diff --git a/lib/flash_internal.c b/lib/flash_internal.c
index 81eac5c..39d7632 100644
--- a/lib/flash_internal.c
+++ b/lib/flash_internal.c
@@ -72,7 +72,7 @@ bool flash_internal_read(uint32_t address, uint8_t *buffer, size_t size)
 	return true;
 }
 
-int8_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size, bool preserve)
+int32_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size, bool preserve)
 {
 	// sanity checks
 	if (buffer == NULL || size == 0 || size % 2) {
@@ -105,6 +105,7 @@ int8_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size
 		}
 	}
 
+	uint32_t written = 0; // number of bytes written
 	flash_unlock(); // unlock flash to be able to write it
 	while (size) { // write page by page until all data has been written
 		// verify of we need to erase the flash before writing it
@@ -149,6 +150,7 @@ int8_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size
 					flash_lock(); // lock back flash to protect it
 					return -8;
 				}
+				written += 2;
 			}
 		} else { // simply copy data until end of page (or end of data)
 			if (erase) {
@@ -168,6 +170,7 @@ int8_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size
 					flash_lock(); // lock back flash to protect it
 					return -11;
 				}
+				written += 2;
 				buffer += 2;
 				address += 2;
 				size -= 2;
@@ -176,5 +179,5 @@ int8_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size
 	}
 	flash_lock(); // lock back flash to protect it
 
-	return 0;
+	return written;
 }
diff --git a/lib/flash_internal.h b/lib/flash_internal.h
index ff804d0..247770a 100644
--- a/lib/flash_internal.h
+++ b/lib/flash_internal.h
@@ -32,7 +32,7 @@ bool flash_internal_read(uint32_t address, uint8_t *buffer, size_t size);
  *  @param[in] buffer data to be written
  *  @param[in] size how much data to write, in bytes
  *  @param[in] preserve keep the rest of the page if data needs to be erased
- *  @return 0 on success, else see internal error code
+ *  @return number of bytes written (including preserved data), or negative in case of error
  *  @note the page will be erased if needed to write the data to the flash
  */
-int8_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size, bool preserve);
+int32_t flash_internal_write(uint32_t address, const uint8_t *buffer, size_t size, bool preserve);