diff --git a/drivers/include/mtd_mapper.h b/drivers/include/mtd_mapper.h
index e57d65fb31..9f2ddbc32d 100644
--- a/drivers/include/mtd_mapper.h
+++ b/drivers/include/mtd_mapper.h
@@ -41,7 +41,7 @@
  *         .page_size = PAGE_SIZE,
  *     },
  *     .parent = &parent,
- *     .offset = PAGE_PER_SECTOR * PAGE_SIZE * SECTOR_COUNT / 2
+ *     .sector = SECTOR_COUNT / 2
  * };
  *
  * mtd_dev_t *dev = &region.mtd;
@@ -96,7 +96,7 @@ typedef struct {
 typedef struct {
     mtd_dev_t mtd;                  /**< MTD context                         */
     mtd_mapper_parent_t *parent;    /**< MTD mapper parent device            */
-    uint32_t offset;                /**< Offset address to start this region */
+    uint32_t sector;                /**< first sector of the region */
 } mtd_mapper_region_t;
 
 /**
diff --git a/drivers/mtd_mapper/mtd_mapper.c b/drivers/mtd_mapper/mtd_mapper.c
index 4e11b60bbf..6720f16ea7 100644
--- a/drivers/mtd_mapper/mtd_mapper.c
+++ b/drivers/mtd_mapper/mtd_mapper.c
@@ -45,6 +45,12 @@ static uint32_t _region_size(mtd_mapper_region_t *region)
            region->mtd.sector_count;
 }
 
+static uint32_t _byte_offset(mtd_mapper_region_t *region)
+{
+    return region->mtd.pages_per_sector * region->mtd.page_size *
+           region->sector;
+}
+
 static int _init_target(mtd_mapper_region_t *region)
 {
     mtd_mapper_parent_t *parent = region->parent;
@@ -68,10 +74,7 @@ static int _init(mtd_dev_t *mtd)
     assert(backing_mtd->sector_count >= region->mtd.sector_count);
 
     /* offset + region size must not exceed the backing device */
-    assert(region->offset + _region_size(
-               region) <=
-           backing_mtd->pages_per_sector * backing_mtd->sector_count *
-           backing_mtd->page_size);
+    assert(region->sector + region->mtd.sector_count <= backing_mtd->sector_count);
 
     /* avoid unused variable warning if compiled with NDEBUG */
     (void)backing_mtd;
@@ -92,7 +95,7 @@ static int _write(mtd_dev_t *mtd, const void *src, uint32_t addr,
     }
 
     _lock(region);
-    int res = mtd_write(region->parent->mtd, src, addr + region->offset, count);
+    int res = mtd_write(region->parent->mtd, src, addr + _byte_offset(region), count);
     _unlock(region);
     return res;
 }
@@ -106,7 +109,7 @@ static int _read(mtd_dev_t *mtd, void *dest, uint32_t addr, uint32_t count)
     }
 
     _lock(region);
-    int res = mtd_read(region->parent->mtd, dest, addr + region->offset, count);
+    int res = mtd_read(region->parent->mtd, dest, addr + _byte_offset(region), count);
     _unlock(region);
     return res;
 }
@@ -120,7 +123,7 @@ static int _erase(mtd_dev_t *mtd, uint32_t addr, uint32_t count)
     }
 
     _lock(region);
-    int res = mtd_erase(region->parent->mtd, addr + region->offset, count);
+    int res = mtd_erase(region->parent->mtd, addr + _byte_offset(region), count);
     _unlock(region);
     return res;
 }
diff --git a/tests/mtd_mapper/main.c b/tests/mtd_mapper/main.c
index e103fac797..a756e236c2 100644
--- a/tests/mtd_mapper/main.c
+++ b/tests/mtd_mapper/main.c
@@ -131,7 +131,7 @@ static mtd_mapper_region_t _region_a = {
         .page_size = PAGE_SIZE,
     },
     .parent = &_parent,
-    .offset = 0,
+    .sector = 0,
 };
 
 static mtd_mapper_region_t _region_b = {
@@ -142,7 +142,7 @@ static mtd_mapper_region_t _region_b = {
         .page_size = PAGE_SIZE,
     },
     .parent = &_parent,
-    .offset = PAGE_PER_SECTOR * PAGE_SIZE * SECTOR_COUNT / 2,
+    .sector = SECTOR_COUNT / 2,
 };
 
 static mtd_dev_t *_dev_a = &_region_a.mtd;