diff --git a/man/gnt-backup.sgml b/man/gnt-backup.sgml
index 25f367388f5d95208f69b5e8f38d9b7a3ab72137..796148d5364e87c85a0e42f061dde2018fd29721 100644
--- a/man/gnt-backup.sgml
+++ b/man/gnt-backup.sgml
@@ -122,7 +122,12 @@
 
       <para>
         The <option>-s</option> option specifies the disk size for
-        the instance, in gibibytes (defaults to 20 GiB).
+        the instance, in mebibytes (defaults to
+        <constant>20480MiB</constant> =
+        <constant>20GiB</constant>). You can also use one of the
+        suffixes <literal>m</literal>, <literal>g</literal> or
+        <literal>t</literal> to specificy the exact the units used;
+        these suffixes map to mebibytes, gibibytes and tebibytes.
       </para>
 
       <para>
diff --git a/scripts/gnt-backup b/scripts/gnt-backup
index daf7c90dd99845fa7b6520220eecb04aea26dffe..6ad2e703df6b1d7ef06bebc67092753adb2cd465 100755
--- a/scripts/gnt-backup
+++ b/scripts/gnt-backup
@@ -106,7 +106,8 @@ import_opts = [
   make_option("-n", "--node", dest="node",
               help="Target node and optional secondary node",
               metavar="<pnode>[:<snode>]"),
-  cli_option("-s", "--os-size", dest="size", help="Disk size",
+  cli_option("-s", "--os-size", dest="size", help="Disk size, in MiB unless"
+             " a suffix is used",
              default=20 * 1024, type="unit", metavar="<size>"),
   cli_option("--swap-size", dest="swap", help="Swap size",
              default=4 * 1024, type="unit", metavar="<size>"),