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Unitwise is a Ruby library for unit measurement conversion and math.

For an over the top example, consider a car (2800 lb) completing the quarter mile in 10 seconds (with uniform acceleration).

require 'unitwise'

distance = Unitwise(0.25, 'mile')   # => #<Unitwise::Measurement value=0.25 unit=mile>
time     = Unitwise(10,   'second') # => #<Unitwise::Measurement value=10 unit=second>
mass     = Unitwise(2800, 'pound')  # => #<Unitwise::Measurement value=2800 unit=pound>

acceleration = 2.0 * distance / time ** 2
# => #<Unitwise::Measurement value=0.005 unit=[mi_us]/s2>

force = (mass * acceleration).to_lbf
# => #<Unitwise::Measurement value=2297.5084316991147 unit=lbf>

power = (force * distance / time).to_horsepower
# => #<Unitwise::Measurement value=551.4031264140402 unit=horsepower>

speed = ((2.0 * acceleration * distance) ** 0.5).convert_to("mile/hour")
# => #<Unitwise::Measurement value=180.0 unit=mile/hour>

RubyTapas subscribers can also view a screencast: 225-Unitwise


Unitwise is based on the Unified Code for Units of Measure(UCUM), which aims to maintain a cross-platform list of units and their conversions. This gives Unitwise a few key advantages:

  • An enormous list of units. At the time of writing, there are 96 metric units, 211 non-metric units, and 24 unit prefixes. Whatever unit/units you need, they are here.

  • An accurate and up to date set of units. The units, prefixes, and conversions are maintained by UCUM, and are imported into this library with a rake task.

One of the objectives of Unitwise was that it should comprehend any combination of units. For instance it needed to understand that a unit of 'kilogram.(meter/second)2' was equivalent to 'kilogram.meter.(meter/second2)'. This resulted in two unique features:

  • An expression grammar built with a PEG parser. This makes expression parsing more efficient and allows nested parentheses. For example, this is possible: '(kilogram.(meter/second)2)2'

  • Smart compatibility detection. Each unit is reduced down to its most elementary atoms to determine compatibility with another unit. For example, it knows that 'meter/second2' should be considered compatible with 'kilogram.foot.minute-2/pound'.



Measurements can be instantiated with Unitwise().

require 'unitwise'

Unitwise(2.3, 'kilogram') # => #<Unitwise::Measurement value=2.3 unit=kilogram>
Unitwise(100, 'pound')    # => #<Unitwise::Measurement value=100 unit=pound>


Unitwise is able to convert any unit within the UCUM spec to any other compatible unit.

Unitwise(5.0, 'kilometer').convert_to('mile')
# => #<Unitwise::Measurement value=3.106849747474748 unit=mile>

The prettier version of convert_to(unit) is appending the unit code, name, etc. to a to_ message name.

Unitwise(26.2, 'mile').to_kilometer
# => #<Unitwise::Measurement value=42.164897129794255 unit=kilometer>


It also has the ability to compare measurements with the same or different units.

Unitwise(12, 'inch') == Unitwise(1, 'foot') # => true
Unitwise(1, 'meter') > Unitwise(1, 'yard')  # => true

Again, you have to compare compatible units. For example, comparing two temperatures will work, comparing a mass to a length would fail.

SI abbreviations

You can use shorthand for SI units.

Unitwise(1000, 'm') == Unitwise(1, 'km')  # => true
Unitwise(1, 'ml') == Unitwise(0.001, 'l') # => true

Complex Units

Units can be combined to make more complex ones. There is nothing special about them -- they can still be converted, compared, or operated on.

speed = Unitwise(60, 'mile/hour')
# => #<Unitwise::Measurement value=60 unit=mile/hour>

# => #<Unitwise::Measurement value=26.822453644907288 unit=m/s>

Exponents and parenthesis are supported as well.

Unitwise(1000, 'kg.s-1.(m/s)2').to_kilowatt
# => #<Unitwise::Measurement value=1.0 unit=kilowatt>


You can add or subtract compatible measurements.

