diff --git a/include/patty/ax25/frame.h b/include/patty/ax25/frame.h
index 4ca9385..84b10a1 100644
--- a/include/patty/ax25/frame.h
+++ b/include/patty/ax25/frame.h
@@ -7,6 +7,11 @@
 #define PATTY_AX25_FRAME_DEFAULT_MAXLEN 256
 #define PATTY_AX25_FRAME_DEFAULT_WINDOW   8
 
+#define PATTY_AX25_FRAME_SIZE(hops, mode, infolen) \
+    ((sizeof(patty_ax25_addr) * (2 + hops)) \
+   + (mode == PATTY_SOCK_SABME? 2: 1) \
+   + (infolen > 0? 1 + infolen: 0))
+
 enum patty_ax25_frame_type {
     PATTY_AX25_FRAME_UNKNOWN,
     PATTY_AX25_FRAME_INFO,