Ohm’s law states that the current through a conductor is proportional to the voltage across it, with resistance as the constant of proportionality. It applies to resistors and most metallic conductors at constant temperature.
How to use it
Enter the supply voltage and the resistance in Ω, kΩ or MΩ. Read current in A, mA or µA and power in W or mW.
Choosing a resistor rating
Pick a resistor whose power rating is at least double the calculated dissipation to keep it cool and reliable.
Inputs explained
| Field | What to enter |
|---|---|
| Voltage | In V |
| Resistance | Units: Ω, kΩ, MΩ |
What each result means
- Current = [Voltage] ÷ [Resistance]
- Power dissipated = [Voltage] × [Current]
Example values
How the main result changes when Voltage changes and everything else stays at the example values above:
| Voltage (V) | Current (A) |
|---|---|
| 6 | 0.0273 |
| 9 | 0.0409 |
| 12 | 0.0545 |
| 15 | 0.0682 |
| 18 | 0.0818 |
| 24 | 0.1091 |
Formula
I = V ÷ RP = V × I = I² R = V² ÷ RWorked example
12 V across a 220 Ω resistor.
I = 12 ÷ 220 ≈ 0.0545 A = 54.5 mA. P = 12 × 0.0545 ≈ 0.65 W, so use a 1 W or larger resistor.
Key terms
- Voltage (V)
- Electric potential difference, measured in volts.
- Current (I)
- Rate of flow of electric charge, measured in amperes.
- Resistance (R)
- Opposition to current, measured in ohms (Ω).
Frequently asked questions
Does Ohm’s law apply to LEDs?
Not directly; LEDs are non-linear. Use the supply minus the LED forward voltage to size a series resistor.
What is the power formula?
P = V × I, equivalently I²R or V²/R.
Why does a resistor get hot?
Electrical energy is converted to heat at the rate P.
What formula does the Ohm’s Law Calculator use?
I = V ÷ R.
Can I work backwards from the result?
Yes. Type the result you want into Current and the calculator finds the matching voltage.