Fling Units

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A type safe Dart library for unit conversion. Pass around fully abstracted measurements to keep your code simple!

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We designed fling_units to simplify working with any measurement unit within a code base by abstracting measurements to basic types that can be "interpreted" into any valid unit of that type, all in a type-safe way. This means you will never pass around a measurement of the wrong type or unit again without seeing red squiggles in your IDE.

Have a look at the examples below for more details on what is possible, and let me know how we can improve fling_units by creating an issue!


Design goals

  • Simplicity: Casual users must be able to pick up the basic features. We prefer idiomatic approaches since users will already have experience with them. The library will not offer confusing or difficult options.

  • Safety: We will protect against invalid or incorrect usage. We favor compile-time checks over runtime checks. If something is possible with the library, it must be valid in the real world.

  • Correctness: Conversions must be accurate. The limiting factor in accuracy is the measurement itself, not the library. Bug fixes take top priority. Full test coverage is required.

  • Minimalism: We will not depend on larger libraries or other difficult dependencies. Including the library into a project must be easy and must not require any configuration or complicated setup.

Things that matter less

  • Performance: While we will not try to make the library slower than it needs to be, we will not make small performance enhancements at the cost of other design goals.

  • Memory: We will not reduce memory footprint at the cost of one of the other design goals, but we will keep the library as small and unobtrusive as possible.


Create an instance of the dimension type you want to measure:

void main() {
  Distance distanceToSeattle = kilo.meters(
      246); // you can use metric prefixes with any unit
  Distance distanceToTheMoon = Distance.sum(
      [miles(238900), feet(42), inches(6.3)]);
  Mass massOfTheMoon = yocto.grams(73.5);

Convert to any other measurement type within that dimension:

void main() {
  double distanceToSeattleInMiles = distanceToSeattle.as(miles);
  double distanceToSeattleInHectoInches = distanceToSeattle.as(hecto.inches);
  double massOfTheMoonInShortTons = massOfTheMoon.as(shortTons);

Perform basic arithmetic:

void main() {
  // multiplication
  Distance distanceToSeattleAndBack = distanceToSeattle * 2;

  // addition
  Distance distanceToTheMoon = distanceToUpperAtmosphere +

  // support for infinite measurements
  Distance distanceToTheEndsOfTheUniverse = Distance.infinite();

  // compare measurements within the same dimension
  bool useTheCar = distanceToTravel >= miles(1.5);

Built-in ordering:

void main() {
  [Distance.zero(), Distance.infinite(), meters(3), feet(3), yards(-2)].sort();
  // produces [yards(-2), zero(), feet(3), meters(3), infinite()]

Abstract away the specific units your code needs by passing around the encapsulated types. It doesn't matter which units different parts of your code require; you can combine them seamlessly:

Distance computeTotalDistanceWithWiggleRoom(final Distance targetDistance) {
  return targetDistance + meters(3.0);

The more common derived units, e.g. Area and Volume, have full support:

void main() {
  Area myBackYardSize = Area.of(feet(100), feet(70));
  double myBackYardSizeInSquareInches = myBackYardSize.asArea(

  Area yourBackYardSize = Area.square(meters)(100);
  bool offerToHostSummerBarbecue = myBackYardSize > yourBackYardSize;

You can create your own basic derived units (with all the basic features you'd expect) from existing units:

void main() {
  var fuelEconomy = kilo.meters(100).per(liters(6));
  double milesPerGallon = fuelEconomy.as(miles, gallons);

  var stateRequiredFuelEconomy = inches(24).per(teaspoons(2));
  bool switchCars = fuelEconomy < stateRequiredFuelEconomy;

Ensure type safety at compile time:

void main() {
  // none of these lines will compile!
  var nonsense = miles(123) + grams(18);
  var impossible = meters(3).as(celcius);
  var silliness = grams(5) < seconds(3);

Express the certainty in your measurements by setting a precision. Conversions will automatically provide appropriate significant digits. Set the precision when you create the measurement, or later on:

void main() {
  var myHeight = meters(1.5, precision: Precision(2));
  var myHeightInInches = myHeight.as(inches); // 59.0
  var myWeight = kilo.grams(61.234);
  var myPreciseWeight = myWeight.withPrecision(Precision(3)); // 61.2

Extensions allow you to define measurements from any number for a more natural syntax:

void main() {
  var distanceToTheStore = 3.kilo.meters;
  var distanceToYourHouse = 2.5.miles;

Measurements and units alike produce sensible toString() output to facilitate debugging:

void main() {
  print(3.miles); // "3.0 mi"
  print(kilo.meters); // "km"
  print(4.miles.per(2.hours)); // "2.0 mi/h"

Supported Features


With some exceptions (due to the nature of those measurements), all measurements support:

  • all standard metric prefixes for each unit
  • customizable precision for each measurement
  • Comparable for built-in ordering among similar measurements
  • compare two similar measurements (>, <, ==, !=, >=, <=)
  • add or subtract two similar measurements (+, -)
  • multiply or divide measurements by a scalar (*, /)
  • negate measurements (unary -)
  • measurement comparisons (~/, compareMagnitude)
  • visitor pattern compatible
  • extensions on num for more natural instantiation

SI Prefixes

Work with any unit:

  • quecto
  • ronto
  • yocto
  • zepto
  • atto
  • femto
  • pico
  • nano
  • micro
  • milli
  • centi
  • deci
  • deka
  • hecto
  • kilo
  • mega
  • giga
  • tera
  • peta
  • exa
  • zetta
  • yotta
  • ronna
  • quetta

Distance Units

  • meters
  • inches
  • feet
  • yards
  • miles
  • nautical miles

Area Units

  • any Distance unit x any other Distance unit

Volume Units

  • liters
  • teaspoon (Imperial / US)
  • tablespoon (Imperial / US)
  • fluid ounce (Imperial / US)
  • cup (Imperial / US / US Legal)
  • pint (Imperial / US)
  • quart (Imperial / US)
  • gallon (Imperial / US)
  • cubic foot
  • cubic inch

Temperature Units

  • kelvin
  • celcius
  • fahrenheit

Time units

  • seconds
  • minutes
  • hours
  • days

Mass units

  • grams
  • tonnes
  • ounces
  • pounds
  • short (US) tons
  • long (UK) tons
  • atomic mass units
  • daltons
  • electron rest mass

Electric Charge units

  • amperes

Luminous Intensity units

  • candela
  • candlepower
  • Hefnerkerze

Pressure units

  • Pascals
  • bars
  • Baryes
  • standard atmospheres
  • technical atmospheres
  • mmHg
  • inHg
  • PSI

Angle units

  • turns (rotations)
  • radians
  • gradians
  • degrees
  • arc minutes
  • arc seconds

Frequency units

  • hertz


A type-safe measurement framework.