diff --git a/src/filo-blockdev.adb b/src/filo-blockdev.adb
index e0c4ebb..8658e05 100644
--- a/src/filo-blockdev.adb
+++ b/src/filo-blockdev.adb
@@ -7,8 +7,8 @@
    is
       use Interfaces.C;
 
-      Sector      : constant unsigned_long  := unsigned_long (Offset / 512);
-      Byte_Offset : constant unsigned_long  := unsigned_long (Offset rem 512);
+      Sector      : constant unsigned_long  := unsigned_long (Offset / BLOCK_SIZE);
+      Byte_Offset : constant unsigned_long  := unsigned_long (Offset rem BLOCK_SIZE);
       Byte_Len    : constant unsigned_long  := unsigned_long (Buffer'Length);
       Buf         : constant System.Address := Buffer'Address;
 
@@ -19,7 +19,7 @@
    begin
       -- With 32-bit longs, the current C interface
       -- can't access more than 512 * 2 ** 32:
-      if Offset / 512 > Blockdev_Length (unsigned_long'last) then
+      if Offset / BLOCK_SIZE > Blockdev_Length (unsigned_long'last) then
          Success := False;
          return;
       end if;