Ohm's Law Calculator

Fill in any two of voltage, current, resistance and power and the other two follow. There are six ways to pick two from four and this handles all of them, telling you which rearrangement it used. Every answer is checked against all three identities, because V = IR, P = VI and P = I squared R have to agree.

Fill in any two boxes. The other two follow.

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How this Ohm's law calculator works

Fill in any two of voltage, current, resistance and power, and the other two follow. There are exactly six ways to pick two from four, and this page handles all of them, telling you which rearrangement it used rather than just handing back numbers. Every answer also carries a free correctness check, because the three identities V = IR, P = VI and P = I2R all have to agree, and if they ever did not you would want to know.

The formula

V = IR = P ÷ I = √(PR)
I = V ÷ R = P ÷ V = √(P ÷ R)
R = V ÷ I = V² ÷ P = P ÷ I²
P = VI = I²R = V² ÷ R

This is the power wheel, and every line is the same relationship viewed from a different corner. V is voltage in volts, I current in amps, R resistance in ohms and P power in watts. The reason there are twelve expressions rather than four is that Ohm's law (V = IR) and the power law (P = VI) can be substituted into each other in every direction, which is exactly why knowing any two quantities pins down all four.

Worked example

A 120 V supply across a 240 ohm heater element. I = V ÷ R = 120 ÷ 240 = 0.5 A, and P = V2 ÷ R = 14,400 ÷ 240 = 60 W. Those two numbers describe a perfectly ordinary 60 watt incandescent bulb.

Now the part the textbook version leaves out. That 240 ohms is the resistance hot, at around 2,500 degrees. Measure the same bulb cold with a multimeter and you will read about 20 ohms, because tungsten's resistance rises steeply with temperature.

So at the instant you flip the switch, the bulb briefly draws 120 ÷ 20 = 6 A, twelve times its running current, until the filament heats up in a few tens of milliseconds. That inrush is why bulbs almost always fail at switch-on rather than partway through an evening, and it is why a cold resistance reading looks so wrong against the numbers on the box.

P = I squared R is the one to memorise

Of the twelve expressions above, one earns its keep more than the rest. Power dissipated in a resistance goes as the square of the current. Double the current and you quadruple the heat. That single relationship explains most of what goes wrong in practical electrical work.

It is why an undersized wire overheats while a longer one at the same current does not get hotter per foot. It is why a loose connection is dangerous out of all proportion to its size: the resistance of the bad joint might be a fraction of an ohm, but all of the circuit's current passes through it, and the heat lands in one spot rather than spread along a run. It is why voltage drop costs real money on a long run. And it is why high-voltage transmission exists at all: pushing the same power at a hundred times the voltage means a hundredth of the current, and therefore a ten-thousandth of the loss in the line.

Ohm's law is not a law

Newton's laws describe how everything behaves. Ohm's law describes how some materials behave, and calling it a law is a historical accident that misleads a lot of people. A material is called ohmic if its resistance stays constant as voltage changes, which metals do rather well at a fixed temperature. Plenty of things do not.

A diode passes essentially nothing below about 0.6 V and then passes essentially anything, so its resistance is not a number at all. An LED behaves the same way, which is precisely why an LED wired straight across a battery destroys itself and why it always needs a current-limiting resistor. A thermistor changes resistance with temperature on purpose, and is used as a sensor for exactly that reason. A filament lamp is ohmic at any fixed temperature and wildly non-ohmic across its operating range, as the worked example shows.

None of this makes the calculator wrong; it makes it a snapshot. The numbers here describe the circuit at the operating point you specified. For a resistor, a heating element or a length of wire, that snapshot is the whole story and stays true. For anything with a junction in it, it is one point on a curve, and moving along that curve is what the rest of electronics is about.

Frequently asked questions

What is Ohm's law?

Voltage equals current times resistance, or V = IR. Rearranged, current is voltage divided by resistance and resistance is voltage divided by current. Combined with the power law P = VI it gives twelve expressions in total, the so-called power wheel, which is why knowing any two of the four quantities fixes all four.

How do I calculate watts from volts and amps?

Multiply them: P = VI. A 120 V circuit drawing 0.5 A uses 60 W. If you have resistance instead of one of those, use P = V squared divided by R, or P = I squared times R. All three give the same answer, which is what the consistency check on every result here is confirming.

Why does P = I squared R matter more than the other formulas?

Because power goes as the square of current, so doubling the current quadruples the heat. That one relationship explains most practical electrical trouble: why an undersized wire overheats, why a loose connection is dangerous out of all proportion to its size, and why high-voltage transmission exists. Pushing the same power at a hundred times the voltage means a hundredth of the current and a ten-thousandth of the loss in the line.

Why is a light bulb's cold resistance so different from its hot one?

Because tungsten's resistance rises steeply with temperature. A 60 W 120 V bulb has a hot resistance of 240 ohms at around 2,500 degrees, but reads about 20 ohms cold on a multimeter. At the instant you flip the switch it therefore draws around 6 A rather than its running 0.5 A, a twelve-fold inrush that lasts a few tens of milliseconds. That surge is why bulbs almost always fail at switch-on rather than partway through use.

Does Ohm's law always work?

No, and the name oversells it. It describes materials that are ohmic, meaning their resistance stays constant as voltage changes, which metals do well at a fixed temperature. A diode passes almost nothing below about 0.6 V and then almost anything, so it has no single resistance. An LED behaves the same way, which is why it needs a current-limiting resistor and destroys itself without one. A thermistor changes resistance deliberately. For those, this calculator describes one point on a curve rather than the whole device.

How do I find resistance from power and voltage?

R equals voltage squared divided by power. A 60 W device on 120 V has a resistance of 14,400 divided by 60, which is 240 ohms. Going the other way, from power and current, R equals power divided by current squared. Fill in whichever two you actually know above and the page picks the right rearrangement.

What happens if resistance is zero?

That is a short circuit, and Ohm's law returns infinity, which is the equation's honest way of saying the question is now about the supply rather than the circuit. Real current in a short is limited by the source's internal resistance and by the wiring, and it is the job of a fuse or breaker to interrupt it before something melts. The same applies at the other extreme: an open circuit has infinite resistance and zero current, which is why no resistance can be worked out from a voltage alone.

Can I use this for AC circuits?

For purely resistive AC loads, yes, using RMS values for voltage and current, which is what any AC meter reads. For anything with motors, transformers or electronic supplies you also need power factor, because volts times amps then gives apparent power in VA rather than real power in watts. Our watts to amps calculator handles that distinction, and reactance also means the relevant quantity becomes impedance rather than plain resistance.

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