diff --git a/app/models/spree/line_item_decorator.rb b/app/models/spree/line_item_decorator.rb
index ca3c9cc848..5b7c307ba6 100644
--- a/app/models/spree/line_item_decorator.rb
+++ b/app/models/spree/line_item_decorator.rb
@@ -7,7 +7,7 @@ Spree::LineItem.class_eval do
   attr_accessible :max_quantity, :final_weight_volume
   attr_accessible :final_weight_volume, :price, :as => :api
 
-  before_create :inherit_units_from_variant
+  before_save :calculate_final_weight_volume, unless: :final_weight_volume_changed?
   after_save :update_units
 
   delegate :unit_description, to: :variant
@@ -83,9 +83,13 @@ Spree::LineItem.class_eval do
 
   private
 
-  def inherit_units_from_variant
-    if final_weight_volume || variant.andand.unit_value
-      self.final_weight_volume = final_weight_volume || ((variant.andand.unit_value) * quantity)
+  def calculate_final_weight_volume
+    if quantity_changed?
+      if final_weight_volume.present?
+        self.final_weight_volume = final_weight_volume * quantity / quantity_was
+      elsif variant.andand.unit_value
+        self.final_weight_volume = ((variant.andand.unit_value) * quantity)
+      end
     end
   end
 end
diff --git a/spec/models/spree/line_item_spec.rb b/spec/models/spree/line_item_spec.rb
index a46b50d6ef..fd2729b2db 100644
--- a/spec/models/spree/line_item_spec.rb
+++ b/spec/models/spree/line_item_spec.rb
@@ -97,23 +97,98 @@ module Spree
         let!(:v) { p.variants.first }
         let!(:o) { create(:order) }
 
-        describe "when no final_weight_volume is set" do
-          let(:li) { build(:line_item, order: o, variant: v, quantity: 3) }
+        context "on create" do
+          context "when no final_weight_volume is set" do
+            let(:li) { build(:line_item, order: o, variant: v, quantity: 3) }
 
-          it "initializes final_weight_volume from the variant's unit_value on create" do
-            expect(li.final_weight_volume).to be nil
-            li.save
-            expect(li.final_weight_volume).to eq 3000
+            it "initializes final_weight_volume from the variant's unit_value" do
+              expect(li.final_weight_volume).to be nil
+              li.save
+              expect(li.final_weight_volume).to eq 3000
+            end
+          end
+
+          context "when a final_weight_volume has been set" do
+            let(:li) { build(:line_item, order: o, variant: v, quantity: 3, final_weight_volume: 2000) }
+
+            it "uses the changed value" do
+              expect(li.final_weight_volume).to eq 2000
+              li.save
+              expect(li.final_weight_volume).to eq 2000
+            end
           end
         end
 
-        describe "when a final_weight_volume has been set" do
-          let(:li) { build(:line_item, order: o, variant: v, quantity: 3, final_weight_volume: 2000) }
+        context "on save" do
+          let!(:li) { create(:line_item, order: o, variant: v, quantity: 3) }
 
-          it "uses the existing value" do
-            expect(li.final_weight_volume).to eq 2000
-            li.save
-            expect(li.final_weight_volume).to eq 2000
+          before do
+            expect(li.final_weight_volume).to eq 3000
+          end
+
+          context "when final_weight_volume is changed" do
+            let(:attrs) { { final_weight_volume: 2000 } }
+
+            context "and quantity is not changed" do
+              before do
+                li.update_attributes(attrs)
+              end
+
+              it "uses the value given" do
+                expect(li.final_weight_volume).to eq 2000
+              end
+            end
+
+            context "and quantity is changed" do
+              before do
+                attrs.merge!( quantity: 4 )
+                li.update_attributes(attrs)
+              end
+
+              it "uses the value given" do
+                expect(li.final_weight_volume).to eq 2000
+              end
+            end
+          end
+
+          context "when final_weight_volume is not changed" do
+            let(:attrs) { { price: 3.00 } }
+
+            context "and quantity is not changed" do
+              before do
+                li.update_attributes(attrs)
+              end
+
+              it "does not change final_weight_volume" do
+                expect(li.final_weight_volume).to eq 3000
+              end
+            end
+
+            context "and quantity is changed" do
+              context "and a final_weight_volume has been set" do
+                before do
+                  expect(expect(li.final_weight_volume).to eq 3000)
+                  attrs.merge!( quantity: 4 )
+                  li.update_attributes(attrs)
+                end
+
+                it "calculates a final_weight_volume from the variants unit_value" do
+                  expect(li.final_weight_volume).to eq 4000
+                end
+              end
+
+              context "and a final_weight_volume has not been set" do
+                before do
+                  li.update_attributes(final_weight_volume: nil)
+                  attrs.merge!( quantity: 1 )
+                  li.update_attributes(attrs)
+                end
+
+                it "calculates a final_weight_volume from the variants unit_value" do
+                  expect(li.final_weight_volume).to eq 1000
+                end
+              end
+            end
           end
         end
       end
diff --git a/spec/models/spree/order_populator_spec.rb b/spec/models/spree/order_populator_spec.rb
index b32e87f7f9..7ba59fb47a 100644
--- a/spec/models/spree/order_populator_spec.rb
+++ b/spec/models/spree/order_populator_spec.rb
@@ -23,9 +23,10 @@ module Spree
           li.should be
           li.quantity.should == 1
           li.max_quantity.should == 2
+          li.final_weight_volume.should == 1.0
         end
 
-        it "updates a variant's quantity and max quantity" do
+        it "updates a variant's quantity, max quantity and final_weight_volume" do
           order.add_variant v, 1, 2
 
           op.populate({variants: {v.id.to_s => {quantity: '2', max_quantity: '3'}}}, true)
@@ -33,6 +34,7 @@ module Spree
           li.should be
           li.quantity.should == 2
           li.max_quantity.should == 3
+          li.final_weight_volume.should == 2.0
         end
 
         it "removes a variant" do