diff --git a/src/detail/align_up.hpp b/src/detail/align_up.hpp
new file mode 100644
index 00000000..89510a20
--- /dev/null
+++ b/src/detail/align_up.hpp
@@ -0,0 +1,32 @@
+//
+// Copyright (c) 2024 Christian Mazakas
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/CPPAlliance/http_proto
+//
+
+#ifndef BOOST_HTTP_PROTO_DETAIL_ALIGN_UP_HPP
+#define BOOST_HTTP_PROTO_DETAIL_ALIGN_UP_HPP
+
+#include <cstddef>
+
+namespace boost {
+namespace http_proto {
+namespace detail {
+
+constexpr
+inline
+std::size_t
+align_up(std::size_t s, std::size_t A)
+{
+  return A * (
+        (s + A - 1) / A);
+}
+
+} // detail
+} // http_proto
+} // boost
+
+#endif
diff --git a/src/detail/header.cpp b/src/detail/header.cpp
index d92b0772..62deadeb 100644
--- a/src/detail/header.cpp
+++ b/src/detail/header.cpp
@@ -27,6 +27,8 @@
 #include <string>
 #include <utility>
 
+#include "align_up.hpp"
+
 namespace boost {
 namespace http_proto {
 namespace detail {
@@ -209,9 +211,7 @@ bytes_needed(
         size = 19;
     static constexpr auto A =
         alignof(header::entry);
-    // round up to alignof(A)
-    return A * (
-        (size + A - 1) / A) +
+    return align_up(size, A) +
             (count * sizeof(
                 header::entry));
 }