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Ohm's Law Calculator

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Two equations tie together everything a resistive circuit does: V = IR relates voltage, current and resistance, and P = VI adds power to the set. Between them they mean that knowing any two of the four fixes the other two exactly. Fill in the two you know, in whatever prefix your meter reads them in, and this works out the rest — showing which rearrangement it used, and what wattage of resistor the answer calls for.

Fill in any two boxes and leave the other two empty.

Try:

How to use the ohm's law calculator

  1. 1Fill in exactly two of the four boxes and leave the other two empty.
  2. 2Choose the unit beside each value — milliamps and amps are a factor of a thousand apart, and mixing them up is the commonest source of a wrong answer here.
  3. 3Press Calculate.
  4. 4Read all four quantities in the grid, and the rearranged formula underneath showing how each unknown was found.
  5. 5Check the suggested resistor wattage before you fit anything: it allows twice the calculated dissipation, which is the headroom a resistor needs to stay in tolerance.

Examples

A resistor on a bench supply

Input
12 V across 470 Ω
Result
25.53 mA, 306.4 mW — fit a 1 W resistor

A quarter-watt part would be running at 122% of its rating here, which is how resistors end up discoloured and out of tolerance.

Working backwards from a measured current

Input
5 V, 20 mA
Result
250 Ω, 100 mW

The classic indicator LED case, though a real LED is not a resistor — see the LED resistor calculator for the version that accounts for forward voltage.

A heating element from its rating

Input
230 V, 2 kW
Result
8.696 A, 26.45 Ω

Appliance ratings give voltage and power, so this is the pair that finds the current a circuit has to carry.

About the ohm's law calculator

The four quantities, and what each one physically is

Voltage is the push: the difference in electrical potential between two points, measured in volts. It is always a difference between two places, which is why a multimeter has two probes and why asking for the voltage at a single point means nothing until a reference is named. Current is the flow that results, measured in amperes — roughly six quintillion electrons past a point each second, at one amp. Resistance is how much the material objects to that flow, in ohms.

Power is the fourth, and it is the one that burns things. It is the rate at which electrical energy turns into something else — heat in a resistor, light and heat in a lamp, movement and heat in a motor. Every one of those includes heat, which is why the wattage figure matters more than beginners expect: a circuit that works perfectly on paper can still fail in seconds if a component is quietly turning a watt into warmth in a package designed to shed a quarter of one.

Ohm's law itself dates from Georg Ohm's work in 1827, and it is worth being precise about what it claims. It is not a universal law of nature in the way conservation of energy is — it is a description of how certain materials behave, and plenty of things disobey it. A diode, a transistor and a filament lamp all have a resistance that changes with the conditions, which is why this page describes a resistor and not those.

Where the arithmetic stops describing the circuit

The most common surprise is a filament lamp. A 60 W incandescent bulb measures around 80 Ω cold, which by these equations would draw nearly three amps from a 230 V supply and dissipate 660 W. It does not, because within a few milliseconds of switch-on the filament reaches white heat and its resistance rises by a factor of ten or more. The steady-state resistance is what matters in operation, and the cold value is why filament lamps fail at the moment you turn them on rather than while they are running.

The second is temperature more generally. Copper gains about 0.4% resistance per degree Celsius, so a cable that is comfortable at room temperature is measurably more resistive when it has been carrying current for an hour. Over a long run this is enough to matter, which is why the voltage drop calculator on this site takes a conductor temperature rather than assuming one.

The third is that none of this addresses safety. These formulas will tell you what current a load draws and how much heat a resistor makes, and they will do it just as cheerfully for a figure that would melt a cable or kill somebody. Anything involving mains voltage is work for a qualified electrician in most jurisdictions, and the arithmetic being correct is not the same as the installation being safe.

Frequently asked questions

Why does it insist on exactly two values?
Because two is what the mathematics needs, and three is what makes the answer ambiguous. With two knowns there is precisely one solution for the remaining pair. With three, the values you typed may not satisfy the equations at all — 5 V, 1 A and 10 Ω describe no circuit that exists — and there would be no way to know which of the three you actually measured and which you mistyped. Refusing is more useful than silently picking two of them.
What happens if I enter a resistance of zero?
You get an explanation rather than a number, because none exists. A perfect short has no current predicted by these equations — the division has a zero denominator. What limits the current in a real short is the internal resistance of the supply and the resistance of the wire itself, and neither of those is anywhere in this model. Enter the actual resistance of the path, however small it is.
Does this work for mains and other AC circuits?
For a purely resistive load, yes, provided you use RMS values — which is what a multimeter shows and what a supply is rated at. Heaters, filament lamps and kettles behave this way. Anything with a coil or a large capacitance does not: motors, transformers, fluorescent ballasts and switch-mode supplies draw current out of step with the voltage, so the true power is lower than volts times amps by the power factor. This page does not model that, and using it on a motor will overstate the power.
Why suggest a resistor wattage larger than the calculated power?
A part run at its full rating gets hot, and heat is what pushes a resistor out of specification and eventually opens it. Manufacturers' derating curves start reducing the permitted dissipation above about 70 °C ambient, so a component sitting inside a warm enclosure has less headroom than its label suggests. Doubling is the standard bench habit, and it costs a fraction of a penny.
Which formula does it use for power?
Whichever one avoids a quantity you did not give. Power can be written three ways — VI, I²R and V²/R — and they agree exactly, so the choice is only about not needing an intermediate result. Entering current and resistance uses I²R directly rather than finding the voltage first and multiplying, which keeps the working shown on screen as short as the arithmetic actually is.

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