diff --git a/thin-provisioning/thin_generate_mappings.cc b/thin-provisioning/thin_generate_mappings.cc
index 97402ce..1a124bb 100644
--- a/thin-provisioning/thin_generate_mappings.cc
+++ b/thin-provisioning/thin_generate_mappings.cc
@@ -129,10 +129,10 @@ thin_generate_mappings_cmd::usage(std::ostream &out) const
 	    << "  {-h|--help}\n"
 	    << "  {-o|--output} <output device or file>\n"
 	    << "  {--dev-id} <dev-id>\n"
-	    << "  {--offset} <offset>\n"
-            << "  {--io-size} <io_size>\n"
+	    << "  {--offset} <offset in sectors>\n"
+	    << "  {--io-size} <io-size in sectors>\n"
 	    << "  {--rw write|trim|randwrite|randtrim|randtw}\n"
-            << "  {--size} <size>\n"
+	    << "  {--size} <size in sectors>\n"
 	    << "  {-V|--version}" << endl;
 }