What Ohm's law actually says
Ohm's law describes a fixed relationship between three things in any simple electrical circuit: the voltage pushing current round it, the current that actually flows, and the resistance standing in its way. Push harder (more voltage) and more current flows; add more resistance and less current flows for the same push. Georg Ohm published this relationship in 1827 after a careful series of experiments with wires of different lengths and thicknesses, and it has stayed the starting point for circuit analysis ever since. This calculator covers the three-way relationship and, since the three rarely turn up in isolation, the power a circuit uses or dissipates alongside them, so it takes any two of voltage, current, resistance or power and works out the other two.
The formula and its rearrangements
Ohm's law itself is one short equation, most often written as:
Rearranged, the same equation answers the other two questions it can be asked:
Power, the rate at which a circuit uses electrical energy, follows directly once any two of voltage, current and resistance are known. The plain definition is voltage multiplied by current, and substituting Ohm's law into that definition gives two further, equally valid versions:
Whichever two figures you actually have to hand decides which version does the work, which is exactly why this calculator lets you pick any two of the four rather than forcing you to have voltage and resistance specifically.
The Ohm's law wheel
Because voltage, current, resistance and power connect through only four short equations plus a definition, electronics teaching has long summarised the whole set as a single reference chart, usually called the Ohm's law wheel or Ohm's law pie chart: four quadrants, one per quantity, each listing the three ways to reach that quantity from whichever other two you know. The table above this section is exactly that wheel, laid out as rows instead of a circle. Look up the quantity you're missing, find the column that matches the two you already have, and the formula for the exact gap you have is right there rather than something to derive from scratch each time.
The water-pipe analogy
Electricity is invisible, which is why almost every introduction to it reaches for water flowing through a pipe. Voltage is the pressure pushing the water along, current is the rate the water actually flows past a point, measured in litres per second rather than volts and amps, and resistance is the narrowness of the pipe itself: a wide pipe lets water through easily at a given pressure, a narrow or partly blocked one holds it back and needs more pressure to shift the same flow. Squeeze the pipe (raise the resistance) at constant pressure and the flow drops; open a valve wider (lower the resistance) and it rises, exactly mirroring how more resistance cuts current at a fixed voltage and less resistance allows more. The analogy has its limits, electric current does not pool or compress the way water can, but for getting an intuitive feel for why the three quantities move together the way they do, it holds up well enough that it has survived in classrooms for well over a century.
Not everything obeys Ohm's law
It's worth being upfront that Ohm's law is a property of certain materials and components, not a universal law of physics that every circuit must follow. A component is called ohmic when its resistance stays roughly constant regardless of the voltage across it, so current rises in a straight line as voltage rises; plain resistors and lengths of metal wire, held at a steady temperature, behave this way closely enough for everyday calculations. Plenty of common components do not. An incandescent light bulb's filament heats up as current flows through it, and its resistance climbs sharply with temperature, so the current does not rise in a straight line with voltage the way a fixed resistor's would. LEDs and semiconductor diodes are more different still: they conduct almost no current at all below a threshold forward voltage, then current rises steeply and non-linearly once that threshold is passed, a curve no single resistance value describes. This calculator's arithmetic is exactly right for resistors, wiring and other ohmic loads; treat a result for a bulb's hot filament, an LED or any semiconductor as a rough, single-point estimate rather than a description of how it behaves across a range of voltages.
Choosing which two figures to enter
Component datasheets and battery labels are usually the quickest source for a figure to start from. A battery or power supply's rating is a voltage; a resistor's printed colour bands or stamped value give resistance directly; an appliance's rating plate or a component's datasheet often gives current or power outright rather than making you calculate it first. If you only have two of the four and they happen to be current and power rather than the more familiar voltage-and-resistance pairing, this calculator handles that combination exactly the same way, by picking the appropriate rearrangement rather than insisting on one particular pair.
The reference points in the table above are there to sanity-check whatever you enter: a torch bulb or hobby LED circuit runs on a battery in the 1.5 to 12 V range and draws current in the tens of milliamps, while anything wired into a wall socket sits at mains voltage, 230 V in the UK and much of Europe, 120 V in the US, and typically draws whole amps rather than milliamps. If your result lands far outside the range you expected for the kind of circuit you're picturing, it's usually a sign that a decimal point or a unit toggle, mA against A, or kΩ against Ω, slipped somewhere on the way in.
Reading the results panel
The headline names whichever of the four quantities is missing and gives its value; the line beneath it gives the other missing quantity and states which rearrangement of the formula actually produced both. Below that sits the full working with your own numbers substituted step by step, a table listing all four quantities side by side with which two you supplied and which two were derived, a chart placing your current against a couple of everyday examples, and the real-world reference table and Ohm's law wheel described above.
A note on safety
This calculator is built for electronics hobby projects, coursework and general learning, working with batteries, LEDs and low-voltage circuits on a bench. Anything involving mains wiring, the sockets and circuits actually built into a building, is a job for a qualified, registered electrician; mains voltage and fault current can kill, and no calculator substitutes for the training and testing that job requires.
Questions people ask
What's the difference between voltage, current and resistance?
Voltage is the electrical pressure pushing current round a circuit, current is the actual rate of flow of that electric charge, and resistance is how strongly a component opposes the flow. Ohm's law, V = I × R, is simply the fixed relationship between the three in an ohmic circuit.
Why does this calculator ask for power as well as the three Ohm's law quantities?
Because power, the rate a circuit uses electrical energy, follows directly once any two of voltage, current and resistance are known, and datasheets or rating plates often give power or current rather than making you calculate one from the other two first. Letting you pick any two of the four, rather than insisting on voltage and resistance specifically, covers that case directly.
What does it mean if my resistance comes out at zero?
Zero ohms describes a dead short: Ohm's law calls for infinite current at zero resistance, which is exactly the fault condition a fuse or circuit breaker is designed to interrupt. No real wire or component has exactly zero resistance in practice, so a genuine zero-ohm result almost always signals a wiring fault rather than a normal working state.
Does Ohm's law apply to every component?
No. It applies closely to ohmic components, plain resistors and wiring held at a steady temperature, where resistance stays roughly constant as voltage changes. LEDs, semiconductor diodes and a light bulb's hot filament are all non-ohmic: their resistance shifts with voltage, temperature or both, so a single resistance value only ever gives a rough, single-point estimate for them.
Can I use this for mains-voltage circuits?
The arithmetic works at any voltage, but actually working on mains wiring, the circuits built into a building, is a job for a qualified, registered electrician, not something to attempt from a calculator result. This tool is aimed at low-voltage hobby electronics, batteries and coursework.
Why do my current and resistance toggles show mA, kΩ and MΩ?
Everyday circuits span a wide range: a hobby LED draws current in the tens of milliamps while a phone charger draws whole amps, and resistors range from single ohms to millions of them. The mA/A and Ω/kΩ/MΩ toggles let you enter a figure in whichever unit it's actually printed in, without doing the conversion by hand first.
What's the water-pipe analogy actually comparing?
Voltage to water pressure, current to the rate water actually flows, and resistance to how narrow the pipe is. Squeezing the pipe at constant pressure cuts the flow, exactly as raising resistance at constant voltage cuts current; the comparison is a teaching aid rather than an exact physical equivalence.
This calculator covers Ohm's law and its power rule for ohmic components (resistors and plain wiring); it does not model non-ohmic parts such as LEDs, diodes or a lit filament, and it is not a substitute for qualified electrical work on mains circuits.