diff --git a/tools/image-data b/tools/image-data
index fe46e310..e73b6d7d 100755
--- a/tools/image-data
+++ b/tools/image-data
@@ -3,10 +3,10 @@
 
 if [ $# -eq 0 ]
 then
-  echo "Fill disk of some capacity (in MB) from stdin"
+  echo "Fill disk of some capacity (in ~1 MB units) from stdin"
   echo "usage: image-data capacity"
   exit 1
 fi
 
-dd if=/dev/zero of=data.img count=$(($1*2016))  # 32 * 63 sectors/track
+dd if=/dev/zero of=data.img count=$(($1*2016))  # 32 tracks * 63 sectors/track of 512-byte sectors
 dd of=data.img conv=notrunc