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Release notes for 2.0 are here.

elm-units is useful if you want to store, pass around, convert between, compare, or do arithmetic on:

  • Durations (seconds, milliseconds, hours...)
  • Angles (degrees, radians, turns...)
  • Lengths (meters, feet, inches, miles, light years...)
  • Temperatures (Celsius, Fahrenheit, kelvins)
  • Pixels (whole or partial)
  • Speeds (pixels per second, miles per hour...) or any other rate of change
  • Any of the other built-in quantity types: areas, accelerations, masses, forces, pressures, currents, voltages...
  • Or even values in your own custom units, such as 'number of tiles' in a tile-based game

It is aimed especially at engineering/scientific/technical applications but is designed to be generic enough to work well for other fields such as games and finance. The core of the package consists of a generic Quantity type and many concrete types such as Length, Angle, Duration, Temperature, and Speed, which you can use to add some nice type safety to data types and function signatures:

type alias Camera =
    { fieldOfView : Angle
    , shutterSpeed : Duration
    , minimumOperatingTemperature : Temperature

canOperateAt : Temperature -> Camera -> Bool
canOperateAt temperature camera =
        |> Temperature.greaterThan

You can construct values of these types from any units you want, using provided functions such as:

Length.feet : Float -> Length
Length.meters :  Float -> Length
Duration.seconds : Float -> Duration
Angle.degrees : Float -> Angle
Temperature.degreesFahrenheit : Float -> Temperature

You can later convert back to plain numeric values, also in any units you want (which do not have to be the same units used when initially constructing the value!):

Length.inCentimeters : Length -> Float
Length.inMiles : Length -> Float
Duration.inHours : Duration -> Float
Angle.inRadians : Angle -> Float
Temperature.inDegreesCelsius : Temperature -> Float

This means that (among other things!) you can use these functions to do simple unit conversions:

Duration.hours 3 |> Duration.inSeconds
--> 10800

Length.feet 10 |> Length.inMeters
--> 3.048

Speed.milesPerHour 60 |> Speed.inMetersPerSecond
--> 26.8224

Temperature.degreesCelsius 30
    |> Temperature.inDegreesFahrenheit
--> 86

Additionally, types like Length are actually type aliases of the form Quantity number units (Length is Quantity Float Meters, for example, meaning that it is internally stored as a number of meters), and there are many generic functions which let you work directly with any kind of Quantity values:

Length.feet 3
    |> Quantity.lessThan (Length.meters 1)
--> True

Duration.hours 2
  |> (Duration.minutes 30)
  |> Duration.inSeconds
--> 9000

-- Some functions can actually convert between units!
-- Multiplying two Length values gives you an Area
Length.centimeters 60
    |> Quantity.times
        (Length.centimeters 80)
--> Area.squareMeters 0.48

    [ Angle.radians 1
    , Angle.degrees 10
    , Angle.turns 0.5
--> [ Angle.degrees 10 , Angle.radians 1 , Angle.turns 0.5 ]

Ultimately, what this does is let you pass around and manipulate Length, Duration or Temperature etc. values without having to worry about units. When you initially construct a Length, you need to specify what units you're using, but once that is done you can:

  • Store the length inside a data structure
  • Pass it around between different functions
  • Compare it to other lengths
  • Add and subtract it to other lengths
  • Multiply it by another length to get an area, or divide by a duration to get a speed

...and much more, all without having to care about units at all. All calculations will be done in an internally consistent way, and when you finally need to actually display a value on screen or encode to JSON, you can extract the final result in whatever units you want.

Table of contents


Assuming you have installed Elm and started a new project, you can install elm-units by running

elm install ianmackenzie/elm-units

in a command prompt inside your project directory.



To take code that currently uses raw Float values and convert it to using elm-units types, there are three basic steps:

  • Wherever you store a Float, such as in your model or in a message, switch to storing a Duration or Angle or Temperature etc. value instead.
  • Whenever you have a Float (from an external package, JSON decoder etc.), use a function such as Duration.seconds, Angle.degrees or Temperature.degreesFahrenheit to turn it into a type-safe value.
  • Whenever you need a Float (to pass to an external package, encode as JSON etc.), use a function such as Duration.inMillliseconds, Angle.inRadians or Temperature.inDegreesCelsius to extract the value in whatever units you want.
  • Where you do math with Float values, switch to using Quantity functions like or Quantity.greaterThan. If this becomes impractical, there are other approaches.

