diff --git a/CHANGELOG.md b/CHANGELOG.md
index 382d486..4d96b89 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ Features:
 Bugs:
 * Fresh installs of Squib were broken due to two hidden dependencies, gio2 and gobject-introspection. (#172)
 * Embedding icons in text show unicode placeholders on some OSs. This is a workaround until we get a better solution for embedding icons. See #170, #171, and #176. For that matter, see #103, #153, and #30 if you really want the whole story.
+* Unit conversion is supported when using `extends` in layouts, as promised in the docs (#173)
 
 ## v0.10.0 / 2016-05-06
 
diff --git a/lib/squib/api/settings.rb b/lib/squib/api/settings.rb
index 6cb181e..739df60 100644
--- a/lib/squib/api/settings.rb
+++ b/lib/squib/api/settings.rb
@@ -14,7 +14,7 @@ module Squib
 
     # DSL method. See http://squib.readthedocs.io
     def use_layout(file: 'layout.yml')
-      @layout = LayoutParser.load_layout(file, @layout)
+      @layout = LayoutParser.new(@dpi).load_layout(file, @layout)
     end
 
   end
diff --git a/lib/squib/deck.rb b/lib/squib/deck.rb
index 8b93f1b..315a194 100644
--- a/lib/squib/deck.rb
+++ b/lib/squib/deck.rb
@@ -68,7 +68,7 @@ module Squib
       @width         = Args::UnitConversion.parse width, dpi
       @height        = Args::UnitConversion.parse height, dpi
       cards.times{ |i| @cards << Squib::Card.new(self, @width, @height, i) }
-      @layout = LayoutParser.load_layout(layout)
+      @layout = LayoutParser.new(dpi).load_layout(layout)
       enable_groups_from_env!
       if block_given?
         instance_eval(&block) # here we go. wheeeee!
diff --git a/lib/squib/layout_parser.rb b/lib/squib/layout_parser.rb
index 1841ad1..d7f6f8b 100644
--- a/lib/squib/layout_parser.rb
+++ b/lib/squib/layout_parser.rb
@@ -5,16 +5,21 @@ module Squib
   # @api private
   class LayoutParser
 
+    def initialize(dpi = 300)
+      @dpi = dpi
+    end
+
     # Load the layout file(s), if exists
     # @api private
-    def self.load_layout(files, initial = {})
+    def load_layout(files, initial = {})
       layout = initial
       Squib::logger.info { "  using layout(s): #{files}" }
       Array(files).each do |file|
         thefile = file
         thefile = builtin(file) unless File.exists?(file)
         if File.exists? thefile
-          yml = layout.merge(YAML.load_file(thefile) || {}) # load_file returns false on empty file
+          # note: YAML.load_file returns false on empty file
+          yml = layout.merge(YAML.load_file(thefile) || {})
           yml.each do |key, value|
             layout[key] = recurse_extends(yml, key, {})
           end
@@ -25,15 +30,17 @@ module Squib
       return layout
     end
 
+    private
+
     # Determine the file path of the built-in layout file
-    def self.builtin(file)
+    def builtin(file)
       "#{File.dirname(__FILE__)}/layouts/#{file}"
     end
 
     # Process the extends recursively
     # :nodoc:
     # @api private
-    def self.recurse_extends(yml, key, visited)
+    def recurse_extends(yml, key, visited)
       assert_not_visited(key, visited)
       return yml[key] unless has_extends?(yml, key)
       return yml[key] unless parents_exist?(yml, key)
@@ -43,9 +50,9 @@ module Squib
       parent_keys.each do |parent_key|
         from_extends = yml[key].merge(recurse_extends(yml, parent_key, visited)) do |key, child_val, parent_val|
           if child_val.to_s.strip.start_with?('+=')
-            parent_val + child_val.sub('+=', '').strip.to_f
+            add_parent_child(parent_val, child_val)
           elsif child_val.to_s.strip.start_with?('-=')
-            parent_val - child_val.sub('-=', '').strip.to_f
+            sub_parent_child(parent_val, child_val)
           else
             child_val # child overrides parent when merging, no +=
           end
@@ -57,17 +64,29 @@ module Squib
       return h
     end
 
+    def add_parent_child(parent, child)
+      parent_pixels = Args::UnitConversion.parse(parent, @dpi).to_f
+      child_pixels = Args::UnitConversion.parse(child.sub('+=', ''), @dpi).to_f
+      parent_pixels + child_pixels
+    end
+
+    def sub_parent_child(parent, child)
+      parent_pixels = Args::UnitConversion.parse(parent, @dpi).to_f
+      child_pixels = Args::UnitConversion.parse(child.sub('-=', ''), @dpi).to_f
+      parent_pixels - child_pixels
+    end
+
     # Does this layout entry have an extends field?
     # i.e. is it a base-case or will it need recursion?
     # :nodoc:
     # @api private
-    def self.has_extends?(yml, key)
+    def has_extends?(yml, key)
       !!yml[key] && yml[key].key?('extends')
     end
 
