In this article we are going to lear how to create custom units and dimensions to extend the ones that Swift offers.
## Creating a Custom Unit
To create a custom unit we need to use the initializer of the dimension we want to add a new unit to.
So if we want to add a new unit to `UnitIlluminance`, we can do the following:
```swift
let footCandleUnit = UnitIlluminance(symbol: "fc", converter: UnitConverterLinear(coefficient: 10.76))
```
> A coefficient of '10.76' means that whenever a measurement of this unit is converted to the base unit, which is `.lux`, it will be multiplied by '10.76'.
Now we can convert `.lux` to `.footCandle`:
```swift
let footCandle = Measurement(value: 2, unit: footCandleUnit)
let lumen = footCandle.converted(to: .lux)
```
## Extending an Existing Dimension
The second method uses an extension to add custom units.
```swift
extension UnitIlluminance {
  static let footCandle = UnitIlluminance(symbol: "fc", converter: UnitConverterLinear(coefficient: 10.76))
}
```
We declare the new unit exactly as previously, passing a symbol and a coefficient. Now we can use this new unit in a slightly different way.
```swift
let footCandle = Measurement(value: 2, unit: UnitIlluminance.footCandle)
let lumen = footCandle.converted(to: .lux)
```
## Creating a Custom Dimension
Swift allow us to create a whole new dimension by creating a subclass of Dimension.
Here is the example that Apple used in its documentation:
```swift
class CustomRadioactivityUnit: Dimension {
    static let becquerel = CustomRadioactivityUnit(symbol: "Bq", UnitConverterLinear(coefficient: 1.0))
    static let curie = CustomRadioactivityUnit(symbol: "Ci", UnitConverterLinear(coefficient: 3.7e10))
    static let baseUnit = self.becquerel
}
```
1. First you create the units of the dimension, as we've seen previously.
2. Then you define the base unit of the new dimension using the `baseUnit` property.
## Conclusion
As we've seen, creating custom units and dimensions is an easy task, and most of the time the built in dimensions are just what we need.
Thanks for reading.

Learn how to use the measurements APIs to create custom units and convert units of measure easily and effortlessly with Swift.

Create Custom Units and Dimensions in Swift (Part 2)

Most developers are not aware that Apple introduced the units and measurements APIs. They are part of the Foundation framework and available on every Apple platform.
Let's get started by seeing what `Unit`, `Dimension`, and `Measurement` are.
## Unit
* The [`Unit`](https://developer.apple.com/documentation/foundation/unit) class represents a unit that has a string to describe it called `symbol`.
## Dimension
* The [`Dimension`](https://developer.apple.com/documentation/foundation/dimension) class inherits from `Unit`.
* It has multiple subclasses, each representing a different dimension (e.g. `UnitAcceleration`, `UnitDuration`, and `UnitLength`).
* Each of these subclasses defines a base unit and multiple additional units that can be converted to other units of the same dimension by using a `UnitConverter`.
* A list of all dimension subclasses can be found in [Apple's documentation](https://developer.apple.com/documentation/foundation/dimension).
## Measurement
* The [`Measurement`](https://developer.apple.com/documentation/foundation/measurement) struct wraps a double value together with a unit.
## Usage
Now lets see how `Measurement` works.
If you're 1,73 meters tall, you'd create a `Measurement` instance like this:
```swift
let heightInMeters = Measurement(value: 1.73, unit: UnitLength.meters)
```
Then you can convert it to other units like this:
```swift
let heightInInches = heightInMeters.converted(to: UnitLength.inches)
```
You should see “28.7 in” in your output, showing that the conversion process has worked.
The `UnitLength` class, like all unit subclasses, spans a huge range of units from old to futuristic. For example, you can convert feet to astronomical units, which is equal to the average distance between the Earth and the Sun, or about 150 million kilometers:
```swift
let heightAUs = heightInFeet.converted(to: UnitLength.astronomicalUnits)
```
Every unit works exactly the same as the one we've seen. Here are some more examples:
```swift
// Convert fahrenheit to celsius
let fahrenheit = Measurement(value: 6, unit: UnitTemperature.fahrenheit)
let celsius = fahrenheit.converted(to: .celsius)
 
// Convert kilometersPerHour to milesPerHour
let kilometersPerHour = Measurement(value: 35, unit: UnitSpeed.kilometersPerHour)
let milesPerHour = kilometersPerHour.converted(to: .milesPerHour)
```
## Conclusion
As we have seen Swift provides an easy-to-use and powerful way of defining, using, and converting measurements.
Since there are currently 21 built-in dimensions with about 200 units combined, this will be enough for most apps. But there is also the possibility to define and add special units or even whole new dimensions. We will cover this in the next article.
Thanks for reading!

Measurement

Create Custom Units and Dimensions in Swift (Part 2)

Units and Measurement Conversion in Swift (Part 1)