Ohm's Law Explained: Voltage, Current & Resistance (V = IR)

What Is Ohm's Law?

Ohm's Law describes the relationship between three things that exist in every electronic circuit: voltage, current, and resistance. It says that the current flowing through a conductor between two points is directly proportional to the voltage across those two points, and inversely proportional to the resistance between them. In plain terms: more voltage pushes more current, and more resistance chokes off current.

The Ohm's Law Formula (V = IR)

Ohm's Law is usually written as:

V = I × R

V is voltage, measured in volts (V)
I is current, measured in amps (A)
R is resistance, measured in ohms (Ω)

Because it's a simple algebraic relationship, you can rearrange the formula to solve for whichever value you don't know:

To find voltage: V = I × R
To find current: I = V ÷ R
To find resistance: R = V ÷ I

Ohm's Law Triangle

A common memory trick is the "Ohm's Law triangle": picture V on top, with I and R side by side underneath it. Cover up the value you want to find, and the triangle shows you the other two values multiplied or divided to get it. Cover V and you see I and R next to each other, meaning V = I × R. Cover I and you see V over R, meaning I = V ÷ R.

Worked Example

Say you have a 9V battery connected to a resistor, and you measure 0.03A (30mA) of current flowing through the circuit. To find the resistance:

R = V ÷ I = 9 ÷ 0.03 = 300 ohms

That's exactly the kind of calculation you'd run before wiring up an LED, a button, or any other component that needs current limited to a safe level.

Why Ohm's Law Matters When Building Circuits

Every time you add a resistor to protect an LED, choose a pull-down resistor for a button, or size a current-limiting resistor for a sensor, you're applying Ohm's Law — whether or not you do the math by hand. Understanding it helps you avoid two of the most common beginner mistakes: drawing too much current through a component (which can damage it or your board) and using a resistor value so high that a circuit barely works at all.

Don't want to do the math by hand? Try our free Ohm's Law Calculator — enter any two values and it solves for the rest.

We use these exact calculations when designing our open-source kits. Want to see basic circuit wiring in practice? Check out the Mighty Mini Macro Pad build guide, which walks through soldering and wiring a real GPIO circuit step by step.

Frequently Asked Questions

What units does Ohm's Law use?

Voltage is measured in volts (V), current in amps (A), and resistance in ohms (Ω). Keeping units consistent is the most common source of calculation errors — for example, converting milliamps (mA) to amps before plugging numbers into the formula.

Does Ohm's Law apply to AC circuits?

Ohm's Law applies directly to the resistive part of an AC circuit. Once capacitors or inductors are involved, you need the AC version of the relationship, which uses impedance in place of plain resistance.

What's a typical resistor value for an LED?

It depends on the supply voltage and the LED's forward voltage, but values between 220Ω and 1kΩ are common for standard 5mm LEDs running off 3.3V–5V logic. The more precise formula is R = (V_supply − V_fwd) ÷ I, where V_fwd is the LED's forward voltage — you subtract that voltage drop before dividing by the current you want to allow, rather than using the full supply voltage.

Want to keep going? Check out What Is a Resistor? next, or browse all electronics tutorials.