Ohm's law calculator

What is Ohm’s law?

Ohm’s law describes the relationship between the voltage across a conductor, the electric current flowing through it, and its electrical resistance. It states that, for many materials at a constant temperature, the current is directly proportional to the voltage and inversely proportional to the resistance. This simple but powerful rule, named after the German physicist Georg Ohm, sits at the heart of nearly every electrical and electronic circuit you will ever encounter.

This calculator lets you solve for any one of the three quantities once you know the other two. Choose whether you want to find voltage, current, or resistance, enter the remaining values, and the result appears instantly. It is a handy tool for students learning circuit theory, hobbyists wiring up electronics, and engineers double-checking a quick calculation.

The importance of Ohm’s law

Ohm’s law is the foundation for analyzing direct-current circuits and many alternating-current circuits as well. By tying together voltage, current, and resistance, it allows you to predict how a circuit will behave before you build it. If you know the supply voltage and the resistance of a component, you can work out the current that will flow, and therefore whether a wire, resistor, or device can safely handle it.

The law is equally useful in reverse. Engineers routinely size resistors to limit current, calculate the voltage drop across a length of cable, or determine the resistance of an unknown component from measured voltage and current. Because the relationship is linear for ohmic materials, the arithmetic stays straightforward, which is exactly why Ohm’s law remains one of the first things taught in any electronics course.

Formula

The core relationship of Ohm’s law is written as:

where:

• $V$ is the voltage across the conductor, measured in volts (V),
• $I$ is the current flowing through it, measured in amperes (A),
• $R$ is the resistance of the conductor, measured in ohms (Ω).

Rearranging the same equation lets you solve for the other two quantities:

These three forms are mathematically identical; you simply pick the one whose unknown you need.

Examples

1. Finding voltage: A current of 2 A flows through a 3 Ω resistor. The voltage across it is:

   $$V = I\,R = 2 \, \text{A} \times 3 \, \Omega = 6 \, \text{V}$$

2. Finding current: A 12 V battery is connected across a 4 Ω resistor. The current that flows is:

   $$I = \frac{V}{R} = \frac{12 \, \text{V}}{4 \, \Omega} = 3 \, \text{A}$$

3. Finding resistance: A component carries 3 A when 12 V is applied across it. Its resistance is:

   $$R = \frac{V}{I} = \frac{12 \, \text{V}}{3 \, \text{A}} = 4 \, \Omega$$

Notes

• Ohm’s law applies to ohmic materials, whose resistance stays constant over a range of voltages. Components such as diodes and transistors are non-ohmic and do not follow it directly.
• Resistance generally changes with temperature, so the law holds most accurately when the conductor’s temperature is steady.
• Keep your units consistent: volts, amperes, and ohms. Mixing in milliamps or kilohms without converting will give the wrong answer.

FAQs

What does Ohm’s law state?

Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across those points and inversely proportional to the resistance, expressed as $V = I\,R$.

How do I rearrange Ohm’s law to find current?

Divide the voltage by the resistance: $I = V / R$. For example, 12 V across 4 Ω gives a current of 3 A.

What units does Ohm’s law use?

Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). Using these SI units keeps the equation $V = I\,R$ consistent.

Does Ohm’s law work for all materials?

No. It holds for ohmic conductors, whose resistance is constant. Non-ohmic devices such as diodes, lamps near their operating limit, and semiconductors have a resistance that changes with voltage or current.

Can Ohm’s law be used in AC circuits?

A generalized form applies to AC circuits, where resistance is replaced by impedance to account for capacitors and inductors. For purely resistive AC loads, the basic $V = I\,R$ relationship still works.

Why does resistance matter in a circuit?

Resistance limits how much current flows for a given voltage. Choosing the right resistance protects components, sets operating points, and controls how energy is dissipated as heat.

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