Unitwise(2.0, 'meter') + Unitwise(3.0, 'inch') - Unitwise(1.0, 'yard')
# => #<Unitwise::Measurement value=1.1618 unit=meter>

You can multiply or divide measurements and numbers.

Unitwise(110, 'volt') * 2
# => #<Unitwise::Measurement value=220 unit=volt>

You can multiply or divide measurements with measurements.

Unitwise(20, 'milligram') / Unitwise(1, 'liter')
# => #<Unitwise::Measurement value=20 unit=mg/l>

Exponentiation is also supported.

(Unitwise(10, 'cm') ** 3).to_liter
# => #<Unitwise::Measurement value=1 unit=liter>

Unit Names and Atom Codes

This library is based around the units in the UCUM specification, which is extensive and well thought out. However, not all of our unit systems throughout the world and history are consistent or logical. UCUM has devised a system where each unit has a unique atom code to try and solve this. The previous code examples don't show this, because for the most part you won't need it. Unitwise can figure out most of the units by their name or symbol. If you find you need to (or just want to be explicit) you use the UCUM atom codes without any modification.

Just for example, you can see here that there are actually a few versions of inch and foot:

Unitwise(1, '[ft_i]') == Unitwise(1, '[ft_us]') # => false

Unitwise(3, '[in_br]') == Unitwise(3, '[in_i]') # => false

Available Units

If you are looking for a particular unit, you can search with a string or Regexp.

# => [ ... ]

You can also get the official list from the UCUM website in XML format at unitsofmeasure.org/ucum-essence.xml or a YAML version within this repo github.com/joshwlewis/unitwise/tree/master/data.

UCUM designations

UCUM defines several designations for it's units: names, primary_code, secondary_code, and symbol. You can see them all when inspecting an atom:

# => #<Unitwise::Atom names=["British thermal unit"], primary_code="[Btu]", secondary_code="[BTU]", symbol="btu", scale=#<Unitwise::Scale value=1 unit=[Btu_th]>, classification="heat", property="energy", metric=false, special=false, arbitrary=false, dim="L2.M.T-2">

When initializing a measurement, you can use any of the designations:

Unitwise(1, '[Btu]') == Unitwise(1, 'British thermal unit')
# => true
Unitwise(1, 'btu') == Unitwise(1, "[BTU]")
# => true

When inspecting or printing (to_s) that measurement, it will remember the desigation you used. However, if you want to print it with another designation, that's also possible:

temperature = Unitwise(10, "Cel")
temperature.to_s # => "10 Cel"
temperature.to_s(:names) # => "10 degree Celsius"
temperature.to_s(:symbol) # => "10 °C"

There is on caveat here. You must use the same designation for each atom in a complex unit. Meaning you can't mix designations within a unit.

Unitwise(1, "m/s")          # Works, both atoms use their primary_code
Unitwise(1, "meter/second") # Works, both atoms use a name
Unitwise(1, "meter/s")      # Does not work, mixed designations (name and primary_code)
Unitwise(1, "meter") / Unitwise(1, "s") # Also works

Adding custom units

While UCUM's list of units is rather exhaustive, there may still be occasions where you need custom or uncommon measurements. You can add them yourself with Unitwise.register, which will allow you to convert to or from the new unit.

For example, if your app needed to pour "3 fingers" of bourbon, you could register an atom for that:

  names: ["finger", "fingers"],
  symbol: "🥃",
  primary_code: "fng",
  secondary_code: "fng",
  scale: {
    value: 1.0,
    unit_code: '[foz_us]'
  property: 'fluid volume'

Unitwise(1, "gallon").to_fingers
# => #<Unitwise::Measurement value=0.153721590464621430998E3 unit=fingers>

Unitwise(1, "🥃").to_cup
# => #<Unitwise::Measurement value=0.125 unit=cup>

Supported Ruby Versions

This library aims to support and is tested against the following Ruby implementations:

If something doesn't work on one of these versions, it's a bug.

This library may inadvertently work (or seem to work) on other Ruby versions or implementations, however support will only be provided for the implementations listed above.


Add this line to your application's Gemfile:

gem 'unitwise'

And then execute:

$ bundle

Or install it yourself as:

$ gem install unitwise


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request