diff --git a/app/models/issue.rb b/app/models/issue.rb
index 9cf2953..f35016e 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -1549,14 +1549,38 @@ class Issue < ActiveRecord::Base
         unless Issue.use_status_for_done_ratio? && p.status && p.status.default_done_ratio
           child_count = p.children.count
           if child_count > 0
-            average = p.children.where("estimated_hours > 0").average(:estimated_hours).to_f
-            if average == 0
-              average = 1
+            # Calculate the average hours for all child tasks using the childrens total_estimated_hours.
+            sum = 0
+            p.children.each do |child|
+              sum += child.total_estimated_hours
             end
-            done = p.children.joins(:status).
-              sum("COALESCE(CASE WHEN estimated_hours > 0 THEN estimated_hours ELSE NULL END, #{average}) " +
-                  "* (CASE WHEN is_closed = #{self.class.connection.quoted_true} THEN 100 ELSE COALESCE(done_ratio, 0) END)").to_f
-            progress = done / (average * child_count)
+            if sum == 0
+              average_hours = 1
+            else
+              average_hours = sum / child_count
+            end
+
+            # Calculate the total work done for the children
+            # Each child contribute to the total work with its total_estimate_hours multiplied
+            # by its done_ratio or 100 if the child is closed.
+            # If the childs total_estiamted_hours is zero then use the average_hours for
+            # that child.
+            done = 0
+            p.children.each do |child|
+              child_hours = child.total_estimated_hours
+              if child_hours == 0 then
+                child_hours = average_hours
+              end
+              # calculate the contribution to done for the child
+              if child.closed? then
+                done += child_hours * 100
+              else
+                done += child_hours * child.done_ratio
+              end
+            end
+
+            # Calculate the progress
+            progress = done / (average_hours * child_count)
             p.done_ratio = progress.round
           end
         end