Ohm's Law Calculator

Ohm's Law Calculator - Solve for Voltage, Current, Resistance & Power

Ohm's Law is the single most fundamental relationship in electrical circuits, connecting voltage, current, and resistance in a simple proportional formula. Combined with the power equations, knowing any two of the four values - voltage, current, resistance, or power - lets you calculate the other two, which is essential for everything from choosing the right resistor for an LED circuit to sizing a home appliance's wiring.

Understanding the Relationship Between the Three Core Quantities

Voltage (measured in volts) is the electrical "pressure" pushing current through a circuit. Current (measured in amperes) is the rate of flow of electric charge. Resistance (measured in ohms) opposes that flow. Ohm's Law ties them together: for a fixed resistance, increasing voltage increases current proportionally; for a fixed voltage, increasing resistance decreases current proportionally. This is why a circuit with high resistance - like a thin wire or a resistor - allows less current to flow at the same voltage than a circuit with low resistance.

Bringing Power Into the Picture

Power, measured in watts, tells you the rate at which electrical energy is converted into heat, light, or motion. Because power depends on both voltage and current, there are three equivalent ways to calculate it depending on which two values you already know: P = V × I when you know voltage and current directly, P = I²R when you know current and resistance, or P = V²/R when you know voltage and resistance. All three formulas always agree with each other for the same circuit.

Practical Uses of Ohm's Law

• Choosing a resistor for an LED: Given your supply voltage, the LED's forward voltage, and its rated current, Ohm's Law tells you exactly what resistor value limits the current to a safe level.
• Sizing wiring and fuses: Knowing the voltage and the resistance (or power draw) of an appliance tells you the current it will draw, which determines the wire gauge and fuse rating needed.
• Diagnosing circuit faults: If a circuit draws far more or less current than expected for its voltage, Ohm's Law helps identify whether resistance has unexpectedly changed - for example, from a short circuit (very low resistance, high current) or a broken connection (very high resistance, near-zero current).
• Estimating appliance running costs: Power (watts) calculated from voltage and current is the same figure used to estimate electricity costs - see the Electricity Bill Calculator to take it a step further.

A Note on AC Circuits and Impedance

This calculator applies directly to DC circuits and to purely resistive AC circuits (like incandescent bulbs or heating elements). For AC circuits containing inductors or capacitors - such as motors or electronic power supplies - the simple resistance term is replaced by impedance, which also accounts for the phase relationship between voltage and current. The underlying V = IR relationship still holds, just with impedance (Z) in place of resistance (R).