View on GitHub


Test Coverage
# [Unitwise](//

[![Gem Version](](
[![Build Status](](
[![Dependency Status](](
[![Coverage Status](](
[![Code Climate](](

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:

## Rationale

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'.

## Usage

### Initialization:

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>

### Conversion

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>

### Comparison

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>

### Math

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

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

# => [ ... ]

You can also get the official list from the UCUM website in XML format at
or a YAML version within this repo

### 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

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

* Ruby 2.2
* Ruby 2.3
* Ruby 2.4
* [JRuby](

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.

## Installation

Add this line to your application's Gemfile:

    gem 'unitwise'

And then execute:

    $ bundle

Or install it yourself as:

    $ gem install unitwise

## Contributing

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