diff --git a/NEWS b/NEWS
index 4a48ae145335c..24c1b6a15ec69 100644
--- a/NEWS
+++ b/NEWS
@@ -5,6 +5,9 @@ PHP                                                                        NEWS
 - DOM:
   . Added Dom\Element::$outerHTML. (nielsdos)
 
+- Output:
+  . Fixed calculation of aligned buffer size. (cmb)
+
 - PDO_PGSQL:
   . Added Iterable support for PDO::pgsqlCopyFromArray. (KentarouTakeda)
   . Implement GH-15387 Pdo\Pgsql::setAttribute(PDO::ATTR_PREFETCH, 0) or
diff --git a/main/php_output.h b/main/php_output.h
index 25a8d370707b5..55bf62f62237a 100644
--- a/main/php_output.h
+++ b/main/php_output.h
@@ -81,8 +81,8 @@ typedef enum _php_output_handler_hook_t {
 } php_output_handler_hook_t;
 
 #define PHP_OUTPUT_HANDLER_INITBUF_SIZE(s) \
-( ((s) > 1) ? \
-	(s) + PHP_OUTPUT_HANDLER_ALIGNTO_SIZE - ((s) % (PHP_OUTPUT_HANDLER_ALIGNTO_SIZE)) : \
+( ((s) > 0) ? \
+	ZEND_MM_ALIGNED_SIZE_EX(s, PHP_OUTPUT_HANDLER_ALIGNTO_SIZE) : \
 	PHP_OUTPUT_HANDLER_DEFAULT_SIZE \
 )
 #define PHP_OUTPUT_HANDLER_ALIGNTO_SIZE		0x1000
diff --git a/tests/output/gh16135.phpt b/tests/output/gh16135.phpt
new file mode 100644
index 0000000000000..5c880ef3a2a8c
--- /dev/null
+++ b/tests/output/gh16135.phpt
@@ -0,0 +1,27 @@
+--TEST--
+GH-16135: output buffer over-allocated for aligned chunk sizes
+--FILE--
+<?php
+ob_start(null, 0);
+ob_start(null, 1);
+ob_start(null, 2);
+ob_start(null, 8191);
+ob_start(null, 8192);
+ob_start(null, 8193);
+var_dump(array_map(fn ($s) => $s["buffer_size"], ob_get_status(true)));
+?>
+--EXPECT--
+array(6) {
+  [0]=>
+  int(16384)
+  [1]=>
+  int(4096)
+  [2]=>
+  int(4096)
+  [3]=>
+  int(8192)
+  [4]=>
+  int(8192)
+  [5]=>
+  int(12288)
+}