Ohm's Law Calculator

Calculate voltage, current, resistance, or power from any two known values using Ohm's Law and power formulas.

Enter any two values. The calculator solves for the remaining two.

Voltage (V)

Current (A)

Voltage (V)

12 V

Current (A)

2 A

Resistance (Ω)

6 Ω

Power (W)

24 W

Formula

V = V (given)
I = I (given)
R = V / I
P = V × I

This calculator is for educational purposes only. Always consult a licensed electrician and local electrical codes before installation or repair.

How It Works

1
Pick the two values you know
Choose any two of voltage (V), current (A), resistance (Ω), or power (W). These are the values you measured, read from a datasheet, or fixed by design.
2
Enter them in the selectors
Type the numeric values and choose the matching unit label for each. The calculator blocks invalid combinations, for example picking the same variable twice.
3
Read all four results and the formula
The calculator fills in the remaining two variables and shows the derived formula it used, so you can check the work by hand.

Ohm's Law: The Core Relationship That Powers Every Circuit

Georg Ohm published his law in 1827 and was immediately attacked for it. Critics called the work "a web of naked fancies," the Prussian Minister of Education declared him unworthy to teach science, and he resigned his teaching post. Twenty years later the physics establishment caught up. The Royal Society awarded him the Copley Medal in 1841, and by the 1850s V = I × R was on every electrical engineer's desk. It still is. The law is simple: current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. Combined with the power formula P = V × I, the four values voltage, current, resistance, and power lock together. Any two fix the other two. That relationship lets you size a resistor for an LED, calculate the power dissipation in a heating element, work out a battery's run time, or check whether a fuse will blow before the wiring melts. Ohm's Law is not universal. It holds for ohmic conductors such as copper, aluminum, and most metals at stable temperatures, and modern experiments have confirmed it down to silicon wires four atoms wide and one atom high. It breaks down for semiconductor diodes, transistors, tungsten filaments whose resistance climbs with heat, varistors, and any device deliberately engineered to be non-linear. Knowing which side of that line your component falls on is the difference between a design that works and one that doesn't.

Common pitfalls

Applying V = IR to a diode or LED. A silicon diode's current roughly doubles for every 60 mV above V_f, so a supply held 0.2 V over the 2 V forward drop can push four times the rated current and destroy the junction. Use the LED resistor calculator instead.
Forgetting that resistance rises with temperature. A tungsten incandescent filament's cold resistance is roughly 1/10 of its hot resistance, so cold inrush current is 10x the steady-state number. Copper climbs about 0.4% per degree C, which shifts a 75 C conductor about 22% above its 20 C resistance.
Plugging peak AC voltage into P = V x I. For sinusoidal AC, use RMS values. V_peak x I_peak / 2 = V_rms x I_rms only for purely resistive loads; for motors or power supplies you must also multiply by power factor, so P = V x I x PF.
Treating 'watts' as the only power number on an AC circuit. The utility's wiring and transformer must carry apparent power S = V x I (volt-amperes), not just real power P. A 0.7 PF motor pulling 1000 W draws 1430 VA from the panel.

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R. Combined with the power formula P = V × I, any two of the four values — voltage, current, resistance, power — determine the other two.

When does Ohm's Law not apply?

Ohm's Law holds for ohmic conductors (most metals at constant temperature). It does not apply to non-ohmic devices such as diodes, transistors, filaments whose resistance changes strongly with temperature, or any semiconductor where current is not linear in voltage.

How is power related to voltage, current, and resistance?

Electrical power in watts is given by P = V × I. Substituting Ohm's Law produces two derived forms: P = I² × R and P = V² / R. These forms are equivalent — use whichever uses the values you already know.

Can I use this for AC circuits?

Yes, but voltages and currents must be RMS values, and the result represents apparent power (volt-amperes) unless you're working with a purely resistive load. For reactive loads (motors, transformers), you must include the power factor: P = V × I × PF.

What if I enter impossible values?

The calculator flags physically impossible combinations — for example, zero voltage with non-zero power, or two values of the same type. It also handles special cases like zero current (open circuit → infinite resistance) gracefully.

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