diff --git a/src/css/simplemde.css b/src/css/simplemde.css
index fb0e4c7..e749c08 100644
--- a/src/css/simplemde.css
+++ b/src/css/simplemde.css
@@ -1,4 +1,5 @@
 .CodeMirror {
+    box-sizing: border-box;
     height: auto;
     min-height: 300px;
     border: 1px solid #ddd;
@@ -325,4 +326,4 @@
 
 .CodeMirror .CodeMirror-placeholder {
     opacity: .5;
-}
\ No newline at end of file
+}