Ohm’s Law Explained

Have you ever wondered why a small battery lights up an LED but barely warms a heater coil? The answer is Ohm’s law, a simple rule that ties voltage, current, and resistance together in every circuit. Ohm’s law says voltage equals current multiplied by resistance, written as V = I x R. This guide covers the formula, its two rearrangements, and a worked example you can check by hand.

What Is Ohm’s Law?

Ohm’s law is one of the first rules every electronics student learns. It explains how voltage, current, and resistance relate inside a simple electric circuit.

These three quantities show up in almost every circuit you will ever see. Here is what each one means and how it is measured:

• Voltage (V): measured in volts, the electrical push that drives current through a wire.
• Current (I): measured in amps, the actual flow of electric charge past a point.
• Resistance (R): measured in ohms, anything that slows that flow of charge down.

Once you know any two of these numbers, Ohm’s law lets you calculate the third one every time.

The Ohm’s Law Formula and Triangle Trick

The Ohm’s law formula is short and easy to remember: V equals I times R. Voltage equals current multiplied by resistance, every single time.

A popular way to remember this is the Ohm’s law triangle. Picture a triangle with V on top, and I and R sitting side by side on the bottom.

Cover the letter you want to find, and the triangle shows you what to do with the other two. Cover V, and you see I next to R, which means I times R. Cover I, and you see V over R, which means V divided by R.

The Two Rearrangements You Need

Sometimes you already know voltage and resistance, but need current. Other times you know voltage and current, but need resistance. Ohm’s law rearranges easily for both cases.

• Find current: I = V / R
• Find resistance: R = V / I

Both rearrangements come from the same original formula, V = I x R. You are just moving the pieces around to isolate whichever value you do not yet know.

Ohm’s Law as a Water Analogy

It can help to picture electricity like water flowing through a pipe. This is only an analogy, not a literal description of electricity, but it makes the idea easier to picture.

Voltage acts like water pressure pushing the flow forward. Current acts like the amount of water actually passing a point each second. Resistance acts like a narrow section of pipe that slows the flow down.

Squeeze the pipe narrower, and less water gets through at the same pressure. Raise the pressure, and more water pushes through that same narrow spot. Ohm’s law captures that same relationship, but for electricity instead of water.

How Current Flows in a Simple Circuit

A basic circuit needs three things to work: a voltage source, a path for current, and some resistance. Picture a battery connected by wires to a light bulb.

The battery is the voltage source. It pushes current around the loop, through the wires, and through the bulb.

The bulb itself acts as the resistance. It resists the flow of current, and that resistance is part of what makes the bulb glow.

Worked Example: Solving for Each Quantity

Let’s use real numbers to see Ohm’s law in action. Working through each rearrangement by hand helps the formula stick.

Say a circuit has a 12 volt battery connected to a 4 ohm resistor. To find current, use I = V / R. That gives I = 12 / 4 = 3 amps.

Now say a circuit carries 2 amps of current through a 6 ohm resistor. To find voltage, use V = I x R. That gives V = 2 x 6 = 12 volts.

Finally, say a circuit has 9 volts pushing 3 amps of current. To find resistance, use R = V / I. That gives R = 9 / 3 = 3 ohms.

Each answer checks out with simple multiplication or division. Practice with your own numbers using our Resistance Calculator to check your work instantly.

Comparing Low Resistance and High Resistance

Voltage does not act alone. The same voltage can push very different amounts of current, depending on resistance.

A low resistance circuit lets current flow easily, so more amps flow at the same voltage. A high resistance circuit fights that flow, so fewer amps flow at the same voltage.

This is why a short circuit, which has almost no resistance, can cause a dangerous surge of current. It is also why adding resistance is a common way to safely limit current in a circuit.

Where Ohm’s Law Connects to Power

Ohm’s law also connects to electrical power, measured in watts. Power equals voltage times current, or P = V x I, and it combines with Ohm’s law to describe how much energy a circuit uses. For the full breakdown of power calculations, see our guide on How to Calculate Power in Watts.

Common Mistakes to Avoid

A few small mix-ups trip up beginners again and again. Watching for these keeps your calculations accurate.

• Mixing up units: always plug in volts, amps, and ohms, not other unit prefixes, without converting first.
• Forgetting to rearrange first: isolate the unknown value fully before plugging numbers into the formula.
• Confusing Ohm’s law with power: V = I x R finds voltage, current, or resistance, while P = V x I finds power.
• Assuming resistance never changes: some materials change resistance as they heat up, so real circuits can drift from the ideal formula.

Ohm’s law also assumes a simple, resistor-like circuit. It does not directly describe force or motion, which are covered in our guide on Newton’s Laws and Force Explained.

Ohm’s Law in Real-World Circuits

Not every component follows Ohm’s law perfectly at every temperature. A light bulb filament heats up as current flows through it, and its resistance rises as it gets hotter. Because resistance is not perfectly fixed in that case, current does not scale in a perfectly straight line with voltage. Engineers call this non-ohmic behavior, and Ohm’s law still gives a useful estimate even when a real part is not perfectly ohmic.

How you wire resistors together also changes total resistance. In a series circuit, resistances simply add together, so total resistance goes up. In a parallel circuit, resistances combine differently, so total resistance actually goes down. Either way, once you know the total resistance of the circuit, Ohm’s law still applies to the whole thing.

You will also see resistance measured in different sized units on real parts. Everyday resistors are often labeled in ohms, kilohms, and megohms. A kilohm equals 1,000 ohms, and a megohm equals 1,000,000 ohms, and you can spot these labels on resistors inside phone chargers, remote controls, and computer circuit boards.

Frequently Asked Questions About Ohm’s Law

Sources

This article is for general education only. Formulas and examples use standard physics conventions and idealized conditions (no friction or air resistance unless noted), so real-world results can vary. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 13, 2026.