diff --git a/lib/beerxml/hop.rb b/lib/beerxml/hop.rb
index 385b43c..98d2de5 100644
--- a/lib/beerxml/hop.rb
+++ b/lib/beerxml/hop.rb
@@ -21,11 +21,11 @@ class Beerxml::Hop < Beerxml::Model
   property :id, Serial
   belongs_to :recipe, :required => false
 
-  def tinseth(post_boil_og, batch_size) # batch size is gallons for now
+  def tinseth(post_boil_og, batch_size) # batch size is gallons or Unit
     bigness = 1.65 * 0.000125**(post_boil_og - 1)
     boil_factor = (1 - 2.72 ** (-0.04 * time.in_minutes.to_f)) / 4.15
     utilization = bigness * boil_factor
-    ibus = utilization * (aau * 0.01 * 7490) / batch_size
+    ibus = utilization * (aau * 0.01 * 7490) / U(batch_size, 'gallons').to_f
     ibus.round
   end
   alias_method :ibus, :tinseth
diff --git a/lib/beerxml/recipe.rb b/lib/beerxml/recipe.rb
index e344ccc..ccb6b14 100644
--- a/lib/beerxml/recipe.rb
+++ b/lib/beerxml/recipe.rb
@@ -45,4 +45,9 @@ class Beerxml::Recipe < Beerxml::Model
 
   # these are not used in the xml
   property :id, Serial
+
+  def tinseth
+    hops.inject(0) { |v, h| v + h.tinseth(og, batch_size) }.round
+  end
+  alias_method :ibus, :tinseth
 end
diff --git a/lib/beerxml/unit.rb b/lib/beerxml/unit.rb
index 7e41453..cd4b333 100644
--- a/lib/beerxml/unit.rb
+++ b/lib/beerxml/unit.rb
@@ -1,6 +1,7 @@
+module Beerxml
 # Unit system with conversions between units of the same type. Very much
 # a work in progress.
-class Beerxml::Unit
+class Unit
   attr_reader :type, :unit
 
   Units = {}
@@ -22,6 +23,7 @@ def in!(new_unit, *args)
     @unit = new_unit
     self
   end
+  alias_method :to!, :in!
   alias_method :unit=, :in!
 
   def base_unit
@@ -33,7 +35,13 @@ def to_f
   end
 
   def initialize(amount, unit = nil)
-    if !unit
+    if amount.is_a?(Unit)
+      if unit
+        amount = amount.to(unit).to_f
+      else
+        amount, unit = amount.to_f, amount.unit
+      end
+    elsif !unit
       amount, unit = amount.to_s.split(/\s+/, 2)
     end
     unit = unit.to_s
@@ -117,6 +125,7 @@ def self.included(k)
   module Volume
     def self.included(k)
       k.add_type('volume', 'liters', 'l', 'liter')
+      k.add('volume', 0.26417, 'gallons', 'gallon')
     end
   end
   include Volume
@@ -138,6 +147,7 @@ def self.included(k)
   end
   include Temperature
 end
+end
 
 def U(*a)
   Beerxml::Unit.new(*a)
diff --git a/spec/parsing_spec.rb b/spec/parsing_spec.rb
index 8c8cd4f..39707ab 100644
--- a/spec/parsing_spec.rb
+++ b/spec/parsing_spec.rb
@@ -1,16 +1,6 @@
 require "#{File.dirname(__FILE__)}/spec_helper"
 
 describe "beerxml.com examples" do
-  def filename(example)
-    "examples/beerxml.com/#{example}.xml"
-  end
-  def read_file(example)
-    File.read(filename(example))
-  end
-  def read_xml(example)
-    Nokogiri::XML(read_file(example))
-  end
-
   it "should parse the first recipe and its hops" do
     recipe = Beerxml::Recipe.new.from_xml(read_xml("recipes").root.children[1])
 
diff --git a/spec/recipes_spec.rb b/spec/recipes_spec.rb
new file mode 100644
index 0000000..ea6c7cc
--- /dev/null
+++ b/spec/recipes_spec.rb
@@ -0,0 +1,9 @@
+require "#{File.dirname(__FILE__)}/spec_helper"
+
+describe Beerxml::Recipe do
+  it "should calculate IBUs using the tinseth method" do
+    recipe = Beerxml::Recipe.new.from_xml(read_xml("recipes").root.children[1])
+    recipe.should be_valid
+    recipe.ibus.should == 44
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 0a6e037..b35c1fc 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -3,4 +3,13 @@
 require 'rspec'
 
 RSpec.configure do |c|
+  def filename(example)
+    "examples/beerxml.com/#{example}.xml"
+  end
+  def read_file(example)
+    File.read(filename(example))
+  end
+  def read_xml(example)
+    Nokogiri::XML(read_file(example))
+  end
 end
diff --git a/spec/units_spec.rb b/spec/units_spec.rb
index 39e7606..6309945 100644
--- a/spec/units_spec.rb
+++ b/spec/units_spec.rb
@@ -41,6 +41,17 @@ def near?(rhs, prec = 0.01)
     w.should == U('53.97 oz')
   end
 
+  it "should allow constructing from another compatible Unit object" do
+    w = U(53.97, 'oz')
+    w2 = U(w) # defaults to oz, same units as copying object
+    w3 = U(w, 'kg')
+    w.should == w2
+    w.should == w3
+    w2.should == w3
+    w2.should == 53.97
+    w3.should == 1.53
+  end
+
   describe "equality" do
     it "should compare between units" do
       w = U(3.5, 'kg')