diff --git a/main.go b/main.go
index f4dc812..de4d73f 100644
--- a/main.go
+++ b/main.go
@@ -411,7 +411,7 @@ func handleGUI(w http.ResponseWriter, r *http.Request) {
   <style>
     body { font-family: sans-serif; max-width: 1000px; margin: auto; padding: 2em; background: #f9fafb; }
     h1 { font-size: 1.5em; margin-bottom: 1em; }
-    input, button { padding: 0.7em; margin: 0.3em 0; width: 100%; border-radius: 0.4em; border: 1px solid #ccc; }
+    input, button { padding: 0.7em; margin: 0.3em 0; width: 100%; border-radius: 0.4em; border: 1px solid #ccc; box-sizing: border-box; }
     button { background: #2563eb; color: white; border: none; cursor: pointer; }
     button:hover { background: #1d4ed8; }
     #result, #metrics, #history { background: white; border: 1px solid #ddd; padding: 1em; border-radius: 0.4em; margin-top: 1em; white-space: pre-wrap; }