diff --git a/armsrc/spiffs_config.h b/armsrc/spiffs_config.h
index 4f8a5dd26..d0d4eb1d7 100644
--- a/armsrc/spiffs_config.h
+++ b/armsrc/spiffs_config.h
@@ -28,6 +28,7 @@
 #include "printf.h"
 #include "string.h"
 #include "flashmem.h"
+#include "pmflash.h"
 
 //#include <stddef.h>
 //#include <unistd.h>
@@ -236,7 +237,7 @@ typedef uint8_t u8_t;
 // Instead of giving parameters in config struct, singleton build must
 // give parameters in defines below.
 #ifndef SPIFFS_CFG_PHYS_SZ
-#define SPIFFS_CFG_PHYS_SZ(ignore)        (1024 * 64 * (spi_flash_pages64k - 1))
+#define SPIFFS_CFG_PHYS_SZ(ignore)        ((1024 * 64 * spi_flash_pages64k) - (1024 * 4 * (FLASH_RESERVED_TRAILING_4K_SECTORS + 1)))
 #endif
 #ifndef SPIFFS_CFG_PHYS_ERASE_SZ
 #define SPIFFS_CFG_PHYS_ERASE_SZ(ignore)  (4*1024)
diff --git a/include/pmflash.h b/include/pmflash.h
index 8f9f9c741..3a6b361b6 100644
--- a/include/pmflash.h
+++ b/include/pmflash.h
@@ -48,6 +48,8 @@
 # define FLASH_MEM_MAX_4K_SECTOR_P(p64k)  (FLASH_MEM_MAX_SIZE_P(p64k) - 4096)
 #endif
 
+#define FLASH_RESERVED_TRAILING_4K_SECTORS 10
+
 #ifndef FLASH_MEM_ID_LEN
 # define FLASH_MEM_ID_LEN 8
 #endif