# Ohm’s Law Calculator

URL: https://speedycalculator.com/calculators/physics/ohms-law/
Last updated: 2026-10-25

> The Ohm’s law calculator relates voltage, current and resistance with V = I × R. Enter voltage and resistance to get current and the power dissipated by the resistor; type a desired current into the result to find the voltage required.

## Inputs

- Voltage (V)
- Resistance (Ω, kΩ, MΩ)

## Results

- Current
- Power dissipated

## Formula

```
I = V ÷ R
P = V × I = I² R = V² ÷ R
```

## Explanation

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

FieldWhat to enterVoltageIn VResistanceUnits: 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)60.027390.0409120.0545150.0682180.0818240.1091

## Worked 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).

## FAQ

**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.


## Sources

- [Encyclopaedia Britannica – Ohm’s law](https://www.britannica.com/science/Ohms-law)