Ohm's Law Calculator
Find voltage, current or resistance using V = IR — supports any mix of units, auto-scales the result, and always shows the power dissipated.
Step-by-step solution
Enter the two known values and press Calculate
The VIR Triangle
Cover V: I × R
Cover I: V ÷ R
Cover R: V ÷ I
All Formula Variations
Ohm's Law Calculator Explained
Core concepts, classroom applications, and plain-English explanations for all audiences.
When would you actually use this?
Every time you wonder whether a fuse is the right rating — or why swapping a 60W bulb for an 8W LED saves so much on your electricity bill. A kettle draws 2,200 W at 230 V. Ohm's Law tells you it needs a current of 9.6 A — which is why a 13 A fuse is correct, not a 3 A one.
Worked example
"A 60 W light bulb runs on 230 V mains. What current does it draw, and what is its resistance?"
Current: I = P ÷ V = 60 ÷ 230 = 0.26 A
Resistance: R = V ÷ I = 230 ÷ 0.26 = 885 Ω
LED equivalent: 8 W → I = 8 ÷ 230 = 0.035 A (87% less current)
A 3 A fuse is safe for both — but the LED runs on 13× less current than the old bulb.
The V = IR triangle
- → Voltage (V): The "pressure" pushing current around the circuit — measured in volts
- → Current (I): The flow of charge — measured in amps (A). I = V ÷ R
- → Resistance (R): How much the circuit opposes current — measured in ohms (Ω). R = V ÷ I
The right fuse saves lives
A fuse must be rated just above the normal operating current. A 2,000 W hairdryer draws 8.7 A — use a 10 A fuse, not a 3 A (which blows immediately) or a 13 A (which won't blow fast enough if something goes wrong).
What is Ohm's Law?
Ohm's Law is a fundamental principle in electronics that describes the relationship between voltage, current, and resistance in an electrical circuit. Named after German physicist Georg Ohm, it's a cornerstone for understanding how electricity behaves.
Simply put, Ohm's Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them. This relationship is most commonly expressed as V = I x R. Our calculator helps you quickly find any of these values if you know the other two. As a bonus, it also calculates power (P = V x I), a crucial metric in circuit design. The VIR triangle is a handy visual aid to remember the formulas!
Common mistakes to avoid
- Calculating R = 12V / 20mA as 0.6 Ohms:Forgetting to convert milliamps (mA) to amps (A) before calculation. 20mA is 0.02A. R = 12V / 0.02A = 600 Ohms.
- Using V = I / R when trying to find voltage:Mixing up the formula. The VIR triangle helps prevent this. To find V, cover V, leaving I x R. V = I x R.
- Calculating power for a 12V, 2A circuit as 24 Ohms:Confusing the power formula (P = V x I) with the resistance formula (R = V / I). Power is measured in Watts, resistance in Ohms. Power P = 12V x 2A = 24W. Resistance R = 12V / 2A = 6 Ohms.
- Assuming Ohm's Law applies to all electronic components, like diodes or transistors:Ohm's Law only applies to 'ohmic' components where resistance is constant regardless of voltage or current. Non-ohmic components have a resistance that changes with voltage or current. Ohm's Law describes a linear relationship. For components like diodes, the V-I relationship is non-linear, so Ohm's Law doesn't directly apply.
Questions students ask
What does Ohm's Law state?
What are the units of voltage, current and resistance?
How do I use the VIR triangle?
* Cover V: You see I x R, so V = I x R.
* Cover I: You see V / R, so I = V / R.
* Cover R: You see V / I, so R = V / I.
What is the difference between AC and DC for Ohm's Law?
How do I calculate the resistor needed for an LED?
1. Determine your supply voltage (Vs).
2. Find the LED's forward voltage (Vf) and desired forward current (If) from its datasheet (e.g., 2V, 20mA).
3. Calculate the voltage drop across the resistor: Vr = Vs - Vf.
4. Use Ohm's Law to find the resistance: R = Vr / If.
Example: For a 5V supply, 2V LED, 20mA current: R = (5V - 2V) / 20mA = 3V / 0.02A = 150 Ohms.