Ohm's Law Calculator

Ohm's Law, named after German physicist Georg Ohm who published it in 1827, describes the relationship between voltage, current, and resistance in an electrical circuit: V = I × R. Voltage (V, measured in volts) is the electrical pressure that pushes current through a circuit. Current (I, measured in amperes) is the rate of flow of electric charge. Resistance (R, measured in ohms) is how much a component opposes that flow. The law states that the current through a conductor is directly proportional to the voltage across it, and inversely proportional to its resistance.

A common way to visualize Ohm's Law is to compare an electrical circuit to a water system. Voltage is like water pressure — the force pushing water through a pipe. Current is like the flow rate — how much water passes a point per second. Resistance is like the pipe's narrowness — a narrower pipe resists flow more, requiring more pressure to achieve the same flow rate. Just as more pressure or a wider pipe increases water flow, more voltage or less resistance increases electrical current.

Electrical power (P, measured in watts) is the rate at which electrical energy is converted — into heat, light, motion, or other forms. It's calculated as P = V × I. Combining this with Ohm's Law (V = I × R) produces two more useful forms: substituting V = I × R into P = V × I gives P = I² × R, and substituting I = V ÷ R gives P = V² ÷ R. Together, these relationships mean that if you know any two of voltage, current, resistance, or power, you can calculate the other two — which is exactly what this calculator does.

Because V = I × R and P = V × I can be combined and rearranged in different ways, there are twelve total formulas relating voltage, current, resistance, and power, often drawn as a 'wheel' for quick reference. Given voltage and current: R = V÷I and P = V×I. Given voltage and resistance: I = V÷R and P = V²÷R. Given voltage and power: I = P÷V and R = V²÷P. Given current and resistance: V = I×R and P = I²×R. Given current and power: V = P÷I and R = P÷I². Given resistance and power: V = √(P×R) and I = √(P÷R).

Ohm's Law holds precisely for 'ohmic' components — resistors, wires, and other conductors whose resistance stays constant regardless of the voltage or current applied, at a given temperature. Many real-world components are 'non-ohmic': diodes, transistors, light bulb filaments (whose resistance rises as they heat up), and other semiconductor devices have a current-voltage relationship that isn't a straight line. For these components, Ohm's Law is still useful as an approximation or at a specific operating point, but it doesn't describe their full behavior the way it does for a simple resistor.