# How do I use Ohm’s law?

Computes voltage, current, resistance and power from any two of them, with Ohm's law (V = I × R) and electric power (P = V × I).

- Page: https://www.acalculator.org/physics/ohms-law-calculator
- JSON spec: https://www.acalculator.org/physics/ohms-law-calculator.json
- Version: 34c4867b22a1

## Default answer

Example with the default inputs (Voltage 12 V, Resistance 4 Ω): At 12 V across 4 Ω, the current is 3 A and the power is 36 W.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| v | Voltage | The voltage across the resistor, in volts. |
| i | Current | The current through the resistor, in amperes. |
| r | Resistance | The resistance, in ohms. |
| p | Power | The power the resistor turns into heat, in watts. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| V | Voltage | The voltage across the resistor, in volts. |
| I | Current | The current through the resistor, in amperes. |
| R | Resistance | The resistance, in ohms. |
| P | Power | The power the resistor turns into heat, in watts. |

## Method

V = I × R (Ohm’s law) and P = V × I, so also P = I² × R = V² ÷ R.

## Assumptions

- The resistance is ohmic: it stays the same whatever the voltage or current.
- Values are for direct current, or root-mean-square (RMS) values in a purely resistive AC circuit.
- All values are positive.

## Worked examples

1. v = 12, r = 4 gives i = 3, p = 36. Source: hand calculation in content.mdx: I = 12 V ÷ 4 Ω = 3 A; P = 12 V × 3 A = 36 W.
2. p = 60, v = 120 gives i = 0.5, r = 240. Source: hand calculation in content.mdx: I = 60 W ÷ 120 V = 0.5 A; R = 120 V ÷ 0.5 A = 240 Ω.
3. p = 0.25, r = 1,000 gives v = 15.811388, i = 0.015811. Source: hand calculation in content.mdx: V = √(0.25 W × 1,000 Ω) = √250 = 15.81 V; I = 15.81 V ÷ 1,000 Ω = 15.81 mA.
4. i = 0.02, r = 220 gives v = 4.4, p = 0.088. Source: hand calculation in content.mdx: V = 0.02 A × 220 Ω = 4.4 V; P = 4.4 V × 0.02 A = 0.088 W.

## FAQ

### What is Ohm's law?

Ohm's law says that the current through a resistor is proportional to the voltage across it: V = I × R. V is the voltage in volts, I is the current in amperes, and R is the resistance in ohms. One ohm is one volt per ampere.

### How do I find the current from the voltage and the resistance?

Divide the voltage by the resistance: I = V ÷ R. For example, 12 V across 4 Ω gives 12 ÷ 4 = 3 A.

### How do I work out power in watts?

Multiply the voltage by the current: P = V × I. With Ohm's law you can also use P = I² × R or P = V² ÷ R. A 60 W bulb on 120 V draws 60 ÷ 120 = 0.5 A.

### Why does the calculator need two values?

There are two independent equations (V = I × R and P = V × I) and four quantities. Two known values fix the other two. If you enter three or four values that do not fit the equations, the calculator says which ones disagree.

### Does Ohm's law work for AC circuits?

For a purely resistive load, yes, if you use root-mean-square (RMS) voltage and current, which is what a mains voltage such as 120 V or 230 V states. Circuits with capacitors, coils or motors have reactance, and then you need impedance instead of resistance.

### Which parts do not follow Ohm's law?

Parts whose resistance changes with the current, such as diodes, LEDs and filament bulbs as they heat up. For those, V = I × R holds only at one working point.

## Sources

- BIPM, The International System of Units (SI Brochure), 9th edition (2019), Table 4: the volt is W/A and the ohm is V/A, so 1 W = 1 V × 1 A. https://www.bipm.org/en/publications/si-brochure
- NIST Special Publication 811 (2008), Guide for the Use of the International System of Units, Table 3 (SI derived units). https://www.nist.gov/pml/special-publication-811