diff --git a/headers/os/mail/mail_encoding.h b/headers/os/mail/mail_encoding.h
index eac45759f5..aeb877efbd 100644
--- a/headers/os/mail/mail_encoding.h
+++ b/headers/os/mail/mail_encoding.h
@@ -6,7 +6,6 @@
 */
 
 
-#include <size_t.h>
 #include <sys/types.h>
 
 
diff --git a/headers/private/debugger/user_interface/util/UiUtils.h b/headers/private/debugger/user_interface/util/UiUtils.h
index e96a89b97c..0836f7eaea 100644
--- a/headers/private/debugger/user_interface/util/UiUtils.h
+++ b/headers/private/debugger/user_interface/util/UiUtils.h
@@ -6,8 +6,7 @@
 #ifndef UI_UTILS_H
 #define UI_UTILS_H
 
-#include <size_t.h>
-
+#include <stddef.h>
 #include <image.h>
 
 #include "Types.h"
diff --git a/src/add-ons/kernel/file_systems/shared/crc32.cpp b/src/add-ons/kernel/file_systems/shared/crc32.cpp
index f4d6c0c42d..204b2c6b6c 100644
--- a/src/add-ons/kernel/file_systems/shared/crc32.cpp
+++ b/src/add-ons/kernel/file_systems/shared/crc32.cpp
@@ -49,7 +49,7 @@
 #ifdef __HAIKU__
 #include <endian.h>
 #include <kernel.h>
-#include <size_t.h>
+#include <stddef.h>
 #include <stdint.h>
 uint32 calculate_crc32c(uint32 crc32c, const unsigned char *buffer,
 	unsigned int length);