Quick answer
V = I × R. To find current, divide voltage by resistance: 12 V across 24 Ω gives 0.5 A. To find resistance, divide voltage by current: 5 V at 0.02 A needs 250 Ω.
How to use the Ohm's Law Calculator
- Enter two known values, for example voltage and resistance.
- Choose the units next to each field (such as mA or kΩ).
- Press Calculate. The missing value appears along with the power and the formula used.
- Press Clear to start a new calculation.
Example
A 12 V car accessory has a resistance of 24 Ω. How much current does it draw?
I = V ÷ R = 12 V ÷ 24 Ω = 0.5 A
The power is P = V × I = 12 × 0.5 = 6 W. A 1 A fuse would protect this circuit with some margin.
How it works
Ohm's law describes the relationship between voltage (V), current (I) and resistance (R) in a resistive circuit:
| To find | Formula |
|---|---|
| Voltage | V = I × R |
| Current | I = V ÷ R |
| Resistance | R = V ÷ I |
| Power | P = V × I = I² × R = V² ÷ R |
An easy way to remember it is the triangle with V on top and I and R at the bottom: cover the value you want, and the remaining two show the formula.
Ohm's law applies to resistors and other “ohmic” components. Components such as LEDs, diodes and motors don't have a constant resistance, so use dedicated calculators like our LED resistor calculator for those.
Common mistakes with Ohm's law
- Mixing units: 20 mA is 0.02 A, not 20 A. Use the unit menus here so you don't have to convert by hand.
- Using it on LEDs: an LED's resistance changes with current, so V = I × R on its own gives wrong answers. Calculate the series resistor instead.
- Forgetting power: a 100 Ω resistor across 12 V passes 0.12 A and dissipates 1.44 W. A small ¼ W resistor would burn.
- Measuring resistance in a live circuit: a multimeter reads resistance correctly only with the power off and the component isolated.
- Assuming wires have zero resistance: over long runs at low voltage, cable resistance matters. Use the voltage drop calculator for that.
Frequently asked questions
What is Ohm's law?
Ohm's law states that the current through a conductor is proportional to the voltage across it: V = I × R. Double the voltage across a resistor and the current doubles too.
How do I calculate amps from volts and ohms?
Divide the voltage by the resistance: I = V ÷ R. For example, 230 V across 100 Ω gives 2.3 A.
Does Ohm's law work for AC?
Yes, for purely resistive loads such as heaters and incandescent bulbs, using RMS values. For loads with inductance or capacitance, such as motors, you need impedance instead of resistance.
Why does the calculator also show power?
Because power often decides which component to choose. A resistor or wire must be rated for the power it dissipates, or it will overheat.
Can I enter values like 4k7 or 2.2k?
Enter the number (for example 4.7) and choose kΩ in the unit menu. Commas are accepted as decimal separators.