diff --git a/kmod/src/format.h b/kmod/src/format.h
index 4b60a75c..3cefe482 100644
--- a/kmod/src/format.h
+++ b/kmod/src/format.h
@@ -15,6 +15,7 @@
 #define SCOUTFS_BLOCKS_PER_PAGE (PAGE_SIZE / SCOUTFS_BLOCK_SIZE)
 #define SCOUTFS_BLOCK_SECTOR_SHIFT (SCOUTFS_BLOCK_SHIFT - 9)
 #define SCOUTFS_BLOCK_SECTORS (1 << SCOUTFS_BLOCK_SECTOR_SHIFT)
+#define SCOUTFS_BLOCK_MAX (U64_MAX >> SCOUTFS_BLOCK_SHIFT)
 
 /*
  * FS data is stored in segments, for now they're fixed size. They'll