diff --git a/src/cryptobox/core/blockdevice.py b/src/cryptobox/core/blockdevice.py
index 8536520..5473cd8 100644
--- a/src/cryptobox/core/blockdevice.py
+++ b/src/cryptobox/core/blockdevice.py
@@ -389,8 +389,8 @@ class Blockdevice:
         default = 0
         try:
             size_blocks = int(file(os.path.join(self.devdir, 'size')).read())
-            ## size is defined as the number of blocks (512 byte)
-            return int(size_blocks/512)
+            ## size is defined as the number of blocks (512 byte each)
+            return int(size_blocks*512/1024/1024)
         except OSError:
             return default
         except ValueError: