Key takeaways
- Ohm's Law is V = I × R: voltage equals current times resistance.
- Voltage is measured in volts, current in amps, resistance in ohms.
- Rearrange it to find current (I = V ÷ R) or resistance (R = V ÷ I).
- Resistance can never be negative in a real circuit.
In this guide
What is Ohm's Law?
Ohm's Law describes how electricity flows through a circuit. Think of voltage as water pressure pushing electricity along a wire, current as how much electricity is flowing, and resistance as how narrow the pipe is.
Why does Ohm's Law matter?
Every wire, resistor, and fuse in your home is sized using this formula. Get it wrong, and a wire can overheat, or a device simply won't get enough power to work.
A typical car battery supplies 12 V. Ohm's Law is exactly how electricians and hobbyists work out how much current a circuit will draw before they wire anything up.
The formula
| Symbol | What it means | Example |
|---|---|---|
| V | The voltage, in volts (V) | 12 V |
| I | The current, in amps (A) | 2 A |
| R | The resistance, in ohms (Ω) | 6 Ω |
How to calculate voltage step by step
- 1Find the current. Measure or note the current flowing through the circuit, in amps.
- 2Find the resistance. Measure or note the resistance of the component, in ohms.
- 3Multiply current by resistance.
I × Rgives you the voltage. - 4Write the answer in volts. That's the electrical push across that part of the circuit.
Try the calculator below with your own current and resistance to see the working step by step.
Try it yourself
Pre-filled with the example — change any value.
Common mistakes to avoid
- Wrong unit for resistance. Resistance is in ohms (Ω), never volts or amps.
- Forgetting resistance can't be negative. A real resistor always has a positive resistance value.
- Using the wrong version of the formula. If you know V and R, use
I = V ÷ Rfor current — don't try to force V = I × R with the wrong two values.
Tips and tricks
- Draw a simple V-I-R triangle: cover the one you want, and the other two show you whether to multiply or divide.
- A 12 V car battery is a handy everyday reference point — many small electronics run on similar low voltages.
- It also shares an electrical theme with Coulomb's Law, which measures the force between charges rather than the flow of current.
- Once you have voltage and current, the power formula calculator tells you how fast that circuit uses energy.
Where you'll use it in real life
- Home wiring: electricians use Ohm's Law to size wires and fuses so they don't overheat.
- Electronics: hobbyists calculate the right resistor to protect an LED from too much current.
- Troubleshooting: an unexpectedly high or low voltage reading often points straight to a resistance problem.
- Batteries: knowing a device's resistance tells you how much current it will draw from a battery of a given voltage.
Quick summary
To find voltage, multiply the current in amps by the resistance in ohms. The same formula rearranges to find current or resistance if you know the other two values. Use the calculator above to work through your own circuit.
Worked example
A small circuit carries a current of 2 amps through a resistor rated at 6 ohms. What voltage is being applied across the resistor?
- 01V = I × R
- 02V = 2 × 6
- 03V = 12 V
These numbers are pre-filled in the calculator above.
Frequently asked questions
How do I calculate voltage using Ohm's Law?
Multiply the current in amps by the resistance in ohms: V = I × R. The answer comes out in volts.
What is the difference between current and voltage?
Voltage is the electrical push that drives a circuit. Current is the actual flow of electric charge that results from that push.
What unit is resistance measured in?
Ohms, written with the symbol Ω. Higher resistance means the circuit resists the flow of current more.
Can I rearrange Ohm's Law to find current or resistance?
Yes. From V = I × R you can also get I = V ÷ R to find current, or R = V ÷ I to find resistance.
