diff --git a/webui/model/health/health.php b/webui/model/health/health.php
index e5901d4..9e5a1de 100644
--- a/webui/model/health/health.php
+++ b/webui/model/health/health.php
@@ -87,7 +87,7 @@
       }
 
       // average total message size per day, computed over the time period since the first email was archived
-      $averagesizedayraw = ($averagemessagesizeraw + $averagesqlsizeraw + $averagesphinxsizeraw) * $this->data['averagemessagestotalraw'];
+      $averagesizedayraw = ($averagemessagesizeraw + $averagesqlsizeraw + $averagesphinxsizeraw) * $this->data['averagemessagesmonthraw'];
 
       $datapart = 0;
       foreach($this->data['shortdiskinfo'] as $part) {