diff --git a/app/models/spree/line_item_decorator.rb b/app/models/spree/line_item_decorator.rb
index ee4384b460..9db9ae1269 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, :price
   attr_accessible :final_weight_volume, :price, :as => :api
 
-  before_save :calculate_final_weight_volume, unless: :final_weight_volume_changed?
+  before_save :calculate_final_weight_volume, if: :quantity_changed?, unless: :final_weight_volume_changed?
   after_save :update_units
 
   delegate :unit_description, to: :variant
@@ -88,12 +88,10 @@ Spree::LineItem.class_eval do
   private
 
   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
+    if final_weight_volume.present? && quantity_was > 0
+      self.final_weight_volume = final_weight_volume * quantity / quantity_was
+    elsif variant.andand.unit_value.present?
+      self.final_weight_volume = variant.andand.unit_value * quantity
     end
   end
 end
diff --git a/spec/models/spree/line_item_spec.rb b/spec/models/spree/line_item_spec.rb
index ecbd3ae610..6bb18fafa3 100644
--- a/spec/models/spree/line_item_spec.rb
+++ b/spec/models/spree/line_item_spec.rb
@@ -165,27 +165,57 @@ module Spree
             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)
+              context "from > 0" do
+                context "and a final_weight_volume has been set" do
+                  before do
+                    expect(li.final_weight_volume).to eq 3000
+                    attrs.merge!( quantity: 4 )
+                    li.update_attributes(attrs)
+                  end
+
+                  it "scales the final_weight_volume based on the change in quantity" do
+                    expect(li.final_weight_volume).to eq 4000
+                  end
                 end
 
-                it "calculates a final_weight_volume from the variants unit_value" do
-                  expect(li.final_weight_volume).to eq 4000
+                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
 
-              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)
+              context "from 0" do
+                before { li.update_attributes(quantity: 0) }
+
+                context "and a final_weight_volume has been set" do
+                  before do
+                    expect(li.final_weight_volume).to eq 0
+                    attrs.merge!( quantity: 4 )
+                    li.update_attributes(attrs)
+                  end
+
+                  it "recalculates a final_weight_volume from the variants unit_value" do
+                    expect(li.final_weight_volume).to eq 4000
+                  end
                 end
 
-                it "calculates a final_weight_volume from the variants unit_value" do
-                  expect(li.final_weight_volume).to eq 1000
+                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
@@ -196,7 +226,7 @@ module Spree
       describe "generating the full name" do
         let(:li) { LineItem.new }
 
-	context "when display_name is blank" do
+	      context "when display_name is blank" do
           before do
             li.stub(:unit_to_display) { 'unit_to_display' }
             li.stub(:display_name) { '' }
@@ -223,7 +253,7 @@ module Spree
             li.stub(:unit_to_display) { '10kg' }
             li.stub(:display_name) { '10kg Box' }
           end
-        
+
           it "returns display_name" do
             li.full_name.should == '10kg Box'
           end
@@ -234,7 +264,7 @@ module Spree
             li.stub(:unit_to_display) { '1 Loaf' }
             li.stub(:display_name) { 'Spelt Sourdough' }
           end
-        
+
           it "returns unit_to_display" do
             li.full_name.should == 'Spelt Sourdough (1 Loaf)'
           end