diff --git a/synapse/res/templates/sso.css b/synapse/res/templates/sso.css
index 428ee917b3..46b309ea4e 100644
--- a/synapse/res/templates/sso.css
+++ b/synapse/res/templates/sso.css
@@ -55,6 +55,7 @@ main {
   display: block;
   border-radius: 12px;
   width: 100%;
+  box-sizing: border-box;
   margin: 16px 0;
   cursor: pointer;
   text-align: center;