diff --git a/grade/edit/tree/calculation.php b/grade/edit/tree/calculation.php
index 76fea61a8ab..1e984beed3f 100644
--- a/grade/edit/tree/calculation.php
+++ b/grade/edit/tree/calculation.php
@@ -139,7 +139,7 @@ echo '
         </div>
     </fieldset>
     <div class="fitem" style="text-align: center;">
-        <input id="id_addidnumbers" type="submit" value="'.get_string('addidnumbers', 'grades').'" name="addidnumbers" />
+        <input id="id_addidnumbers" type="submit" class="btn btn-secondary" value="' . get_string('addidnumbers', 'grades') . '" name="addidnumbers" />
     </div>
 </form>';