diff --git a/firmware/include/gbb_header.h b/firmware/include/gbb_header.h
index 18bb64b7a0..39e1ac2080 100644
--- a/firmware/include/gbb_header.h
+++ b/firmware/include/gbb_header.h
@@ -18,6 +18,9 @@
 #define GBB_MAJOR_VER      (0x01)
 #define GBB_MINOR_VER      (0x00)
 
+/* Maximum length of a HWID in bytes, counting terminating null. */
+#define GBB_HWID_MAX_SIZE  256
+
 #ifdef __cplusplus
 extern "C" {
 #endif  // __cplusplus