The Quantity type

All values produced by this package (with the exception of Temperature, which is a bit of a special case) are actually values of type Quantity, defined as

type Quantity number units
    = Quantity number

For example, Length is defined as

type alias Length =
    Quantity Float Meters

This means that a Length is internally stored as a Float number of Meters, but the choice of internal units can mostly be treated as an implementation detail.

Having a common Quantity type means that it is possible to define generic arithmetic and comparison operations that work on any kind of quantity; read on!

Basic arithmetic and comparison

You can do basic math with Quantity values:

-- 6 feet 3 inches, converted to meters
Length.feet 6
    |> (Length.inches 3)
    |> Length.inMeters
--> 1.9050000000000002

Duration.hours 1
  |> Quantity.minus (Duration.minutes 15)
  |> Duration.inMinutes
--> 45

-- pi radians plus 45 degrees is 5/8 of a full turn
Quantity.sum [ Angle.radians pi, Angle.degrees 45 ]
    |> Angle.inTurns
--> 0.625

Quantity values can be compared/sorted:

Length.meters 1 |> Quantity.greaterThan (Length.feet 3)
--> True (Length.meters 1) (Length.feet 3)
--> GT

Quantity.max (Length.meters 1) (Length.feet 3)
--> Length.meters 1

Quantity.maximum [ Length.meters 1, Length.feet 3 ]
--> Just (Length.meters 1)

Quantity.sort [ Length.meters 1, Length.feet 3 ]
--> [ Length.feet 3, Length.meters 1 ]

Multiplication and division

There are actually three different 'families' of multiplication and division functions in the Quantity module, used in different contexts:

  • multiplyBy and divideBy are used to multiply (scale) or divide a Quantity by a plain Int or Float, with twice and half for the common cases of multiplying or dividing by 2
  • product, times, over and over_ are used to work with quantities that are products of other quantities:
    • multiply a Length by another Length to get an Area
    • multiply an Area by a Length to get a Volume
    • multiply a Mass by an Acceleration to get a Force
    • divide a Volume by an Area to get a Length
    • divide a Force by a Mass to get an Acceleration
  • rate, per, at, at_ and for are used to work with rates of change:
    • divide Length by Duration to get Speed
    • multiply Speed by Duration to get Length
    • divide Length by Speed to get Duration
  • And one bonus fourth function: ratio, used to divide two quantities with the same units to get a plain Float value

For example, to calculate the area of a triangle:

-- Area of a triangle with base of 2 feet and
-- height of 8 inches
base =
    Length.feet 2

height =
    Length.inches 8

Quantity.half (Quantity.product base height)
    |> Area.inSquareInches
--> 96

Comprehensive support is provided for working with rates of change:

-- How fast are we going if we travel 30 meters in
-- 2 seconds?
speed =
    Length.meters 30 |> Quantity.per (Duration.seconds 2)

-- How far do we go if we travel for 2 minutes
-- at that speed?
Duration.minutes 2 -- duration
  |> speed -- length per duration
  |> Length.inKilometers -- gives us a length!
--> 1.8

-- How long will it take to travel 20 km
-- if we're driving at 60 mph?
Length.kilometers 20
    |> Quantity.at_ (Speed.milesPerHour 60)
    |> Duration.inMinutes
--> 12.427423844746679

-- How fast is "a mile a minute", in kilometers per hour?
Length.miles 1
    |> Quantity.per (Duration.minutes 1)
    |> Speed.inKilometersPerHour
--> 96.56064

-- Reverse engineer the speed of light from defined
-- lengths/durations (the speed of light is 'one light
-- year per year')
speedOfLight =
    Length.lightYears 1
        |> Quantity.per (Duration.julianYears 1)

speedOfLight |> Speed.inMetersPerSecond
--> 299792458

-- One astronomical unit is the (average) distance from the
-- Sun to the Earth. Roughly how long does it take light to
-- reach the Earth from the Sun?
Length.astronomicalUnits 1
    |> Quantity.at_ speedOfLight
    |> Duration.inMinutes
--> 8.316746397269274

