diff --git a/lxd/storage/backend_lxd.go b/lxd/storage/backend_lxd.go
index fff80100ddcd..62ec945d66ca 100644
--- a/lxd/storage/backend_lxd.go
+++ b/lxd/storage/backend_lxd.go
@@ -3278,7 +3278,12 @@ func (b *lxdBackend) GetInstanceUsage(inst instance.Instance) (*VolumeUsage, err
 	}
 
 	sizeStr, ok := rootDiskConf["size"]
-	if ok {
+	if !ok && vol.ContentType() == drivers.ContentTypeBlock {
+		// If size config key is not set, assume default size for block volumes.
+		sizeStr = drivers.DefaultBlockSize
+	}
+
+	if sizeStr != "" {
 		total, err := units.ParseByteSizeString(sizeStr)
 		if err != nil {
 			return nil, err