diff --git a/app/models/issue.rb b/app/models/issue.rb
index e665a46cb..63d4244cd 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -1731,12 +1731,12 @@ class Issue < ActiveRecord::Base
           if children.any?
             child_with_total_estimated_hours = children.select {|c| c.total_estimated_hours.to_f > 0.0}
             if child_with_total_estimated_hours.any?
-              average = child_with_total_estimated_hours.map(&:total_estimated_hours).sum.to_f / child_with_total_estimated_hours.count
+              average = child_with_total_estimated_hours.map(&:total_estimated_hours).sum.to_d / child_with_total_estimated_hours.count
             else
-              average = 1.0
+              average = 1.0.to_d
             end
             done = children.map {|c|
-              estimated = c.total_estimated_hours.to_f
+              estimated = c.total_estimated_hours.to_d
               estimated = average unless estimated > 0.0
               ratio = c.closed? ? 100 : (c.done_ratio || 0)
               estimated * ratio
diff --git a/test/unit/issue_subtasking_test.rb b/test/unit/issue_subtasking_test.rb
index 85772909b..7a1cea675 100644
--- a/test/unit/issue_subtasking_test.rb
+++ b/test/unit/issue_subtasking_test.rb
@@ -217,6 +217,16 @@ class IssueSubtaskingTest < ActiveSupport::TestCase
     end
   end
 
+  def test_parent_done_ratio_with_completed_children_should_not_be_99
+    with_settings :parent_issue_done_ratio => 'derived' do
+      parent = Issue.generate!
+      parent.generate_child!(:estimated_hours => 8.0, :done_ratio => 100)
+      parent.generate_child!(:estimated_hours => 8.1, :done_ratio => 100)
+      # (8.0 * 100 + 8.1 * 100) / (8.0 + 8.1) => 99.99999999999999
+      assert_equal 100, parent.reload.done_ratio
+    end
+  end
+
   def test_done_ratio_of_parent_with_a_child_without_estimated_time_should_not_exceed_100
     with_settings :parent_issue_done_ratio => 'derived' do
       parent = Issue.generate!