diff --git a/daemon/graphdriver/devmapper/README.md b/daemon/graphdriver/devmapper/README.md
index 849cfce64c..ed99d85a93 100644
--- a/daemon/graphdriver/devmapper/README.md
+++ b/daemon/graphdriver/devmapper/README.md
@@ -128,7 +128,8 @@ Here is the list of supported options:
 
  *  `dm.blocksize`
 
-    Specifies a custom blocksize to use for the thin pool.
+    Specifies a custom blocksize to use for the thin pool.  The default
+    blocksize is 512K.
 
     Example use: