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;