Quantified

Pretty quantifiable measurements which feel like ActiveSupport::Duration.

Access whichever included attributes you want like so:

require 'quantified/mass'
require 'quantified/length'

Add methods to Numeric (only plural, must correspond with plural unit names):

Mass.numeric_methods :grams, :kilograms, :ounces, :pounds
Length.numeric_methods :metres, :centimetres, :inches, :feet

Then you can do things like this:

1.feet == 12.inches
# => true

18.inches.to_feet
# => #<Quantified::Length: 1.5 feet>

(2.5).feet.in_millimetres.to_s
# => "762.0 millimetres"

You can easily define new attributes. Here's length.rb:

module Quantified
  class Length < Attribute
    system :metric do
      primitive :metre

      one :centimetre, :is => Length.new(0.01, :metres)
      one :millimetre, :is => Length.new(0.1, :centimetres)
      one :kilometre, :is => Length.new(1000, :metres)
    end

    system :imperial do
      primitive :inch
      one :inch, :is => Length.new(2.540, :centimetres)

      one :foot, :plural => :feet, :is => Length.new(12, :inches)
      one :yard, :is => Length.new(3, :feet)
      one :mile, :is => Length.new(5280, :feet)
    end
  end
end