Measured Build Status

Encapsulates measurements with their units. Provides easy conversion between units. Built in support for weight, length, and volume.

Lightweight and easily extensible to include other units and conversions. Conversions done with Rational for precision.

Since version 3.0.0, the adapter to integrate measured with Ruby on Rails is also a part of this gem. If you had been using measured-rails for that functionality, you should now remove measured-rails from your gem file.


Using bundler, add to the Gemfile:

gem 'measured'

Or stand alone:

$ gem install measured


Initialize a measurement:"12", "g")
> #<Measured::Weight: 12 #<Measured::Unit: g (gram, grams)>>

Convert to return a new measurement:"12", "g").convert_to("kg")
> #<Measured::Weight: 0.012 #<Measured::Unit: kg (kilogram, kilograms) 1000/1 g>>

Agnostic to symbols/strings:, "kg") ==, :kg)
> true

Seamlessly handles aliases:, :oz) =="12", :ounce)
> true

Raises on unknown units:

begin, :stone)
rescue Measured::UnitError
  puts "Unknown unit"

Parse from string without having to split out the value and unit first:

Measured::Weight.parse("123 grams")
> #<Measured::Weight: 123 #<Measured::Unit: g (gram, grams)>>

Parse can scrub extra whitespace and split number from unit:

Measured::Weight.parse(" 2kg ")
> #<Measured::Weight: 2 #<Measured::Unit: kg (kilogram, kilograms) 1000/1 g>>

Perform addition / subtraction against other units, all represented internally as Rational or BigDecimal:, :g) +, :g)
> #<Measured::Weight: 3 #<Measured::Unit: g (gram, grams)>>"2.1", :g) -, :g)
> #<Measured::Weight: 1.1 #<Measured::Unit: g (gram, grams)>>

Multiplication and division by units is not supported, but the actual value can be scaled by a scalar:, :g).scale(0.5)
> #<Measured::Weight: 5 #<Measured::Unit: g (gram, grams)>>, :g).scale(3)
> #<Measured::Weight: 6 #<Measured::Unit: g (gram, grams)>>

In cases of differing units, the left hand side takes precedence:, :g) +, :kg)
> #<Measured::Weight: 2000 #<Measured::Unit: g (gram, grams)>>

Converts units only as needed for equality comparison:

>, :g) ==, :kg)

Extract the unit and the value:

weight ="1.2", "grams")
> #<BigDecimal:7fabf6c1d0a0,'0.12E1',18(18)>
> #<Measured::Unit: g (gram, grams)>

See all valid units:

> ["g", "kg", "lb", "oz"]

Check if a unit is a valid unit or alias:

> true
> true
> false

See all valid units with their aliases:

> ["g", "gram", "grams", "kg", "kilogram", "kilograms", "lb", "lbs", "ounce", "ounces", "oz", "pound", "pounds"]

String formatting:"3.14", "grams").format("%.1<value>f %<unit>s")
> "3.1 g"

If no string is passed to the format method it defaults to "%.2<value>f %<unit>s".

If the unit isn't the standard SI unit, it will include a conversion string."3.14", "kg").format
> "3.14 kg (1000/1 g)""3.14", "kg").format(with_conversion_string: false)
> "3.14 kg"

Active Record

This gem also provides an Active Record adapter for persisting and retrieving measurements with their units, and model validations.

Columns are expected to have the _value and _unit suffix, and be DECIMAL and VARCHAR, and defaults are accepted. Customizing the column used to hold units is supported, see below for details.

class AddWeightAndLengthToThings < ActiveRecord::Migration
  def change
    add_column :things, :minimum_weight_value, :decimal, precision: 10, scale: 2
    add_column :things, :minimum_weight_unit, :string, limit: 12

    add_column :things, :total_length_value, :decimal, precision: 10, scale: 2, default: 0
    add_column :things, :total_length_unit, :string, limit: 12, default: "cm"

A column can be declared as a measurement with its measurement subclass:

class Thing < ActiveRecord::Base
  measured Measured::Weight, :minimum_weight
  measured Measured::Length, :total_length
  measured Measured::Volume, :total_volume

You can optionally customize the model's unit column by specifying it in the unit_field_name option, as follows:

class ThingWithCustomUnitAccessor < ActiveRecord::Base
  measured_length :length, :width, :height,     unit_field_name: :size_unit
  measured_weight :total_weight, :extra_weight, unit_field_name: :weight_unit
  measured_volume :total_volume, :extra_volume, unit_field_name: :volume_unit

Similarly, you can optionally customize the model's value column by specifying it in the value_field_name option, as follows:

class ThingWithCustomValueAccessor < ActiveRecord::Base
  measured_length :length, value_field_name: :custom_length
  measured_weight :total_weight, value_field_name: :custom_weight
  measured_volume :volume, value_field_name: :custom_volume

There are some simpler methods for predefined types:

class Thing < ActiveRecord::Base
  measured_weight :minimum_weight
  measured_length :total_length
  measured_volume :total_volume

This will allow you to access and assign a measurement object:

thing =
thing.minimum_weight =, "g")
thing.minimum_weight_unit     # "g"
thing.minimum_weight_value    # 10

Order of assignment does not matter, and each property can be assigned separately and with mass assignment:

params = { total_length_unit: "cm", total_length_value: "3" }
thing =
thing.total_length.to_s   # 3 cm


Validations are available:

class Thing < ActiveRecord::Base
  measured_length :total_length

  validates :total_length, measured: true

This will validate that the unit is defined on the measurement, and that there is a value.