     # Checks if we have any absentee parents
     # @api private
-    def self.parents_exist?(yml, key)
+    def parents_exist?(yml, key)
       exists = true
       Array(yml[key]['extends']).each do |parent|
         unless yml.key?(parent)
@@ -81,7 +100,7 @@ module Squib
     # Safeguard against malformed circular extends
     # :nodoc:
     # @api private
-    def self.assert_not_visited(key, visited)
+    def assert_not_visited(key, visited)
       if visited.key? key
         raise "Invalid layout: circular extends with '#{key}'"
       end
diff --git a/spec/data/layouts/extends-units-mixed.yml b/spec/data/layouts/extends-units-mixed.yml
new file mode 100644
index 0000000..0cbac7e
--- /dev/null
+++ b/spec/data/layouts/extends-units-mixed.yml
@@ -0,0 +1,8 @@
+parent:
+  x: 0.5in
+  y: 1in
+
+child:
+  extends: parent
+  x: += 1in
+  y: -= 0.5in
diff --git a/spec/data/layouts/extends-units.yml b/spec/data/layouts/extends-units.yml
new file mode 100644
index 0000000..0cbac7e
--- /dev/null
+++ b/spec/data/layouts/extends-units.yml
@@ -0,0 +1,8 @@
+parent:
+  x: 0.5in
+  y: 1in
+
+child:
+  extends: parent
+  x: += 1in
+  y: -= 0.5in
diff --git a/spec/layout_parser_spec.rb b/spec/layout_parser_spec.rb
index 0be202b..b57918b 100644
--- a/spec/layout_parser_spec.rb
+++ b/spec/layout_parser_spec.rb
@@ -3,7 +3,7 @@ require 'spec_helper'
 describe Squib::LayoutParser do
 
   it 'loads a normal layout with no extends' do
-    layout = Squib::LayoutParser.load_layout(layout_file('no-extends.yml'))
+    layout = subject.load_layout(layout_file('no-extends.yml'))
     expect(layout).to eq({ 'frame' => {
             'x' => 38,
             'valign' => :middle,
@@ -15,7 +15,7 @@ describe Squib::LayoutParser do
   end
 
   it 'loads with a single extends' do
-    layout = Squib::LayoutParser.load_layout(layout_file('single-extends.yml'))
+    layout = subject.load_layout(layout_file('single-extends.yml'))
     expect(layout).to eq({ 'frame' => {
             'x' => 38,
             'y' => 38,
@@ -31,7 +31,7 @@ describe Squib::LayoutParser do
   end
 
   it 'applies the extends regardless of order' do
-    layout = Squib::LayoutParser.load_layout(layout_file('pre-extends.yml'))
+    layout = subject.load_layout(layout_file('pre-extends.yml'))
     expect(layout).to eq({ 'frame' => {
             'x' => 38,
             'y' => 38,
@@ -47,7 +47,7 @@ describe Squib::LayoutParser do
   end
 
   it 'applies the single-level extends multiple times' do
-    layout = Squib::LayoutParser.load_layout(layout_file('single-level-multi-extends.yml'))
+    layout = subject.load_layout(layout_file('single-level-multi-extends.yml'))
     expect(layout).to eq({ 'frame' => {
             'x' => 38,
             'y' => 38,
@@ -69,7 +69,7 @@ describe Squib::LayoutParser do
   end
 
   it 'applies multiple extends in a single rule' do
-    layout = Squib::LayoutParser.load_layout(layout_file('multi-extends-single-entry.yml'))
+    layout = subject.load_layout(layout_file('multi-extends-single-entry.yml'))
     expect(layout).to eq({ 'aunt' => {
             'a' => 101,
             'b' => 102,
@@ -93,7 +93,7 @@ describe Squib::LayoutParser do
   end
 
   it 'applies multi-level extends' do
-    layout = Squib::LayoutParser.load_layout(layout_file('multi-level-extends.yml'))
+    layout = subject.load_layout(layout_file('multi-level-extends.yml'))
     expect(layout).to eq({ 'frame' => {
             'x' => 38,
             'y' => 38,
@@ -116,26 +116,26 @@ describe Squib::LayoutParser do
 
   it 'fails on a self-circular extends' do
     file = layout_file('self-circular-extends.yml')
-    expect { Squib::LayoutParser.load_layout(file) }
+    expect { subject.load_layout(file)}
       .to raise_error(RuntimeError, 'Invalid layout: circular extends with \'a\'')
   end
 
