quantity
Facilitates working with physical quantities.
This is a work in progress; this package's API may change significantly.
Some parts are currently unfinished, untested and/or undocumented. Use at your own risk.
Quick start
// Import the core library
import 'package:quantity/quantity_si.dart';
// Construct an Angle in radians
var ang = new Angle(rad: 1.1);
// Construct an Angle in degrees
var ang2 = new Angle(deg: 270);
// Find the difference
var diff = ang2  ang;
// Display the result in degrees
print(diff.valueInUnits(Angle.degrees);
Key Features
 Typed quantities (such as
Length
,Mass
andTemperature
) that are easy to construct and use in calculations.  Encourages use of standard units by isolating nonstandard units in an extension library.
 Support for relative standard uncertainty in quantity values and calculations.
 Allows use of arbitrary precision values (as well as complex and imaginary values).
 Provides access to the latest official constant values published by NIST.
 Integration with Dart's existing
Duration
andDateTime
classes.  Extensive set of quantity types and standard and nonstandard units.
 Support for untyped quantities having unusual dimensions with the
MiscQuantity
class.  Models quantity ranges, including special support for angle ranges and time spans.
 Includes a mutable quantity with streams.
 Metric prefixes for any unit.
Libraries
 astronomical
 Quantity types, units and constants commonly encountered in the field of Astronomy and related sciences.
 electromagnetic
 Quantity types, units and constants commonly encountered in the fields dealing with electromagnetics.
 number

The quantity package accepts both Dart's
num
types and theNumber
s defined by this number library as quantity values and for quantity operations. [...]  quantity
 A superset of all the other libraries available in the quantity package. [...]
 quantity_ext
 Extends the core library with nonSI units, constants, unit synonyms to enable terse constructors and expressions, and auxiliary functions.
 quantity_range
 Models ranges of quantites with start and end values (not necessarily increasing). [...]
 quantity_si
 The core library containing the fundamental quantity classes and types consistent with the International System of Units (SI, in the French acronym).
 thermodynamic
 Quantity types, units and constants commonly encountered in the field of thermodynamics and related sciences.
 universal
 Quantity types and constants with universal application.