diff --git a/block/vmdk.c b/block/vmdk.c
index 1a80e5a..51398c0 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -1092,6 +1092,7 @@ static int vmdk_read(BlockDriverState *bs, int64_t sector_num,
      BDRVVmdkState *s = bs->opaque;
      int ret;
      uint64_t n, index_in_cluster;
+    uint64_t extent_begin_sector, extent_relative_sector_num;
      VmdkExtent *extent = NULL;
      uint64_t cluster_offset;

@@ -1103,7 +1104,9 @@ static int vmdk_read(BlockDriverState *bs, int64_t sector_num,
          ret = get_cluster_offset(
                              bs, extent, NULL,
                              sector_num << 9, 0, &cluster_offset);
-        index_in_cluster = sector_num % extent->cluster_sectors;
+        extent_begin_sector = extent->end_sector - extent->sectors;
+        extent_relative_sector_num = sector_num - extent_begin_sector;
+        index_in_cluster = extent_relative_sector_num % extent->cluster_sectors;
          n = extent->cluster_sectors - index_in_cluster;
          if (n > nb_sectors) {
              n = nb_sectors;
@@ -1154,6 +1157,7 @@ static int vmdk_write(BlockDriverState *bs, int64_t sector_num,
      VmdkExtent *extent = NULL;
      int n, ret;
      int64_t index_in_cluster;
+    uint64_t extent_begin_sector, extent_relative_sector_num;
      uint64_t cluster_offset;
      VmdkMetaData m_data;

@@ -1196,7 +1200,9 @@ static int vmdk_write(BlockDriverState *bs, int64_t sector_num,
          if (ret) {
              return -EINVAL;
          }
-        index_in_cluster = sector_num % extent->cluster_sectors;
+        extent_begin_sector = extent->end_sector - extent->sectors;
+        extent_relative_sector_num = sector_num - extent_begin_sector;
+        index_in_cluster = extent_relative_sector_num % extent->cluster_sectors;
          n = extent->cluster_sectors - index_in_cluster;
          if (n > nb_sectors) {
              n = nb_sectors;