diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index 1b1b5ad..543a467 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -66,6 +66,19 @@
     return -EINVAL; /* flash_area_open will fail on that */
 }
 
+int flash_area_id_to_image_slot(int area_id)
+{
+    switch (area_id) {
+    case FLASH_AREA_IMAGE_PRIMARY:
+        return 0;
+    case FLASH_AREA_IMAGE_SECONDARY:
+        return 1;
+    default:
+        BOOT_LOG_ERR("invalid flash area ID");
+        return -1;
+    }
+}
+
 int flash_area_sector_from_off(off_t off, struct flash_sector *sector)
 {
     int rc;
diff --git a/boot/zephyr/include/flash_map_backend/flash_map_backend.h b/boot/zephyr/include/flash_map_backend/flash_map_backend.h
index 178c34f..c082dd6 100644
--- a/boot/zephyr/include/flash_map_backend/flash_map_backend.h
+++ b/boot/zephyr/include/flash_map_backend/flash_map_backend.h
@@ -52,6 +52,14 @@
 
 int flash_area_id_from_image_slot(int slot);
 
+/**
+ * Converts the specified flash area ID to an image slot index.
+ *
+ * Returns image slot index (0 or 1), or -1 if ID doesn't correspond to an image
+ * slot.
+ */
+int flash_area_id_to_image_slot(int area_id);
+
 /* Retrieve the flash sector a given offset belongs to.
  *
  * Returns 0 on success, or an error code on failure.