   it 'fails on a easy-circular extends' do
     file = layout_file('easy-circular-extends.yml')
-    expect { Squib::LayoutParser.load_layout(file) }
+    expect { subject.load_layout(file)}
       .to raise_error(RuntimeError, 'Invalid layout: circular extends with \'a\'')
   end
 
   it 'hard on a easy-circular extends' do
     file = layout_file('hard-circular-extends.yml')
-    expect { Squib::LayoutParser.load_layout(file) }
+    expect { subject.load_layout(file)}
       .to raise_error(RuntimeError, 'Invalid layout: circular extends with \'a\'')
   end
 
   it 'redefines keys on multiple layouts' do
     a = layout_file('multifile-a.yml')
     b = layout_file('multifile-b.yml')
-    layout = Squib::LayoutParser.load_layout([a, b])
+    layout = subject.load_layout([a, b])
     expect(layout).to eq({
         'title'    => { 'x' => 300 },
         'subtitle' => { 'x' => 200 },
@@ -146,7 +146,7 @@ describe Squib::LayoutParser do
   it 'evaluates extends on each file first' do
     a = layout_file('multifile-extends-a.yml')
     b = layout_file('multifile-extends-b.yml')
-    layout = Squib::LayoutParser.load_layout([a, b])
+    layout = subject.load_layout([a, b])
     expect(layout).to eq({
         'grandparent' => { 'x' => 100 },
         'parent_a'    => { 'x' => 110, 'extends' => 'grandparent' },
@@ -157,12 +157,12 @@ describe Squib::LayoutParser do
   end
 
   it 'loads nothing on an empty layout file' do
-    layout = Squib::LayoutParser.load_layout(layout_file('empty.yml'))
+    layout = subject.load_layout(layout_file('empty.yml'))
     expect(layout).to eq({})
   end
 
   it 'handles extends on a rule with no args' do
-    layout = Squib::LayoutParser.load_layout(layout_file('empty-rule.yml'))
+    layout = subject.load_layout(layout_file('empty-rule.yml'))
     expect(layout).to eq({
       'empty' => nil
     })
@@ -170,14 +170,14 @@ describe Squib::LayoutParser do
 
   it 'logs an error when a file is not found' do
     expect(Squib.logger).to receive(:error).once
-    Squib::LayoutParser.load_layout('yeti')
+    subject.load_layout('yeti')
   end
 
   it 'freaks out if you extend something doesn\'t exist' do
     expect(Squib.logger)
       .to receive(:error)
       .with("Processing layout: 'verbal' attempts to extend a missing 'kaisersoze'")
-    layout = Squib::LayoutParser.load_layout(layout_file('extends-nonexists.yml'))
+    layout = subject.load_layout(layout_file('extends-nonexists.yml'))
     expect(layout).to eq({
       'verbal' => {
           'font_size' => 25,
@@ -186,11 +186,36 @@ describe Squib::LayoutParser do
       })
   end
 
+  it 'does unit conversion when extending' do
+    layout = subject.load_layout(layout_file('extends-units.yml'))
+    expect(layout).to eq({
+      'parent' => { 'x' => '0.5in', 'y' => '1in'},
+      'child'  => { 'x' => 450.0, 'y' => 150.0, 'extends' => 'parent' },
+    })
+  end
+
+  it 'does unit conversion on non-300 dpis' do
+    parser = Squib::LayoutParser.new(100)
+    layout = parser.load_layout(layout_file('extends-units.yml'))
+    expect(layout).to eq({
+      'parent' => { 'x' => '0.5in', 'y' => '1in'},
+      'child'  => { 'x' => 150.0, 'y' => 50.0, 'extends' => 'parent' },
+    })
+  end
+
+  it 'does mixed unit conversion when extending' do
+    layout = subject.load_layout(layout_file('extends-units-mixed.yml'))
+    expect(layout).to eq({
+      'parent' => { 'x' => '0.5in', 'y' => '1in'},
+      'child'  => { 'x' => 450.0, 'y' => 150.0, 'extends' => 'parent' },
+    })
+  end
+
   it 'loads progressively on multiple calls' do
     a = layout_file('multifile-a.yml')
     b = layout_file('multifile-b.yml')
-    layout = Squib::LayoutParser.load_layout(a)
-    layout = Squib::LayoutParser.load_layout(b, layout)
+    layout = subject.load_layout(a)
+    layout = subject.load_layout(b, layout)
     expect(layout).to eq({
         'title'    => { 'x' => 300 },
         'subtitle' => { 'x' => 200 },