diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index d1bfc8f..7cca766 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -73,6 +73,9 @@
 
 OPTIONS = Options()
 
+# The block size that's used across the releasetools scripts.
+BLOCK_SIZE = 4096
+
 # Values for "certificate" in apkcerts that mean special things.
 SPECIAL_CERT_STRINGS = ("PRESIGNED", "EXTERNAL")