Ohm's Law Calculator
Solve for voltage, current, resistance or power — enter any two of the four.
Results update as you type.
About this calculator
Ohm's law is the single most-used relationship in electronics: the voltage across a resistor equals the current through it times its resistance, V = I × R. Rearranged, the current is I = V / R and the resistance is R = V / I. Power — the rate at which the component turns electrical energy into heat or work — adds a second relationship, P = V × I, which combines with Ohm's law to give the two equivalents P = I² × R and P = V² / R. Together these four quantities form the V–I–R–P 'Ohm's law wheel', and knowing any two of them fixes the other two.
This calculator lets you pick which quantity to solve for and enter any two of the others, then returns the full wheel plus the three equivalent power formulas side by side. For example, a 12 V supply driving a 6 Ω resistor gives I = 12 / 6 = 2 A and P = 12 × 2 = 24 W; check it with P = I² × R = 4 × 6 = 24 W and P = V² / R = 144 / 6 = 24 W, all agreeing. Because the tool back-fills every missing value, entering power and current instead is just as valid — 24 W at 2 A returns 12 V and 6 Ω.
Ohm's law holds exactly for any DC resistive circuit and for the resistive part of an AC circuit, which makes it the everyday tool for sizing current-limiting resistors, choosing LED droppers, working out fuse and wire ratings, reading how much a heater or motor will draw, and predicting the heat a resistor must dissipate. It does not by itself describe capacitors, inductors or non-ohmic parts like diodes, where the voltage–current relationship is no longer a straight line.
Frequently asked questions
What is Ohm's law?
Ohm’s law states that the current through a conductor is proportional to the voltage across it: V = I × R. Rearranged, I = V / R and R = V / I.
How do I find electrical power from Ohm’s law?
Power is voltage times current, P = V × I. Using Ohm’s law you can also write it as P = I² × R or P = V² / R — all three give the same watts.
Which two values do I enter?
Pick the quantity you are missing in "Solve for", then enter any two of the other three. To find resistance you can supply voltage and current, or power and current.
Can I use power instead of voltage or current as an input?
Yes. The advanced power field lets you solve from power plus one other quantity — for example, a 24 W load at 2 A returns 12 V and 6 Ω, using V = P / I and R = P / I².
Does Ohm’s law work for AC circuits?
It applies directly to the resistive part of an AC circuit. For circuits with capacitors or inductors you replace resistance with impedance, and power picks up a power factor, but a plain resistor obeys V = I × R at every instant.
Why do my resistor gets hot — how much power does it dissipate?
A resistor turns electrical energy into heat at P = I² × R watts. A 6 Ω resistor carrying 2 A dissipates 4 × 6 = 24 W, so it needs a power rating comfortably above that. Undersizing the wattage is the usual cause of scorched or failed resistors.
Does this handle non-linear parts like diodes or LEDs?
No. Ohm’s law assumes a constant resistance, so it does not model diodes, LEDs or transistors whose current rises non-linearly with voltage. For an LED you instead size a series resistor from the supply voltage minus the LED forward voltage.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/ohms-law/curl
curl "https://calculator.free/api/v1/ohms-law/?solve=r&voltage=12¤t=2&resistance=6"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/ohms-law/?" + new URLSearchParams({
"solve": "r",
"voltage": "12",
"current": "2",
"resistance": "6"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.