Ohm's Law Calculator
Enter any two of voltage, current, resistance and power, and get the other two with the formula used to reach them.
How it works
Ohm's law states that current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: V = I × R. Combined with the power relationship P = V × I, any two of the four quantities determine the other two.
That combination is why the calculator accepts any pair. Give it volts and amps and it derives resistance and watts; give it watts and ohms and it works back to volts and amps. The pair you enter decides which formula is applied, and the tool shows which one it used.
One caveat worth knowing: Ohm's law describes ohmic components — resistors, wire, heating elements. Diodes, LEDs, transistors and lamps are non-ohmic, so their resistance changes with the operating point and a single V/I figure only describes one instant, not the device.
The formula
Ohm's law
V = I × R
Power
P = V × I
Power from current and resistance
P = I² × R
Power from voltage and resistance
P = V² ÷ R
Worked examples
| Scenario | Working | Result |
|---|---|---|
| 12 V across a 6 Ω resistor | I = 12 ÷ 6, P = 12 × 2 | 2 A, 24 W |
| 0.5 A through 100 Ω | V = 0.5 × 100, P = 0.5² × 100 | 50 V, 25 W |
| A 100 W load at 20 V | I = 100 ÷ 20, R = 20² ÷ 100 | 5 A, 4 Ω |
When you'd use it
- Sizing a resistor for a known current
- Checking whether a component will exceed its power rating
- Working out the current a supply must deliver
- Converting between a load's resistance and its wattage
Common questions
Why do I need to enter two values?
Because one value alone is consistent with infinitely many circuits. 12 V could be driving a 1 Ω load or a 1 MΩ load. A second quantity fixes the operating point, and everything else follows from it.
Does Ohm's law work for LEDs and diodes?
Not usefully. Those are non-ohmic — their resistance changes with applied voltage, so V ÷ I gives you the resistance at one particular operating point and nothing more. For an LED, use the LED resistor calculator, which works from the forward voltage instead.
Is the power figure the heat the resistor has to dissipate?
Yes, for a resistor essentially all of it becomes heat. Pick a component rated for at least twice the calculated figure — running a resistor at its rated power makes it very hot and shortens its life.
Does this work for AC circuits?
For purely resistive AC loads, yes, using RMS values. Once capacitance or inductance is involved you need impedance rather than resistance, and real power depends on the power factor — this calculator does not model that.
Is anything I enter sent to a server?
No. Every calculation runs in your browser, so component values, circuit figures and measurements are never uploaded, logged or stored. There is no account and no record of what you typed.