diff --git a/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py b/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
index 69c4d887bf..ff1010a9cc 100644
--- a/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
+++ b/libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py
@@ -572,6 +572,7 @@ def write_mcu_config(f):
             f.write('#define %s\n' % d[7:])
     flash_size = get_config('FLASH_SIZE_KB', type=int)
     f.write('#define BOARD_FLASH_SIZE %u\n' % flash_size)
+    env_vars['BOARD_FLASH_SIZE'] = flash_size
     f.write('#define CRT1_AREAS_NUMBER 1\n')
 
     flash_reserve_start = get_config(