GNU Units -compatible Ruby library for unit conversion of physical quantities. Major features: (1) It uses rich database of GNU Units. (2) It provides normal class interface without modifying built-in classes. Former name is Quanty, the first Ruby units library released in 2001.


Install from gem as:

$ gem install phys-units

Or install from source tree:

$ ruby setup.rb

Load Phys-Units library in your Ruby script:

require 'phys/units'



is the primary class of Phys-Units library. This class represents Physical Quantities with a Unit of measurement. It contains Value and Unit.

  • Value of the quantity is given as the first parameter of Quantity constructor (alias is Quantity[]). It must be a class instance having arithmetic methods, but it is not necessary to be a Numeric. This is a duck typing way.

    Phys::Quantity[2.5,"miles"].value  #=> 2.5
  • Unit is an instance of Phys::Unit class. It is created from the second argument of Quantity constructor.

    Phys::Quantity[2.5,"miles"].unit   #=> #<Phys::Unit 1609.344,{"m"=>1},@expr="5280 ft">


is a class to represent Physical Units of measurement. This class is used in the Phys::Quantity for calculations with Units. It has Factor and Dimension:

  • Factor of the unit is a scale factor relative to its base unit.

    Phys::Unit["km"].factor #=> 1000
  • Dimension of the unit is a hash table containing base units and dimensions as key-value pairs.

    Phys::Unit["N"].dimension #=> {"kg"=>1, "m"=>1, "s"=>-2}


require "phys/units"
Q = Phys::Quantity
U = Phys::Unit

Q[1.23,'km'] + Q[4.56,'m']    #=> Phys::Quantity[1.23456,'km']
Q[123,'mile'] / Q[2,'hr']     #=> Phys::Quantity[61,'mile/hr']
Q[61,'miles/hr'].want('m/s')  #=> Phys::Quantity[27.26944,'m/s']
Q[1.0,'are'] == Q[10,'m']**2  #=> true
Q[70,'tempF'] + Q[10,'degC']  #=> Phys::Quantity[88,'tempF']
Q[20,'tempC'].want('tempF')   #=> Phys::Quantity[68,'tempF']
Math.cos(Q[60,'degree'].to_f) #=> 0.5

U["m/s"] === Q[1,'miles/hr']  #=> true

case Q[1,"miles/hr"]
when U["LENGTH"]
  puts "length"
when U["TIME"]
  puts "time"
when U["VELOCITY"]
  puts "velocity"
  puts "other"
end                    #=> "velocity"


Phys-Units library is differentiated from many other units libraries for Ruby, by the following features:

  • Compatible with GNU Units except currency and nonlinear units.
  • Provides 2331 units, 85 prefixes, including UTF-8 unit names.
  • Defines Units by reading GNU Units text data (See load_units.rb), unlike other libraries which define Units in Ruby code.
  • Provides orthodox design of class interface.
  • No addition or modification to Ruby standard classes in standard usage, to avoid conflicts with other libraries.
  • Calculation of values is through Ruby Numeric arithmetic methods. Phys-Units does not care it.
  • Conversion factors are internally held in Rational form even defined as the decimal form such as `1.0e10'.
  • PI number has a dimension.
  • Japanese Units are available by require 'phys/units/jp'.

Appendix Feature

Mix-in feature provides a simple unit calculator. Note that this usage involves global changes on a build-in class and will cause conflicts with other libraries.

$ irb -r phys/units/mixin

irb> 2.5.miles/hr >> m/s
=> Phys::Quantity[1.1176,"m/s"]

irb> 23.tempC >> tempF
=> Phys::Quantity[73.4,"tempF"]

irb> print_units LENGTH
m                               !
meter                           m
LENGTH                          meter

irb> print_units ENERGY
joule                           N m
J                               joule
prout                           185.5 keV

Platforms tested

  • ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-linux]

Copying License

This program is free software. You can distribute/modify this program under the same terms as GPL3. See "COPYING" file. NO WARRANTY.


Masahiro TANAKA