Does Ohm's Law work for AC circuits?

For purely resistive AC loads, yes - use RMS values for V and I. For circuits with capacitance or inductance, you need impedance (Z) instead of resistance (R), and the math becomes complex (literally - phasors and complex numbers). This calculator covers the resistive case.

Why are there four power formulas?

They are all algebraic substitutions of P = V × I. If you know V and R, use V² ÷ R. If you know I and R, use I² × R. If you know V and I, use V × I directly. The formula you pick depends on which two values you have.

Will this be on my journeyman exam?

Yes. The Texas journeyman exam includes basic Ohm's Law and power calculations. Practice solving them by hand - the PSI exam does not let you bring a calculator app, only a basic calculator they provide.

Ohm's Law Calculator

Enter any two electrical values to instantly calculate the other two. Covers voltage, current, resistance, and power.

Enter Any Two Values

Voltage (V)

Current (I)

Resistance (R)

Power (P)

Enter any two values to calculate the other two.

Ohm's Law Wheel

About this calculator

Ohm's Law is the relationship between voltage, current, and resistance in a circuit, plus the related power equation. Given any two of voltage, current, resistance, or power, the other two can be calculated. Every electrician learns these formulas in apprenticeship and they show up on every journeyman exam. This calculator solves all twelve permutations and shows the formula it used.

Formula

The base relationship is V = I × R. Power adds three more equations by substituting V or I. The full set is captured in the Ohm's Law wheel: any two known values give the other two.

V = I × R
P = V × I
P = I² × R
P = V² ÷ R

How to use

Pick which two values you know - any two of voltage, current, resistance, or power.
Enter the values in their respective fields.
The calculator solves for the remaining two and shows the equation it used.
Switch units between volts/millivolts, amps/milliamps, and ohms/kilohms as needed.

Worked example

Setup

A 12 V automotive circuit feeds a load with 4 ohms of resistance. What is the current and power dissipation?

Calculation

I = V ÷ R = 12 ÷ 4 = 3 A. P = V × I = 12 × 3 = 36 W.

Answer

The circuit draws 3 amps and dissipates 36 watts. A 5-amp fuse would protect this circuit, and the load resistor needs to be rated for at least 36 W (50 W gives a comfortable margin).

Related study guides

Frequently asked questions

Does Ohm's Law work for AC circuits?: For purely resistive AC loads, yes - use RMS values for V and I. For circuits with capacitance or inductance, you need impedance (Z) instead of resistance (R), and the math becomes complex (literally - phasors and complex numbers). This calculator covers the resistive case.
Why are there four power formulas?: They are all algebraic substitutions of P = V × I. If you know V and R, use V² ÷ R. If you know I and R, use I² × R. If you know V and I, use V × I directly. The formula you pick depends on which two values you have.
Will this be on my journeyman exam?: Yes. The Texas journeyman exam includes basic Ohm's Law and power calculations. Practice solving them by hand - the PSI exam does not let you bring a calculator app, only a basic calculator they provide.