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A unit handling library for ruby

Ruby Units

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Kevin C. Olbrich, Ph.D.

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Many technical applications make use of specialized calculations at some point. Frequently, these calculations require unit conversions to ensure accurate results. Needless to say, this is a pain to properly keep track of, and is prone to numerous errors.


The 'Ruby units' gem is designed to simplify the handling of units for scientific calculations. The units of each quantity are specified when a Unit object is created and the Unit class will handle all subsequent conversions and manipulations to ensure an accurate result.


This package may be installed using: gem install ruby-units


unit ="1")             # constant only
unit = Unit("mm")                # unit only (defaults to a scalar of 1)
unit = Unit("1 mm")              # create a simple unit
unit = Unit("1 mm/s")            # a compound unit
unit = Unit("1 mm s^-1")         # in exponent notation
unit = Unit("1 kg*m^2/s^2")      # complex unit
unit = Unit("1 kg m^2 s^-2")     # complex unit
unit = Unit("1 mm")              # shorthand
unit = "1 mm".to_unit            # convert string object
unit = object.to_unit            # convert any object using object.to_s
unit = Unit('1/4 cup')           # Rational number 
unit = Unit('1+1i mm')           # Complex Number


  1. only 1 quantity per unit (with 2 exceptions... 6'5" and '8 lbs 8 oz')
  2. use SI notation when possible
  3. spaces in units are allowed, but ones like '11/m' will be recognized as '11 1/m'.

Unit compatibility:

Many methods require that the units of two operands are compatible. Compatible units are those that can be easily converted into each other, such as 'meters' and 'feet'.

unit1 =~ unit2                  #=> true if units are compatible
unit1.compatible?(unit2)        #=> true if units are compatible

Unit Math:

Unit#+()      # Add. only works if units are compatible
Unit#-()      # Subtract. only works if units are compatible
Unit#*()      # Multiply.  
Unit#/()      # Divide.
Unit#**()     # Exponentiate.  Exponent must be an integer, can be positive,  negative, or zero                        
Unit#inverse  # Returns 1/unit
Unit#abs      # Returns absolute value of the unit quantity.  Strips off the  units
Unit#ceil     # rounds quantity to next highest integer
Unit#floor    # rounds quantity down to next lower integer
Unit#round    # rounds quantity to nearest integer
Unit#to_int   # returns the quantity as an integer

Unit will coerce other objects into a Unit if used in a formula. This means that ..

Unit("1 mm") + "2 mm"  == Unit("3 mm")

This will work as expected so long as you start the formula with a Unit object.

Conversions & comparisons

Units can be converted to other units in a couple of ways.

unit1 = unit >> "ft"              # convert to 'feet'
unit >>= "ft"                     # convert and overwrite original object
unit3 = unit1 + unit2             # resulting object will have the units of unit1
unit3 = unit1 - unit2             # resulting object will have the units of unit1
unit1 <=> unit2                   # does comparison on quantities in base units, throws an exception if not compatible
unit1 === unit2                   # true if units and quantity are the same, even if 'equivalent' by <=>
unit.convert_to('ft')             # convert
unit1 + unit2 >> "ft"             # converts result of math to 'ft'
(unit1 + unit2).convert_to('ft')  # converts result to 'ft'

Any object that defines a 'to_unit' method will be automatically coerced to a unit during calculations.

Text Output

Units will display themselves nicely based on the display_name for the units and prefixes. Since Unit implements a Unit#to_s, all that is needed in most cases is:

"#{Unit('1 mm')}"  #=> "1 mm"

The to_s also accepts some options.

Unit('1.5 mm').to_s("%0.2f")  # "1.50 mm".  Enter any valid format
                                  string.  Also accepts strftime format
Unit('1.5 mm').to_s("in")     # converts to inches before printing
Unit("2 m").to_s(:ft)         # returns 6'7"
Unit("100 kg").to_s(:lbs)     # returns 220 lbs, 7 oz

Time Helpers

Time, Date, and DateTime objects can have time units added or subtracted. + Unit("10 min") 

Several helpers have also been defined. Note: If you include the 'Chronic' gem, you can specify times in natural language.

  Unit('min').since(DateTime.parse('9/18/06 3:00pm'))

Durations may be entered as 'HH:MM:SS, usec' and will be returned in 'hours'.

Unit('1:00')     #=> 1 h
Unit('0:30')     #=> 0.5 h
Unit('0:30:30')  #=> 0.5 h + 30 sec

If only one ":" is present, it is interpreted as the separator between hours and minutes.


[Unit('0 h')..Unit('10 h')].each {|x| p x}

works so long as the starting point has an integer scalar

Math functions

All Trig math functions (sin, cos, sinh, hypot...) can take a unit as their parameter. It will be converted to radians and then used if possible.


Ruby-units makes a distinction between a temperature (which technically is a property) and degrees of temperature (which temperatures are measured in).

Temperature units (i.e., 'tempK') can be converted back and forth, and will take into account the differences in the zero points of the various scales. Differential temperature (e.g., '100 degC'.unit) units behave like most other units.

Unit('37 tempC').convert_to('tempF')   #=> 98.6 tempF

Ruby-units will raise an exception if you attempt to create a temperature unit that would fall below absolute zero.

Unit math on temperatures is fairly limited.

Unit('100 tempC') + Unit('10 degC')   # '110 tempC'.unit
Unit('100 tempC') - Unit('10 degC')   # '90 tempC'.unit
Unit('100 tempC') + Unit('50 tempC')  # exception  
Unit('100 tempC') - Unit('50 tempC')  # '50 degC'.unit
Unit('50 tempC')  - Unit('100 tempC') # '-50 degC'.unit
Unit('100 tempC') * [scalar]          # '100*scalar tempC'.unit
Unit('100 tempC') / [scalar]          # '100/scalar tempC'.unit
Unit('100 tempC') * [unit]            # exception
Unit('100 tempC') / [unit]            # exception
Unit('100 tempC') ** N                # exception

Unit('100 tempC').convert_to('degC')  #=> Unit('100 degC')   

This conversion references the 0 point on the scale of the temperature unit

Unit('100 degC').convert_to('tempC')  #=> '-173 tempC'.unit

These conversions are always interpreted as being relative to absolute zero. Conversions are probably better done like this...

Unit('0 tempC') + Unit('100 degC') #=> Unit('100 tempC')

Defining Units

It is possible to define new units or redefine existing ones.

Define New Unit

The easiest approach is to define a unit in terms of other units.

Unit.define("foobar") do |foobar|
  foobar.definition   = Unit("1 foo") * Unit("1 bar")   # anything that results in a Unit object
  foobar.aliases      = %w{foobar fb}                   # array of synonyms for the unit
  foobar.display_name = "Foobar"                        # How unit is displayed when output

Redefine Existing Unit

Redefining a unit allows the user to change a single aspect of a definition without having to re-create the entire definition. This is useful for changing display names, adding aliases, etc.

Unit.redefine!("cup") do |cup|
  cup.display_name  = "cup"

Namespaced Class

Sometimes the default class 'Unit' may conflict with other gems or applications. Internally ruby-units defines itself using the RubyUnits namespace. The actual class of a unit is the RubyUnits::Unit. For simplicity and backwards compatiblity, the '::Unit' class is defined as an alias to '::RubyUnits::Unit'.

To load ruby-units without this alias...

require 'ruby_units/namespaced'

When using bundler...

gem 'ruby-units', require: 'ruby_units/namespaced'

Note: when using the namespaced version, the Unit('unit string') helper will not be defined.

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