Ohm's law states that the voltage across a resistor equals the current through it times the resistance: V = I × R. The Toolenza calculator solves the law for whichever quantity you don't know, and also returns power dissipation (P = V × I) — so a single calculation gives you all four electrical quantities a circuit-builder cares about.

The three forms of the same equation

Depending on what you know, rearrange:

• V = I × R — voltage from current + resistance
• I = V ÷ R — current from voltage + resistance
• R = V ÷ I — resistance from voltage + current
• P = V × I = I²R = V²/R — power, three equivalent forms

The "Ohm's triangle" mnemonic (V on top, I and R below) is the visual shortcut beginners use; the calculator does the algebra so you don't need it.

Real-world use cases

• Picking a series resistor for an LED. A red LED at 20 mA forward current with a 2 V forward drop, driven from 5 V: drop across the resistor is 3 V, so R = 3 V ÷ 0.02 A = 150 Ω. Power dissipated = 3 V × 0.02 A = 60 mW — well within a ¼-watt resistor's rating.
• Sizing a fuse. If a circuit draws 12 A at 12 V, you need a fuse rated above 12 A (typically 15 A with safety margin) and conductors rated for the same.
• Checking a circuit with a multimeter. Measure two of the three quantities and use the law to predict the third. Big mismatch = something wrong (cold solder joint, wrong resistor value, short).
• Power budget for a battery-powered project. Total current draw × supply voltage = power, which divided into battery watt-hours gives runtime.

Limits

Ohm's law applies to linear (ohmic) components — resistors, copper wire, most loads at normal temperatures. It breaks down for diodes, transistors, inductors and capacitors under AC, and any component whose resistance changes with current (light bulbs warming up, thermistors). For those, you need the device's I-V curve or impedance model.