# Ohm's law calculator

> Ohm's law calculator: enter any two of voltage, current, resistance and power to get the other two, shown in the units you pick (mA, kΩ…).

Interactive version: https://www.calcopenly.com/science/ohms-law-calculator
Subject: Science calculators

Ohm's law says the current through a resistor equals the voltage across it divided by its resistance, I = V/R. Combined with the power law P = VI, any two of voltage, current, resistance and power fix the other two: P = I²R and P = V²/R follow by substitution. Solved values are shown in the unit you choose, so a milliamp current does not appear as 0.002 A.

The default, 12 V across 6 Ω, gives 2 A and 24 W. The same arithmetic sizes fuses and cables (a 2,300 W kettle on 230 V draws 10 A) and checks resistor ratings (a 100 Ω, ¼ W resistor carries at most 50 mA).

The law holds for ohmic conductors, whose resistance stays constant as the current changes. Diodes, LEDs and filament lamps are not ohmic, so a single resistance value describes them only at one operating point.

## Inputs

- **I know** (options: Voltage and resistance, Voltage and current, Current and resistance, Voltage and power, Current and power, Resistance and power)
- **Voltage**
- **Voltage unit** (options: mV, V, kV)
- **Current**
- **Current unit** (options: µA, mA, A, kA)
- **Resistance**: Accepts 4.7k or 2.2M
- **Resistance unit** (options: mΩ, Ω, kΩ, MΩ)
- **Power**
- **Show voltage in** (options: mV, V, kV)
- **Show current in** (options: µA, mA, A, kA)
- **Show resistance in** (options: mΩ, Ω, kΩ, MΩ)
- **Show power in** (options: mW, W, kW)

## Results

- Current (A) — main result
- Voltage (V)
- Resistance (Ω)
- Power (W)
- Conductance (S)

## Formula

$$
V = IR,\qquad P = VI = I^2R = \frac{V^2}{R}
$$

## Worked examples

### 12 V across 6 Ω

- I know: Voltage and resistance
- Voltage: 12
- Voltage unit: V
- Resistance: 6
- Resistance unit: Ω
- Show current in: A
- Show power in: W
- **Current: 2 A**
- **Power: 24 W**
- Checked against: Python 3.8 decimal: I = 12/6, P = 12²/6

### Kettle: 230 V drawing 10 A

- I know: Voltage and current
- Voltage: 230
- Voltage unit: V
- Current: 10
- Current unit: A
- Show resistance in: Ω
- Show power in: W
- **Resistance: 23 Ω**
- **Power: 2,300 W**
- Checked against: Python 3.8 decimal: R = 230/10, P = 230 × 10

### Quarter-watt 100 Ω resistor at full rating

- I know: Resistance and power
- Resistance: 100
- Resistance unit: Ω
- Power: 0.25 W
- Show voltage in: V
- Show current in: A
- **Current: 0.05 A**
- **Voltage: 5 V**
- Checked against: Python 3.8 decimal: I = √(0.25/100), V = √(0.25 × 100)

### 5 V, 500 mW

- I know: Voltage and power
- Voltage: 5
- Voltage unit: V
- Power: 500 mW
- Show current in: A
- Show resistance in: Ω
- **Current: 0.1 A**
- **Resistance: 50 Ω**
- Checked against: Python 3.8 decimal: I = 0.5/5, R = 25/0.5

### 20 mA dissipating 40 mW

- I know: Current and power
- Current: 20
- Current unit: mA
- Power: 40 mW
- Show voltage in: V
- Show resistance in: Ω
- **Voltage: 2 V**
- **Resistance: 100 Ω**
- Checked against: Python 3.8 decimal: V = 0.04/0.02, R = 0.04/0.02²

### 2 mA through 4.7k (typed with the k suffix)

- I know: Current and resistance
- Current: 2
- Current unit: mA
- Resistance: 4.7k
- Resistance unit: Ω
- Show voltage in: V
- Show power in: W
- **Voltage: 9.4 V**
- **Power: 0.0188 W**
- Checked against: Python 3.8 decimal: V = 0.002 × 4700, P = 0.002² × 4700

## Questions

### How do you calculate current from voltage and resistance?

Divide the voltage by the resistance: I = V/R. With 12 V across 6 Ω the current is 2 A; with 5 V across 1 kΩ it is 0.005 A, or 5 mA. Convert kilohms to ohms (×1,000) before dividing, or pick kΩ and mA in the unit selectors and let the calculator convert.

### How do you convert watts to amps?

Divide the power by the voltage: I = P/V. A 2,300 W kettle on a 230 V supply draws 10 A, and a 60 W load on 12 V draws 5 A. For AC this holds for resistive loads such as heaters and kettles; motors and power supplies with a power factor below 1 draw more current than P/V.

### What is the formula for electrical power?

P = VI, which becomes P = I²R or P = V²/R once Ohm's law is substituted. A 100 Ω resistor with 5 V across it dissipates 5²/100 = 0.25 W, exactly the limit of a ¼ W part, so a 0.5 W resistor is the safer choice there.

### Does Ohm's law apply to LEDs and light bulbs?

Not with a single fixed resistance. An LED conducts almost nothing below its forward voltage (about 2 V for red, about 3 V for blue and white) and then current rises steeply, so it needs a series resistor. A filament lamp's resistance rises as the tungsten heats, so the V²/P value, 882 Ω for a 60 W, 230 V bulb, applies only when it is lit.

### What is conductance?

Conductance is the reciprocal of resistance, G = 1/R, measured in siemens (S). A 6 Ω resistor has a conductance of 1/6 ≈ 0.167 S. Conductances in parallel add directly, which is why it is convenient for parallel networks: 1/R_total = G₁ + G₂ + …

### How accurate is the Ohm's law calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 6 worked examples whose answers come from independent sources; for example, “12 V across 6 Ω” is checked against Python 3.8 decimal: I = 12/6, P = 12²/6.

### Where does the method come from?

OpenStax University Physics Volume 2, §9.4 Ohm's law and §9.5 Electrical energy and power; HyperPhysics — Ohm's law, power.

## Sources

- [OpenStax University Physics Volume 2, §9.4 Ohm's law and §9.5 Electrical energy and power](https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law)
- [HyperPhysics — Ohm's law, power](http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html)