Programming language: Swift
License: MIT License
Tags: Utility    
Latest version: v4.0.0

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MKUnits is extremely precise unit conversion library for Swift. It provides units of measurement of physical quantities and simplifies manipulation of them.

NB For Objective-C implementation, please refer to MKUnits pod 2.2.1 or visit archived branch.


MKUnits is available under the MIT license. See the LICENSE file for more info.


MKUnits is available through CocoaPods. To install it, simply add the following line to your Podfile:

pod "MKUnits"


Let's create 1.5 kilograms [kg],

let kilograms = 1.5.kilogram()

0.5 [kg] in grams [g]

let grams = 500.gram()

and 10 pounds [lb] (which is 4.5359237 [kg]) <!--and 4.5359237 [kg] in pounds [lb].-->

let pounds = 10.pound()

Then we add all of the above togehter and subtract 0.0001 [kg] in milligrams [mg].

let milligrams = 100.milligram()
var result = kilograms + grams + pounds - milligrams

The result amount is 6.5358237 [kg].

Now we subtract 0.5358237 [kg] in ounces [oz], which is 18.900624805 [oz] according to Google converter, but as MKUnits is very precise, it is in fact 18.900624805483390296005199558361177 [oz].

let ounces = 0.5358237.kilogram().converted(MassUnit.ounce)
result = result - ounces

The result amount is ~6 [kg]; 6.00000000000000000000000000000000003 [kg] to be exact.

Now we want the result to be in stones [st], so:

result = result.converted(MassUnit.stone)
// 0.94483873964811055873038869091017890993 st

As the result is too precise for our needs, we want to round it.

<!--We can do that by either creating new MKQuantity object with rounded amount,-->

let rounded = result.rounded(3)
// 0.945 st

<!--or by rounding the amount of existing MKQuantity.-->

<!--```swift--> <!--id amount_in_stones_1 = [result amountWithPrecision:3];--> <!--// 0.945-->

<!--id amount_in_stones_2 = [[result amount] mk_roundedWithPrecision:3];;--> <!--// 0.945--> <!--```-->

Supported Units

At the moment MKUnits supports the following group units:

  • Area (base unit: square meter)
  • Mass (base unit: kilogram)
  • Length (base unit: meter)
  • Time (base unit: second)
  • Volume (base unit: litre)

You can easily extend MKUnits by adding new group units or units.

Extending MKUnits

Adding a new group unit

To add a new group unit, simply create a class that extends Unit and follow the convention below.

Please make sure that unit symbols are unique across your new group unit.

public final class NewUnit: Unit {

    public static var unitA: NewUnit {
        return NewUnit(
            name: "unit A",
            symbol: "uA",
            ratio: NSDecimalNumber.one() // as it is a base unit

    public static var unitB: NewUnit {
        return NewUnit(
            name: "unit B",
            symbol: "uB",
            ratio: NSDecimalNumber(mantissa: 2, exponent: 0, isNegative: false)

extension NSNumber {

    public func unitA() -> Quantity {
        return Quantity(amount: self, unit: NewUnit.unitA)

    public func unitB() -> Quantity {
        return Quantity(amount: self, unit: NewUnit.unitB)

Adding new units to existing group unit

To add additional units to existing group unit simply create a category for that group unit.

Do not sublcass as units are only interconvertible with units belonging to the same group unit.

extension NewUnit {
    public static var unitC: NewUnit {
        return NewUnit(
            name: "unit C",
            symbol: "uC",
            ratio: NSDecimalNumber(mantissa: 4, exponent: 0, isNegative: false)

extension NSNumber {
    public func unitC() -> Quantity {
        return Quantity(amount: self, unit: NewUnit.unitC)


  1. Fork it.
  2. Create your feature branch (git checkout -b new-feature).
  3. Commit your changes (git commit -am 'Added new-feature').
  4. Push to the branch (git push origin new-feature).
  5. Create new Pull Request.

*Note that all licence references and agreements mentioned in the MKUnits README section above are relevant to that project's source code only.