diff --git a/fio.1 b/fio.1
index 0838d07..b7046d5 100644
--- a/fio.1
+++ b/fio.1
@@ -257,7 +257,9 @@
 Unless \fBnrfiles\fR and \fBfilesize\fR options are given, this amount will be
 divided between the available files for the job. If not set, fio will use the
 full size of the given files or devices. If the the files do not exist, size
-must be given.
+must be given. It is also possible to give size as a percentage between 1 and
+100. If size=20% is given, fio will use 20% of the full size of the given files
+or devices.
 .TP
 .BI fill_device \fR=\fPbool "\fR,\fB fill_fs" \fR=\fPbool
 Sets size to something really large and waits for ENOSPC (no space left on