diff --git a/source/active_application.cpp b/source/active_application.cpp
index de21506451e..18cdf7374ac 100755
--- a/source/active_application.cpp
+++ b/source/active_application.cpp
@@ -257,7 +257,7 @@ bool eraseActiveFirmware(uint32_t firmwareSize)
     else /* header contiguous with app */
     {
         /* setup erase of the header + application region */
-        size_needed = fw_metadata_hdr_size + firmwareSize;
+        size_needed = (MBED_CONF_APP_APPLICATION_START_ADDRESS - FIRMWARE_METADATA_HEADER_ADDRESS) + firmwareSize;
         erase_start_addr = FIRMWARE_METADATA_HEADER_ADDRESS;
     }