Rather than true the validation can accept a hash with the following options:

  • message: Override the default "is invalid" message.
  • units: A subset of units available for this measurement. Units must be in existing measurement.
  • greater_than
  • greater_than_or_equal_to
  • equal_to
  • less_than
  • less_than_or_equal_to

All comparison validations require Measured::Measurable values, not scalars. Most of these options replace the numericality validator which compares the measurement/method name/proc to the column's value. Validations can also be combined with presence validator.

Note: Validations are strongly recommended since assigning an invalid unit will cause the measurement to return nil, even if there is a value:

thing =
thing.total_length_value = 1
thing.total_length_unit = "invalid"
thing.total_length  # nil

Units and conversions

SI units support

There is support for SI units through the use of si_unit. Units declared through it will have automatic support for all SI prefixes:

Multiplying Factor SI Prefix Scientific Notation
1 000 000 000 000 000 000 000 000 yotta (Y) 10^24
1 000 000 000 000 000 000 000 zetta (Z) 10^21
1 000 000 000 000 000 000 exa (E) 10^18
1 000 000 000 000 000 peta (P) 10^15
1 000 000 000 000 tera (T) 10^12
1 000 000 000 giga (G) 10^9
1 000 000 mega (M) 10^6
1 000 kilo (k) 10^3
0.001 milli (m) 10^-3
0.000 001 micro (ยต) 10^-6
0.000 000 001 nano (n) 10^-9
0.000 000 000 001 pico (p) 10^-12
0.000 000 000 000 001 femto (f) 10^-15
0.000 000 000 000 000 001 atto (a) 10^-18
0.000 000 000 000 000 000 001 zepto (z) 10^-21
0.000 000 000 000 000 000 000 001 yocto (y) 10^-24

Bundled unit conversion

  • Measured::Weight
    • g, gram, grams, and all SI prefixes
    • t, metric_ton, metric_tons
    • slug, slugs
    • N, newtons, newton
    • long_ton, long_tons, weight_ton, weight_tons, 'W/T', imperial_ton, imperial_tons, displacement_ton, displacement_tons
    • short_ton, short_tons
    • lb, lbs, pound, pounds
    • oz, ounce, ounces
  • Measured::Length
    • m, meter, metre, meters, metres, and all SI prefixes
    • in, inch, inches
    • ft, foot, feet
    • yd, yard, yards
    • mi, mile, miles
  • Measured::Volume
    • l, liter, litre, liters, litres, and all SI prefixes
    • m3, cubic_meter, cubic_meters, cubic_metre, cubic_metres
    • ft3, cubic_foot, cubic_feet
    • in3, cubic_inch, cubic_inches
    • gal, imp_gal, imperial_gallon, imp_gals, imperial_gallons
    • us_gal, us_gallon, us_gals, us_gallons
    • qt, imp_qt, imperial_quart, imp_qts, imperial_quarts
    • us_qt, us_quart, us_quarts
    • pt, imp_pt, imperial_pint, imp_pts, imperial_pints
    • us_pt, us_pint, us_pints
    • oz, fl_oz, imp_fl_oz, imperial_fluid_ounce, imperial_fluid_ounces
    • us_oz, us_fl_oz, us_fluid_ounce, us_fluid_ounces

You can skip these and only define your own units by doing:

gem 'measured', require: 'measured/base'

Shortcut syntax

There is a shortcut initialization syntax for creating instances of measurement classes that can avoid the .new:

Measured::Weight(1, :g)
> #<Measured::Weight: 1 #<Measured::Unit: g (gram, grams)>>

Adding new units

Extending this library to support other units is simple. To add a new conversion, use to define your base unit and conversion units:

Measured::Thing = do
  unit :base_unit,           # Add a unit to the system
    aliases: [:bu]           # Allow it to be aliased to other names/symbols

  unit :another_unit,        # Add a second unit to the system
    aliases: [:au],          # All units allow aliases, as long as they are unique
    value: "1.5 bu"        # The conversion rate to another unit

All unit names are case sensitive.

Values for conversion units can be defined as a string with two tokens "number unit" or as an array with two elements. All values will be parsed as / coerced to Rational. Conversion paths don't have to be direct as a conversion table will be built for all possible conversions.


All units and classes are namespaced by default, but can be aliased in your application.

Weight = Measured::Weight
Length = Measured::Length
Volume = Measured::Volume


Existing alternatives which were considered:

Gem: ruby-units

  • Pros
    • Accurate math and conversion factors.
    • Includes nearly every unit you could ask for.
  • Cons
    • Opens up and modifies Array, Date, Fixnum, Math, Numeric, String, Time, and Object, then depends on those changes internally.
    • Lots of code to solve a relatively simple problem.
    • No Active Record adapter.

Gem: quantified

  • Pros
    • Lightweight.
  • Cons
    • All math done with floats making it highly lossy.
    • All units assumed to be pluralized, meaning using unit abbreviations is not possible.
    • Not actively maintained.
    • No Active Record adapter.

Gem: unitwise

  • Pros
    • Well written.
    • Conversions done with Unified Code for Units of Measure (UCUM) so highly accurate and reliable.
  • Cons
    • Lots of code. Good code, but lots of it.
    • Many modifications to core types.
    • Active Record adapter exists but is written and maintained by a different person/org.
    • Not actively maintained.


  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 a new Pull Request