diff --git a/tempest/api/volume/test_volumes_extend.py b/tempest/api/volume/test_volumes_extend.py
index 5d339c4..ac9a9c7 100644
--- a/tempest/api/volume/test_volumes_extend.py
+++ b/tempest/api/volume/test_volumes_extend.py
@@ -33,7 +33,7 @@
     def test_volume_extend(self):
         # Extend Volume Test.
         volume = self.create_volume(image_ref=self.image_ref)
-        extend_size = volume['size'] + 1
+        extend_size = volume['size'] * 2
         self.volumes_client.extend_volume(volume['id'],
                                           new_size=extend_size)
         waiters.wait_for_volume_resource_status(self.volumes_client,
@@ -48,7 +48,7 @@
         volume = self.create_volume()
         self.create_snapshot(volume['id'])
 
-        extend_size = volume['size'] + 1
+        extend_size = volume['size'] * 2
         self.volumes_client.extend_volume(volume['id'], new_size=extend_size)
 
         waiters.wait_for_volume_resource_status(self.volumes_client,
diff --git a/tempest/api/volume/test_volumes_negative.py b/tempest/api/volume/test_volumes_negative.py
index f139283..866bd87 100644
--- a/tempest/api/volume/test_volumes_negative.py
+++ b/tempest/api/volume/test_volumes_negative.py
@@ -103,21 +103,24 @@
     def test_create_volume_with_nonexistent_volume_type(self):
         # Should not be able to create volume with non-existent volume type
         self.assertRaises(lib_exc.NotFound, self.volumes_client.create_volume,
-                          size='1', volume_type=data_utils.rand_uuid())
+                          size=CONF.volume.volume_size,
+                          volume_type=data_utils.rand_uuid())
 
     @decorators.attr(type=['negative'])
     @decorators.idempotent_id('0c36f6ae-4604-4017-b0a9-34fdc63096f9')
     def test_create_volume_with_nonexistent_snapshot_id(self):
         # Should not be able to create volume with non-existent snapshot
         self.assertRaises(lib_exc.NotFound, self.volumes_client.create_volume,
-                          size='1', snapshot_id=data_utils.rand_uuid())
+                          size=CONF.volume.volume_size,
+                          snapshot_id=data_utils.rand_uuid())
 
     @decorators.attr(type=['negative'])
     @decorators.idempotent_id('47c73e08-4be8-45bb-bfdf-0c4e79b88344')
     def test_create_volume_with_nonexistent_source_volid(self):
         # Should not be able to create volume with non-existent source volume
         self.assertRaises(lib_exc.NotFound, self.volumes_client.create_volume,
-                          size='1', source_volid=data_utils.rand_uuid())
+                          size=CONF.volume.volume_size,
+                          source_volid=data_utils.rand_uuid())
 
     @decorators.attr(type=['negative'])
     @decorators.idempotent_id('0186422c-999a-480e-a026-6a665744c30c')
diff --git a/tempest/api/volume/test_volumes_snapshots_negative.py b/tempest/api/volume/test_volumes_snapshots_negative.py
index ea5f036..0453c0a 100644
--- a/tempest/api/volume/test_volumes_snapshots_negative.py
+++ b/tempest/api/volume/test_volumes_snapshots_negative.py
@@ -50,7 +50,7 @@
     @decorators.idempotent_id('677863d1-34f9-456d-b6ac-9924f667a7f4')
     def test_volume_from_snapshot_decreasing_size(self):
         # Creates a volume a snapshot passing a size different from the source
-        src_size = CONF.volume.volume_size + 1
+        src_size = CONF.volume.volume_size * 2
 
         src_vol = self.create_volume(size=src_size)
         src_snap = self.create_snapshot(src_vol['id'])
@@ -58,7 +58,7 @@
         # Destination volume smaller than source
         self.assertRaises(lib_exc.BadRequest,
                           self.volumes_client.create_volume,
-                          size=src_size - 1,
+                          size=CONF.volume.volume_size,
                           snapshot_id=src_snap['id'])
 
     @decorators.attr(type=['negative'])