diff --git a/common/dmthin b/common/dmthin
index 1b5c8569..3cd206a6 100644
--- a/common/dmthin
+++ b/common/dmthin
@@ -91,6 +91,8 @@ _dmthin_init()
 	if [ -z "$low_water" ]; then
 		low_water=204800	# 100M, in sectors
 	fi
+	# low_water is expressed in blocks of size $cluster_size
+	low_water=$((low_water / cluster_size))
 
 	# Need to make linear metadata and data devs.  From kernel docs:
 	# As a guide, we suggest you calculate the number of bytes to use in the