diff --git a/app/controllers/submissions_controller.rb b/app/controllers/submissions_controller.rb
index c1f1a8f0..618ed5f7 100644
--- a/app/controllers/submissions_controller.rb
+++ b/app/controllers/submissions_controller.rb
@@ -54,7 +54,9 @@ class SubmissionsController < ApplicationController
         zio.write(File.read(File.join(scripts_path, file)))
       end
     end
-    send_data(stringio.string, filename: "#{@submission.exercise.title.tr(' ', '_')}.zip")
+    zip_data = stringio.string
+    response.set_header('Content-Length', zip_data.size)
+    send_data(zip_data, filename: "#{@submission.exercise.title.tr(' ', '_')}.zip")
   end
 
   def download_file
@@ -63,6 +65,7 @@ class SubmissionsController < ApplicationController
     if @file.native_file?
       redirect_to protected_upload_path(id: @file.id, filename: @file.filepath)
     else
+      response.set_header('Content-Length', @file.size)
       send_data(@file.content, filename: @file.name_with_extension, disposition: 'attachment')
     end
   end
@@ -93,6 +96,7 @@ class SubmissionsController < ApplicationController
       url = render_protected_upload_url(id: @file.id, filename: @file.filepath)
       redirect_to AuthenticatedUrlHelper.sign(url, @file)
     else
+      response.set_header('Content-Length', @file.size)
       send_data(@file.content, filename: @file.name_with_extension, disposition: 'inline')
     end
   end
diff --git a/app/models/code_ocean/file.rb b/app/models/code_ocean/file.rb
index 5083f8c4..6d2621b0 100644
--- a/app/models/code_ocean/file.rb
+++ b/app/models/code_ocean/file.rb
@@ -17,6 +17,8 @@ module CodeOcean
     before_validation :hash_content, if: :content_present?
     before_validation :set_ancestor_values, if: :incomplete_descendent?
 
+    attr_writer :size
+
     belongs_to :context, polymorphic: true
     belongs_to :file, class_name: 'CodeOcean::File', optional: true # This is only required for submissions and is validated below
     alias ancestor file
@@ -128,5 +130,13 @@ module CodeOcean
     def visible
       !hidden
     end
+
+    def size
+      @size ||= if native_file?
+                  native_file.size
+                else
+                  content.size
+                end
+    end
   end
 end