Note that the various functions above are not restricted to speed (length per unit time) - any units work:

pixelDensity =
    Pixels.pixels 96 |> Quantity.per (Length.inches 1)

Length.centimeters 3 -- length
    |> pixelDensity -- pixels per length
    |> Pixels.inPixels -- gives us pixels!
--> 113.38582677165354

Argument order

Note that several functions like Quantity.minus and Quantity.lessThan (and their Temperature equivalents) that mimic binary operators like - and < "take the second argument first"; for example,

Quantity.lessThan x y

means y < x, not x < y. This is done for a couple of reasons. First, so that use with |> works naturally; for example,

x |> Quantity.lessThan y

does mean x < y. The 'reversed' argument order also means that things like (Quantity.minus x) [ a, b, c ]

will work as expected - it will result in

[ a - x, b - x, c - x ]

instead of

[ x - a, x - b, x - c ]

which is what you would get if Quantity.minus took arguments in the 'normal' order.

There are, however, several functions that take arguments in 'normal' order, for example:

  • Quantity.difference (compare to minus)
  • Quantity.product (compare to times)
  • Quantity.rate (compare to per)
  • Quantity.ratio

In general the function names try to match how you would use them in English; you would say "the difference of a and b" (and so Quantity.difference a b) but "a minus b" (and so a |> Quantity.minus b).

Custom Functions

Some calculations cannot be expressed using the built-in Quantity functions. Take kinetic energy E_k = 1/2 * m * v^2, for example - the elm-units type system is not sophisticated enough to work out the units properly. Instead, you'd need to create a custom function like

kineticEnergy : Mass -> Speed -> Energy
kineticEnergy (Quantity m) (Quantity v) =
    Quantity (0.5 * m * v^2)

In the implementation of kineticEnergy, you're working with raw Float values so you need to be careful to make sure the units actually do work out. (The values will be in SI units - meters, seconds etc.) Once the function has been implemented, though, it can be used in a completely type-safe way - callers can supply arguments using whatever units they have, and extract results in whatever units they want:

kineticEnergy (Mass.tonnes 1.5) (Speed.milesPerHour 60)
    |> Energy.inKilowattHours
--> 0.14988357119999998

Custom Units

elm-units defines many standard unit types, but you can easily define your own! See CustomUnits for an example.

Understanding quantity types

The same quantity type can often be expressed in multiple different ways. Take the Volume type as an example. It is an alias for

Quantity Float CubicMeters

but expanding the CubicMeters type alias, this is equivalent to

Quantity Float (Cubed Meters)

which expands further to

Quantity Float (Product (Product Meters Meters) Meters)

which could also be written as

Quantity Float (Product (Squared Meters) Meters)

or even

Quantity Float (Product SquareMeters Meters)

and you may see any one of these forms pop up in compiler error messages.

Getting Help

For general questions about using elm-units, try asking in the Elm Slack or posting on the Elm Discourse forums or the Elm subreddit. I'm @ianmackenzie on all three =)


Full API documentation is available.

Climate action

I would like for the projects I work on to be as helpful as possible in addressing the climate crisis. If

  • you are working on a project that helps address the climate crisis (clean energy, public transit, reforestation, sustainable agriculture etc.) either as an individual, as part of an non-profit organization or even as part of a for-profit company, and
  • there is a new feature you would find helpful for that work (or a bug you need fixed) in any of my open-source projects, then

please open a new issue, describe briefly what you're working on and I will treat that issue as high priority.


Yes please! One of the best ways to contribute is to add a module for a new quantity type; see issue #6 for details. I'll add a proper at some point, but some brief guidelines in the meantime:

  • Open a pull request by forking this repository, creating a new branch in your fork, making all changes in that branch, then opening a pull request from that branch.
  • Format code with elm-format 0.8.1.
  • Git commit messages should follow the seven rules of a great Git commit message, although I'm not strict about the 50 or 72 character rules.


BSD-3-Clause © Ian Mackenzie