Ohms Law Calculator

Solve for voltage, current, resistance, and power from any two known values using Ohm's Law (V = IR) and the power equation (P = IV).

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Ohm's Law calculator Enter any 2 of the 4 values below to solve for the remaining 2 using Ohm's Law and the power equation.
Enter any two values Provide any 2 of voltage, current, resistance, or power to solve for the remaining values.

Basic Circuits

Ohm's Law: solve voltage, current, resistance, and power from any two values

Enter any two of voltage, current, resistance, or power and instantly solve for the remaining two using Ohm's Law and the electrical power equation. Ideal for circuit design, component selection, and troubleshooting.

How Ohm's Law relates voltage, current, and resistance

Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, with the constant of proportionality being the resistance. The relationship is expressed as V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

This fundamental law applies to any resistive DC circuit element and is the starting point for most circuit analysis. By rearranging the formula, you can solve for any one of the three quantities when the other two are known.

V = I × R

Voltage equals current multiplied by resistance.

I = V / R

Current equals voltage divided by resistance.

R = V / I

Resistance equals voltage divided by current.

The power equation and its relationship to Ohm's Law

Electrical power measures the rate at which energy is converted in a circuit. The basic power equation is P = I × V. By substituting Ohm's Law into the power equation, two additional forms emerge: P = I² × R and P = V² / R.

These combined relationships mean that knowing any two of the four quantities — voltage, current, resistance, and power — is enough to calculate the other two. This calculator automates that process for all six possible input pairs.

P = I × V

Power equals current multiplied by voltage.

P = I² × R

Power equals current squared multiplied by resistance.

P = V² / R

Power equals voltage squared divided by resistance.

Practical applications

Use these calculations to verify that a resistor can handle the power dissipated through it, to size a power supply for a known load, to check that wire gauge is adequate for the expected current, or to determine the resistance needed to limit current in an LED circuit.

For example, if you know a 12 V supply drives a 100 ohm load, the calculator shows the current is 0.12 A and the power dissipated is 1.44 W, which helps you choose a resistor rated for at least 1.44 watts.

Limitations of this calculator

Ohm's Law applies to purely resistive DC circuits. It does not account for reactance in AC circuits, non-linear components such as diodes and transistors, or temperature-dependent resistance changes.

For AC circuit analysis involving capacitors or inductors, impedance calculations are needed instead. This calculator is intended for estimation and educational use, not as a substitute for professional circuit design validation.

Frequently asked questions

What is Ohm's Law and when does it apply?

Ohm's Law (V = IR) describes the linear relationship between voltage, current, and resistance in a purely resistive circuit. It applies to metallic conductors and standard resistors at constant temperature but does not hold for non-linear components like diodes or semiconductors.

 Can I use this calculator for AC circuits?

This calculator solves for DC resistive circuits. In AC circuits, impedance replaces resistance and includes reactive components from capacitors and inductors. The basic V = IR relationship still applies if you substitute impedance for resistance, but phase angles and complex arithmetic are involved.

 Why does the calculator need exactly two inputs?

The system of Ohm's Law and the power equation has four unknowns. Two independent equations require exactly two known values to produce a unique solution for the remaining two. Providing fewer leaves the system under-determined; providing more can create contradictions.

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