diff --git a/IPython/frontend/html/notebook/static/css/notebook.css b/IPython/frontend/html/notebook/static/css/notebook.css
index 9278c71..a8dc299 100644
--- a/IPython/frontend/html/notebook/static/css/notebook.css
+++ b/IPython/frontend/html/notebook/static/css/notebook.css
@@ -183,7 +183,7 @@ div.code_cell {
 
 div.prompt {
     /* This needs to be wide enough for 3 digit prompt numbers: In[100]: */
-    width: 80px;
+    width: 11ex;
     /* This 0.4em is tuned to match the padding on the CodeMirror editor. */
     padding: 0.4em;
     margin: 0px;