Power (P): Measured in watts (W). The rate of energy consumption. A 60W light bulb consumes 60 joules per second.

The Formulas

From Ohm's Law and the power equation, twelve formula variations exist:

V = I × R, V = P / I, V = √(P × R)

I = V / R, I = P / V, I = √(P / R)

R = V / I, R = V² / P, R = P / I²

P = V × I, P = I² × R, P = V² / R

Practical Example

A 12V battery powers a circuit with 4Ω resistance. Current: I = 12V / 4Ω = 3A. Power: P = 12V × 3A = 36W. This means 36 watts of power is dissipated in the resistor. If you know only current (3A) and power (36W), the calculator finds V = 36/3 = 12V and R = 12/3 = 4Ω.

Applications in Electronics

Use Ohm's Law to size resistors for LED circuits, calculate wire gauge requirements, determine fuse ratings, design voltage dividers, and analyze any DC circuit. It is the first tool every electronics student and hobbyist learns to use.

Frequently Asked Questions

Q: Does Ohm's Law apply to AC circuits?

A: For purely resistive AC loads, Ohm's Law applies directly using RMS values. For circuits with capacitors and inductors, impedance (Z) replaces resistance, and the relationship becomes V = I × Z. Impedance includes both resistance and reactance and is calculated using complex numbers.

Q: What is the difference between DC and AC resistance?

A: DC resistance is straightforward opposition to direct current. AC impedance includes resistance plus reactance from capacitors and inductors, which varies with frequency. At DC (0 Hz), a capacitor has infinite impedance and an inductor has zero impedance. This calculator focuses on DC and resistive AC circuits.

Q: How do I choose the right resistor for an LED?

A: Use R = (V_supply - V_LED) / I_LED. For a 5V supply with a standard red LED (2V forward voltage, 20mA current): R = (5-2) / 0.02 = 150Ω. Choose the next standard value up (150Ω) and verify the power: P = 0.02² × 150 = 0.06W. A 1/8W resistor works fine.