diff --git a/app/models/group_workload.rb b/app/models/group_workload.rb
index d0d9332..7d2cb08 100644
--- a/app/models/group_workload.rb
+++ b/app/models/group_workload.rb
@@ -5,6 +5,8 @@
 # the group user dummy.
 #
 class GroupWorkload
+  include RedmineWorkload::WlUserSorting
+
   attr_reader :time_span, :user_workload
 
   ##
@@ -52,11 +54,12 @@ def define_group_members
   end
 
   ##
-  # Sorting of users lastname and their class name in order to ensure that the
-  # GroupUserDummy will come first.
+  # Sorting by the class name first ensures that the GroupUserDummy comes
+  # before the real users. Within a class the name is used as rendered by
+  # Redmine, see RedmineWorkload::WlUserSorting.
   #
   def sorted_user_workload
-    user_workload_with_availabilities.sort_by { |user, _data| [user.class.name, user.lastname] }.to_h
+    user_workload_with_availabilities.sort_by { |user, _data| [user.class.name, *user_sort_key(user)] }.to_h
   end
 
   ##
diff --git a/app/models/wl_user_selection.rb b/app/models/wl_user_selection.rb
index dc11a42..93fa9f0 100644
--- a/app/models/wl_user_selection.rb
+++ b/app/models/wl_user_selection.rb
@@ -4,6 +4,8 @@
 # Presenter organising users to be used in views/workloads/_filers.erb.
 #
 class WlUserSelection
+  include RedmineWorkload::WlUserSorting
+
   attr_reader :groups
 
   ##
@@ -36,7 +38,7 @@ def selected
   # Prepares users to be used in filters
   # @return [Array(User)] An array of user objects.
   def allowed_to_display
-    users_allowed_to_display.sort_by(&:lastname)
+    users_allowed_to_display.sort_by { |user| user_sort_key(user) }
   end
 
   def all_user_ids
diff --git a/lib/redmine_workload.rb b/lib/redmine_workload.rb
index 9f8265d..692edbc 100644
--- a/lib/redmine_workload.rb
+++ b/lib/redmine_workload.rb
@@ -11,3 +11,4 @@
 require File.expand_path('redmine_workload/wl_issue_state', __dir__)
 require File.expand_path('redmine_workload/wl_user_data_finder', __dir__)
 require File.expand_path('redmine_workload/wl_user_data_defaults', __dir__)
+require File.expand_path('redmine_workload/wl_user_sorting', __dir__)
diff --git a/lib/redmine_workload/wl_user_sorting.rb b/lib/redmine_workload/wl_user_sorting.rb
new file mode 100644
index 0000000..65b2899
--- /dev/null
+++ b/lib/redmine_workload/wl_user_sorting.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module RedmineWorkload
+  ##
+  # Ordering of users and group dummies in the workload views.
+  #
+  # Redmine renders a user's name according to the 'Users display format'
+  # setting, so the list has to be ordered by that name rather than by the
+  # last name. Otherwise an installation using 'Firstname Lastname' shows an
+  # apparently unsorted list.
+  #
+  module WlUserSorting
+    ##
+    # @param user [User|GroupUserDummy] Object responding to name and id.
+    # @return [Array] Sort key following Setting.user_format.
+    #
+    def user_sort_key(user)
+      [user.name.to_s.downcase, user.id.to_i]
+    end
+  end
+end
diff --git a/test/unit/user_selection_test.rb b/test/unit/user_selection_test.rb
index cb40dc2..03ab10f 100644
--- a/test/unit/user_selection_test.rb
+++ b/test/unit/user_selection_test.rb
@@ -43,6 +43,14 @@ def all_group_member_ids
       (@fixture_group_member_ids + @group_member_ids.flatten).uniq.sort
     end
 
+    ##
+    # Ids in the order WlUserSelection would render them.
+    #
+    def displayed_ids(current_user)
+      groups = WlGroupSelection.new(user: current_user, groups: [@group1.id, @group2.id, @group3.id])
+      WlUserSelection.new(user: current_user, group_selection: groups).allowed_to_display.map(&:id)
+    end
+
     test 'should return all users if the current user is admin' do
       current_user = User.generate!(admin: true)
       groups = WlGroupSelection.new(user: current_user, groups: [@group1.id, @group2.id, @group3.id])
@@ -80,6 +88,24 @@ def all_group_member_ids
       assert_equal expected, current
     end
 
+    test 'should order users by the display name format' do
+      current_user = User.generate!(admin: true)
+      zoe_adams = User.generate!(firstname: 'Zoe', lastname: 'Adams')
+      adam_zimmer = User.generate!(firstname: 'Adam', lastname: 'Zimmer')
+      [zoe_adams, adam_zimmer].each { |user| user.groups << @group1 }
+      ids = [zoe_adams.id, adam_zimmer.id]
+
+      # A new selection per block, otherwise User#name serves its memoized value
+      # from before the setting changed.
+      with_settings user_format: 'lastname_comma_firstname' do
+        assert_equal [zoe_adams.id, adam_zimmer.id], displayed_ids(current_user) & ids
+      end
+
+      with_settings user_format: 'firstname_lastname' do
+        assert_equal [adam_zimmer.id, zoe_adams.id], displayed_ids(current_user) & ids
+      end
+    end
+
     test 'should return the current user if allowed to :view_own_workloads' do
       current_user = users :users_002 # jsmith
       manager = roles :roles_001 # manager