Calculate voltage, current, or resistance instantly using Ohm's Law (V = I × R) — just enter any two values.
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Ohm's Law is one of the most fundamental laws in electricity, stating that voltage (V) equals current (I) multiplied by resistance (R), i.e., V = I × R. This calculator lets you find any one of the three quantities (voltage, current, or resistance) if the other two are known, just by selecting what you want to find from the dropdown menu. This law is used in practice in designing electrical and electronic circuits, determining the appropriate value of components (such as resistors), and diagnosing electrical faults by measuring any two quantities and finding the third mathematically without needing to measure it directly. First formulated by Georg Simon Ohm in 1827 based on careful experimental measurements, this deceptively simple linear relationship between voltage, current, and resistance remains the single most frequently used equation in all of electrical engineering, underlying nearly every circuit calculation from the simplest single-resistor circuit to complex multi-component networks.
Georg Simon Ohm's 1827 discovery that voltage, current, and resistance relate through a simple linear equation might be the single most consequential finding in the early history of electrical science, precisely because of how broadly and reliably it applies across an enormous range of practical circuits.
The law's power comes from its predictive simplicity: knowing any two of the three quantities immediately gives the third through basic algebra, without needing to physically measure the remaining value. This is genuinely useful in practice — a technician troubleshooting a circuit can measure voltage and current at a test point, calculate the expected resistance, and immediately spot whether a component's actual resistance matches its specified value or has drifted or failed.
Circuit designers use Ohm's Law constantly when selecting component values: knowing the supply voltage and the current a circuit needs to draw (perhaps to properly power an LED, a sensor, or a motor) directly gives the resistance value needed in that branch of the circuit, which is the starting point for choosing an actual resistor from the standard values manufacturers produce.
It's worth noting that Ohm's Law, in its basic V=IR form, applies precisely to what are called 'ohmic' components — those, like standard resistors, where resistance stays constant regardless of the voltage or current applied. Many real components (diodes, transistors, and LEDs among them) are genuinely non-ohmic, meaning their effective resistance changes with operating conditions, which is exactly why calculating an LED's resistor value requires the more involved LED-specific formula rather than simple Ohm's Law alone.
Despite these exceptions, Ohm's Law remains the essential starting point for virtually all circuit analysis, since even circuits containing non-ohmic components are typically analyzed by combining Ohm's Law for the ohmic parts (resistors) with the specific characteristic equations for the non-ohmic parts — making V=IR the foundational building block that every more advanced circuit analysis technique ultimately builds upon.
It's used to relate voltage, current, and resistance in a circuit, letting engineers and hobbyists calculate any one of the three values when the other two are known — essential for designing and troubleshooting circuits.
It applies directly to purely resistive circuits (DC or AC). For circuits with capacitors or inductors, impedance replaces simple resistance, and the Power Factor and RC Circuit calculators on this site can help with those cases.
Dividing by zero resistance or zero current is mathematically undefined, and a real 0 Ω short circuit or 0 A open circuit represents an edge case outside this simple formula's normal use.