Published June 28, 2026

Ohm's Law Explained: Voltage, Current, Resistance, and Power

Ohm's Law is one of the most fundamental principles in electrical work. Whether you're in the field troubleshooting a circuit or sizing components in the office, understanding how voltage, current, resistance, and power relate to each other is essential. Here's what you need to know.

The Four Variables

Ohm's Law involves four quantities that describe the behavior of any electrical circuit:

E — Electromotive Force (Voltage): The electrical pressure that pushes current through a circuit, measured in volts (V).
I — Intensity (Current): The flow of electrons through the circuit, measured in amperes (A).
R — Resistance: The opposition to current flow in a circuit, measured in ohms (Ω).
P — Power: The rate at which electrical energy is consumed or produced, measured in watts (W).

The Basic Relationships

The core of Ohm's Law is simple: voltage equals current multiplied by resistance (E = I × R). From this one relationship — combined with the power formula P = E × I — you can derive all 12 possible formulas for calculating any one of the four variables when you know two others.

For example, if you know the voltage and resistance of a circuit, you can calculate the current. If you know the current and resistance, you can calculate the voltage. Every combination is covered.

Why 12 Formulas?

With four variables and the ability to solve for any one using two known values, there are exactly 12 unique formula combinations — three for each variable. For voltage alone: E = I × R, E = P ÷ I, and E = √(P × R). The same logic applies to current, resistance, and power.

Practical Field Applications

Troubleshooting circuits: If you measure voltage and current, you can calculate the resistance of a load and compare it to nameplate specs to identify a fault.
Sizing heaters and resistive loads: Knowing the wattage and voltage, you can calculate the current draw to ensure the circuit is properly sized.
Checking motor loads: Power calculations help verify that a motor is drawing the expected current under load.
Verifying control circuits: Low-voltage control circuits are easily analyzed using Ohm's Law to confirm relay coil currents and voltage drops.

Common Mistakes

One of the most frequent errors is confusing power (watts) with apparent power (volt-amps) in AC circuits. Ohm's Law in its basic form applies to purely resistive DC circuits. In AC circuits with inductive or capacitive loads, power factor comes into play, and the relationships become more complex. For most field calculations involving resistive loads, however, the basic formulas hold up well.

Use our Ohm's Law Calculator to solve for any unknown — voltage, current, resistance, or power. Enter any two known values in one formula row and get all four results instantly.

The Bottom Line

Ohm's Law isn't just textbook theory — it's a tool you use every time you size a circuit, troubleshoot a load, or verify a calculation. Knowing all 12 formulas and when to use each one is a mark of a well-rounded electrician.

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