About the Ohm's Law Calculator
Ohm's law ties together the three quantities in any simple circuit: voltage (V), current (I) and resistance (R). Give it any two and it finds the third, plus the power being used.
How it works
V = I × R, rearranged as I = V ÷ R and R = V ÷ I. Power follows: P = V × I, which can also be written P = I²R or P = V² ÷ R.
- Enter any two of voltage, current and resistance.
- The calculator works out the third and the power.
- Watch the units: milliamps are thousandths of an amp, kilohms thousands of ohms.
The law holds for resistors and most wiring. It does not hold for components whose resistance changes with current or temperature, such as diodes, LEDs and filament bulbs.
Examples
Worked examples:
- 12 V across 4 Ω: I = 12 ÷ 4 = 3 A, and P = 12 × 3 = 36 W.
- An LED needing 20 mA from 5 V, with a 2 V drop: R = (5 − 2) ÷ 0.02 = 150 Ω.
- A 1,500 W heater on 230 V: I = 1500 ÷ 230 = 6.5 A, so a 10 A circuit is enough.
Related: Analog-to-Digital Converter, Energy Converter, Power Converter.
Ohm's Law Calculator: frequently asked questions
- What is the formula for Ohm's law?
V = I × R. Rearranged: I = V ÷ R to find current, R = V ÷ I to find resistance.
- How do I work out power?
P = V × I. If you only have two of the three quantities, use P = I²R or P = V² ÷ R.
- Does Ohm's law apply to all components?
No. It holds for resistors and ordinary wiring, but not for diodes, LEDs, transistors or filament bulbs, whose resistance changes with current or temperature.