diff --git a/app/models/issue.rb b/app/models/issue.rb
index 5798fef9f..7b67e9f5e 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -1704,7 +1704,7 @@ class Issue < ActiveRecord::Base
               estimated * ratio
             }.sum
             progress = done / (average * children.count)
-            p.done_ratio = progress.round
+            p.done_ratio = progress.floor
           end
         end
       end
diff --git a/test/unit/issue_subtasking_test.rb b/test/unit/issue_subtasking_test.rb
index a50df024c..0bf4ccfbd 100644
--- a/test/unit/issue_subtasking_test.rb
+++ b/test/unit/issue_subtasking_test.rb
@@ -157,6 +157,17 @@ class IssueSubtaskingTest < ActiveSupport::TestCase
     end
   end
 
+  def test_parent_done_ratio_should_round_down_to_the_nearest_decimal
+    with_settings :parent_issue_done_ratio => 'derived' do
+      parent = Issue.generate!
+      parent.generate_child!(:done_ratio => 20)
+      parent.generate_child!(:done_ratio => 20)
+      parent.generate_child!(:done_ratio => 10)
+      # (20 + 20 + 10) / 3 = 16.666...
+      assert_equal 16, parent.reload.done_ratio
+    end
+  end
+
   def test_parent_done_ratio_should_be_weighted_by_estimated_times_if_any
     with_settings :parent_issue_done_ratio => 'derived' do
       parent = Issue.generate!