Ohm's Law Calculator

What is Ohm's Law?

Ohm's Law is the fundamental principle of electrical circuits. It states that the voltage (V) across a conductor equals the product of the current (I) flowing through it and its resistance (R). Stated simply: V = I × R. This relationship, discovered by Georg Simon Ohm in 1827, underlies virtually all circuit analysis and electrical engineering work.

The Four Ohm's Law Formulas

All four electrical quantities are interrelated:

• Voltage: V = I × R
• Current: I = V / R
• Resistance: R = V / I
• Power: P = V × I = I² × R = V² / R

If you know any two values, you can calculate the other two.

How to Use This Calculator

1. Select the quantity you want to solve for (Voltage, Current, Resistance, or Power).
2. Enter any two of the remaining known values.
3. Results update automatically — no button needed.

Practical Examples

Example 1: A 12V battery connected to a 6Ω resistor. Current = 12 / 6 = 2 A. Power = 12 × 2 = 24 W.

Example 2: An LED needs 20 mA at 2V forward voltage, powered from 5V. Resistor = (5 − 2) / 0.02 = 150 Ω.

When to Use Ohm's Law

Ohm's Law applies wherever resistive loads exist: selecting fuse ratings, calculating resistor values for LED circuits, sizing cables, analysing battery discharge, and troubleshooting any DC circuit. Note that it does not directly apply to reactive components (capacitors, inductors) at AC frequencies — use impedance (Z) in those cases.

Common Mistakes

• Mixing units (mA vs A, kΩ vs Ω) without converting first.
• Applying Ohm's Law to non-ohmic devices (diodes, transistors) where resistance varies with voltage.
• Forgetting to account for source impedance in real circuits.

Key Terms

Voltage (V): Electrical potential difference, measured in volts. Current (I): Flow of electric charge, measured in amperes. Resistance (R): Opposition to current flow, measured in ohms (Ω). Power (P): Rate of energy conversion, measured in watts.