diff --git a/writeimg.c b/writeimg.c
index a65f536..c83ce29 100644
--- a/writeimg.c
+++ b/writeimg.c
@@ -22,6 +22,10 @@
 #define CR_YEAR "2026"
 #endif
 
+#ifndef BLOCKSIZE
+#define BLOCKSIZE (4 * 1024 * 1024)
+#endif
+
 // clang-format off
 const char help[] =
     "Usage:\n"
@@ -38,8 +42,6 @@ const char help[] =
 
 const char copyright[] = "Copyright (C) %s Imbus, BSD-2-Clause\n";
 
-#define BLOCKSIZE (4 * 1024 * 1024)
-
 struct write_job {
     char  *filename;
     char  *dev_name;