Ohm's law calculator
Enter any two of voltage, current, resistance and power to get the other two.
- 🇺🇸 US federal & state rules
- See sources
Current and Power
2 A, 24 W
- Voltage V (entered)
- 12 V
- Current I
- 2 A
- Resistance R (entered)
- 6 Ω
- Power P
- 24 W
This is an example. Enter your numbers above and press the button to see your own result.
- Assumes a DC circuit or a purely resistive load. AC loads like motors have a power factor below 1, so real power is V × I × power factor.
Show the math
| Entered V | 12 V |
|---|---|
| Entered R | 6 Ω |
| I = V ÷ R | 2 A |
| P = V² ÷ R | 24 W |
What to do next
How it works
The two values you enter are converted to base units (volts, amps, ohms, watts). Then the calculator picks the matching pair of formulas from Ohm's law (V = IR) and the power law (P = VI).
| To find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage V | I × R | P ÷ I | √(P × R) |
| Current I | V ÷ R | P ÷ V | √(P ÷ R) |
| Resistance R | V ÷ I | V² ÷ P | P ÷ I² |
| Power P | V × I | V² ÷ R | I² × R |
Things to keep in mind
These formulas hold for DC circuits and resistive AC loads like heaters and incandescent bulbs. Motors and power supplies draw more current than P ÷ V suggests because of power factor.
Sources
- NIST: SI units for electric current, volt, ohm and wattChecked 2026-10-19
- BIPM: The International System of Units (SI Brochure), SI prefixesChecked 2026-10-19
Frequently asked questions
What is Ohm's law?
Voltage equals current times resistance, V = I × R. Add the power formula P = V × I and any two of the four values give you the other two.
How do I pick a resistor for an LED?
Divide the leftover voltage by the current you want. With a 5 V supply, a 2 V LED and 20 mA, enter 3 V and 20 mA to get 150 Ω, and pick a resistor rated above the 60 mW it shows.
How many amps does a 1,500 W heater draw on 120 V?
I = P ÷ V = 1,500 ÷ 120 = 12.5 A. That is why space heaters belong on a 15 A circuit by themselves.
What do m, k and M mean?
They are SI prefixes. m (milli) is 1/1,000, k (kilo) is 1,000 and M (mega) is 1,000,000. So 10 mA is 0.01 A and 4.7 kΩ is 4,700 Ω.