# Free online Ohm's Law calculator

> Ohm's Law states that voltage equals current times resistance (V = I × R). This calculator solves for any two unknowns given any two knowns — including power (P = V × I).

Calculate voltage, current, resistance, and power from any two known values using Ohm's Law (V = I × R) and the power equations (P = V × I).

**Interactive version:** https://onlinecalculator.me/everyday/ohm/
**Category:** Everyday

## How to use

1. Enter any **two** of the four values — voltage (V), current (I), resistance (R), or power (P).
2. Leave the other two fields blank.
3. Click **Calculate** to see all four values and the formula used.
4. Click **Clear** to reset.

## Formulas

**Ohm's Law:**
```
V = I × R
```

**Power (Joule's Law):**
```
P = V × I = I² × R = V² / R
```

**All combinations:**

| Given     | Solve for V       | Solve for I       | Solve for R       | Solve for P      |
|-----------|-------------------|-------------------|-------------------|------------------|
| V, I      | —                 | —                 | V / I             | V × I            |
| V, R      | —                 | V / R             | —                 | V² / R           |
| V, P      | —                 | P / V             | V² / P            | —                |
| I, R      | I × R             | —                 | —                 | I² × R           |
| I, P      | P / I             | —                 | P / I²            | —                |
| R, P      | √(P × R)          | √(P / R)          | —                 | —                |

## Worked example

**Inputs:** V = 12 V, R = 100 Ω
```
I = V / R = 12 / 100 = 0.12 A
P = V² / R = 144 / 100 = 1.44 W
```

**Inputs:** I = 2 A, P = 24 W
```
V = P / I = 24 / 2 = 12 V
R = P / I² = 24 / 4 = 6 Ω
```

## Frequently asked questions

### What is Ohm's Law?

Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I × R. It was formulated by Georg Simon Ohm in 1827.

### How is power included?

Power (P) relates to voltage and current by Joule's Law: P = V × I. Combined with Ohm's Law, this gives P = I² × R and P = V² / R. The calculator uses these three relationships to solve for all four quantities from any two known values.

### What units are used?

Voltage is in volts (V), current in amps (A), resistance in ohms (Ω), and power in watts (W). All standard SI units.

### Why does the calculator return no result sometimes?

If the two known values imply a division by zero (for example, voltage = 0 with power given, or resistance = 0 with voltage given), the result is mathematically undefined and the calculator returns an error.

### What is a typical example?

A 12 V car battery connected to a 100 Ω resistor: I = 12 / 100 = 0.12 A, P = 12² / 100 = 1.44 W.

### How do I share my calculation?

Click Share with my numbers to copy a URL with your two input values pre-filled.

## Sources

- [Ohm's law](https://en.wikipedia.org/wiki/Ohm%27s_law) — Wikipedia
- [What Is Ohm's Law?](https://www.fluke.com/en-us/learn/blog/electrical/what-is-ohms-law) — Fluke Corporation

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