diff --git a/drivers/flash/flash_stm32wba_fm.c b/drivers/flash/flash_stm32wba_fm.c
index ff42c26..c248812 100644
--- a/drivers/flash/flash_stm32wba_fm.c
+++ b/drivers/flash/flash_stm32wba_fm.c
@@ -104,7 +104,7 @@
 			     size_t len)
 {
 	int rc;
-	int sect_num = (len / FLASH_PAGE_SIZE) + 1;
+	int sect_num;
 
 	if (!flash_stm32_valid_range(dev, offset, len, true)) {
 		LOG_ERR("Erase range invalid. Offset: %p, len: %zu",
@@ -116,6 +116,9 @@
 		return 0;
 	}
 
+	/* len is a multiple of FLASH_PAGE_SIZE */
+	sect_num = len / FLASH_PAGE_SIZE;
+
 	flash_stm32_sem_take(dev);
 
 	LOG_DBG("Erase offset: %p, page: %ld, len: %zu, sect num: %d",