diff --git a/app/controllers/admin/backups_controller.rb b/app/controllers/admin/backups_controller.rb
index f8b3300f999..0ea55fe1505 100644
--- a/app/controllers/admin/backups_controller.rb
+++ b/app/controllers/admin/backups_controller.rb
@@ -176,13 +176,14 @@ class Admin::BackupsController < Admin::AdminController
     chunk_number = params.fetch(:resumableChunkNumber).to_i
     chunk_size = params.fetch(:resumableChunkSize).to_i
     current_chunk_size = params.fetch(:resumableCurrentChunkSize).to_i
+    previous_chunk_number = chunk_number - 1
 
     # path to chunk file
     chunk = BackupRestore::LocalBackupStore.chunk_path(identifier, filename, chunk_number)
     # upload chunk
     HandleChunkUpload.upload_chunk(chunk, file: file)
 
-    uploaded_file_size = chunk_number * chunk_size
+    uploaded_file_size = previous_chunk_number * chunk_size
     # when all chunks are uploaded
     if uploaded_file_size + current_chunk_size >= total_size
       # merge all the chunks in a background thread
diff --git a/spec/requests/admin/backups_controller_spec.rb b/spec/requests/admin/backups_controller_spec.rb
index e750b6dc512..fa72233a591 100644
--- a/spec/requests/admin/backups_controller_spec.rb
+++ b/spec/requests/admin/backups_controller_spec.rb
@@ -234,10 +234,10 @@ RSpec.describe Admin::BackupsController do
 
     describe "when filename is valid" do
       it "should upload the file successfully" do
+        freeze_time
         described_class.any_instance.expects(:has_enough_space_on_disk?).returns(true)
 
         filename = 'test_Site-0123456789.tar.gz'
-        @paths = [backup_path(File.join('tmp', 'test', "#{filename}.part1"))]
 
         post "/admin/backups/upload.json", params: {
           resumableFilename: filename,
@@ -248,11 +248,102 @@ RSpec.describe Admin::BackupsController do
           resumableCurrentChunkSize: '1',
           file: fixture_file_upload(Tempfile.new)
         }
+        expect_job_enqueued(job: :backup_chunks_merger, args: {
+          filename: filename, identifier: 'test', chunks: 1
+        }, at: 5.seconds.from_now)
 
         expect(response.status).to eq(200)
         expect(response.body).to eq("")
       end
     end
+
+    describe "completing an upload by enqueuing backup_chunks_merger" do
+      let(:filename) { 'test_Site-0123456789.tar.gz' }
+
+      it "works with a single chunk" do
+        freeze_time
+        described_class.any_instance.expects(:has_enough_space_on_disk?).returns(true)
+
+        # 2MB file, 2MB chunks = 1x 2MB chunk
+        post "/admin/backups/upload.json", params: {
+          resumableFilename: filename,
+          resumableTotalSize: '2097152',
+          resumableIdentifier: 'test',
+          resumableChunkNumber: '1',
+          resumableChunkSize: '2097152',
+          resumableCurrentChunkSize: '2097152',
+          file: fixture_file_upload(Tempfile.new)
+        }
+        expect_job_enqueued(job: :backup_chunks_merger, args: {
+          filename: filename, identifier: 'test', chunks: 1
+        }, at: 5.seconds.from_now)
+      end
+
+      it "works with multiple chunks when the final chunk is chunk_size + remainder" do
+        freeze_time
+        described_class.any_instance.expects(:has_enough_space_on_disk?).twice.returns(true)
+
+        # 5MB file, 2MB chunks = 1x 2MB chunk + 1x 3MB chunk with resumable.js
+        post "/admin/backups/upload.json", params: {
+          resumableFilename: filename,
+          resumableTotalSize: '5242880',
+          resumableIdentifier: 'test',
+          resumableChunkNumber: '1',
+          resumableChunkSize: '2097152',
+          resumableCurrentChunkSize: '2097152',
+          file: fixture_file_upload(Tempfile.new)
+        }
+        post "/admin/backups/upload.json", params: {
+          resumableFilename: filename,
+          resumableTotalSize: '5242880',
+          resumableIdentifier: 'test',
+          resumableChunkNumber: '2',
+          resumableChunkSize: '2097152',
+          resumableCurrentChunkSize: '3145728',
+          file: fixture_file_upload(Tempfile.new)
+        }
+        expect_job_enqueued(job: :backup_chunks_merger, args: {
+          filename: filename, identifier: 'test', chunks: 2
+        }, at: 5.seconds.from_now)
+      end
+
+      it "works with multiple chunks when the final chunk is just the remaninder" do
+        freeze_time
+        described_class.any_instance.expects(:has_enough_space_on_disk?).times(3).returns(true)
+
+        # 5MB file, 2MB chunks = 2x 2MB chunk + 1x 1MB chunk with uppy.js
+        post "/admin/backups/upload.json", params: {
+          resumableFilename: filename,
+          resumableTotalSize: '5242880',
+          resumableIdentifier: 'test',
+          resumableChunkNumber: '1',
+          resumableChunkSize: '2097152',
+          resumableCurrentChunkSize: '2097152',
+          file: fixture_file_upload(Tempfile.new)
+        }
+        post "/admin/backups/upload.json", params: {
+          resumableFilename: filename,
+          resumableTotalSize: '5242880',
+          resumableIdentifier: 'test',
+          resumableChunkNumber: '2',
+          resumableChunkSize: '2097152',
+          resumableCurrentChunkSize: '2097152',
+          file: fixture_file_upload(Tempfile.new)
+        }
+        post "/admin/backups/upload.json", params: {
+          resumableFilename: filename,
+          resumableTotalSize: '5242880',
+          resumableIdentifier: 'test',
+          resumableChunkNumber: '3',
+          resumableChunkSize: '2097152',
+          resumableCurrentChunkSize: '1048576',
+          file: fixture_file_upload(Tempfile.new)
+        }
+        expect_job_enqueued(job: :backup_chunks_merger, args: {
+          filename: filename, identifier: 'test', chunks: 3
+        }, at: 5.seconds.from_now)
+      end
+    end
   end
 
   describe "#check_backup_chunk" do