Ohms Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law (V=IR) and the power formula (P=VI).
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How to Use This Calculator
Enter any two of the four electrical values: voltage (V), current (I in amps), resistance (R in ohms), or power (P in watts). The calculator solves for the remaining two values using Ohm's Law and the power formula. For example, enter 120V and 15A to find that the resistance is 8 ohms and the power is 1800W. Or enter 60W and 120V to find that the circuit draws 0.5A through a 240 ohm load. The calculator supports both DC circuits and simplified AC calculations. For AC circuits with reactive components, use the impedance rather than resistance for accurate results. This is the most fundamental electrical calculation and serves as the basis for nearly every other calculator on this site.
Understanding the Concept
Ohm's Law is the foundation of electrical theory and every electrician uses it daily, whether consciously or not. Georg Ohm established that the current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to the resistance of the conductor. This relationship, V = I x R, connects the three fundamental electrical quantities. Combined with the power formula P = V x I, these two equations can be rearranged to solve for any electrical quantity when two others are known. Electricians use Ohm's Law to determine circuit current for breaker sizing, calculate voltage drop across loads, troubleshoot circuits by measuring voltage and current to find unexpected resistance (indicating loose connections or damaged conductors), and verify that equipment is operating within its rated parameters. Understanding Ohm's Law deeply allows an electrician to diagnose problems that less experienced workers cannot. A voltage reading at a receptacle that is lower than expected, combined with a current measurement, immediately reveals whether the problem is excessive circuit resistance from a bad connection or an overloaded conductor.
The Formula Explained
The core Ohm's Law formula is V = I x R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. This rearranges to I = V / R and R = V / I. The power relationships are P = V x I (watts equals volts times amps), P = I squared x R, and P = V squared / R. These six formulas form the complete Ohm's Law wheel. In AC circuits, impedance (Z) replaces resistance (R) when reactive components like inductors or capacitors are present. Impedance combines resistance and reactance: Z = square root of (R squared + X squared), where X is the reactance. For purely resistive loads like heaters and incandescent lamps, resistance and impedance are equal. All values assume steady state conditions. These relationships are described in fundamental electrical theory and apply throughout the NEC wherever circuit calculations are required.
Frequently Asked Questions
What is Ohm's Law in simple terms?
Ohm's Law states that voltage equals current multiplied by resistance (V = I x R). Think of it like water flowing through a pipe: voltage is the water pressure, current is the flow rate, and resistance is how narrow the pipe is. Higher pressure (voltage) pushes more water (current) through the pipe. A narrower pipe (higher resistance) reduces the flow for the same pressure. This simple relationship lets you calculate any one of the three values when you know the other two.
How do I use Ohm's Law to find the current in a circuit?
Divide the voltage by the resistance: I = V / R. For example, a 120V circuit with a 12 ohm heater element draws 120 / 12 = 10 amps. In practice, electricians often use this to verify circuit loading. If you know the supply voltage (120V or 240V) and the equipment resistance from the nameplate or measurement, you can calculate the expected current and compare it to the breaker rating and conductor ampacity.
What is the relationship between watts, volts, and amps?
Power in watts equals voltage multiplied by current: P = V x I. This is the power formula that works alongside Ohm's Law. A 1500W space heater on a 120V circuit draws 1500 / 120 = 12.5 amps. This is critical for determining how many devices can share a circuit. A 20A, 120V circuit can supply a maximum of 2400W (20 x 120), and with the 80% continuous load rule, no more than 1920W of continuous load.
Does Ohm's Law work for AC circuits?
Ohm's Law applies to AC circuits, but resistance must be replaced with impedance for circuits containing inductors or capacitors. For purely resistive loads like heaters, toasters, and incandescent lights, resistance and impedance are the same and Ohm's Law applies directly. For motors, transformers, and electronic equipment, impedance includes both resistance and reactance, and the current may not be in phase with the voltage. For most practical electrician calculations on branch circuits, using Ohm's Law with resistance gives sufficiently accurate results.
Why is Ohm's Law important for electricians?
Ohm's Law is the basis for troubleshooting, circuit design, and code compliance. Electricians use it to calculate expected current for breaker and wire sizing, determine voltage drop across long conductors, diagnose high resistance connections by comparing measured voltage and current to expected values, and verify equipment is operating within specifications. An electrician who understands Ohm's Law can quickly identify whether a problem is caused by low supply voltage, excessive resistance in the circuit, or an overloaded conductor.