diff --git a/lorawan_toa.py b/lorawan_toa.py
index c970071..f69a242 100644
--- a/lorawan_toa.py
+++ b/lorawan_toa.py
@@ -108,8 +108,13 @@ if __name__ == "__main__" :
         p.add_argument("n_sf", metavar="SF", type=int,
             help="Spreading Factor. It should be from 7 to 12.")
         p.add_argument("n_size", metavar="SIZE", type=int,
-            help="""PHY payload size in byte. It's equal to the MAC payload + 5
-                       (MHDR + MIC). e.g. 23 as Join Request.""")
+            help="""PHY payload size in byte.
+                       Remember that PHY payload (i.e. MAC frame) is consist of 
+                       MHDR(1) + MAC payload + MIC(4), or
+                       MHDR(1) + FHDR(7) + FPort(1) + APP + MIC(4).
+                       For example, SIZE for Join Request is going to be 23.
+                       If the size of an application message (APP) is
+                       12, SIZE is going to be 25. """)
         p.add_argument("--band-width", action="store", dest="n_bw", type=int,
             default=125, metavar="NUMBER",
             help="bandwidth in kHz. default is 125 kHz.")