diff --git a/src/include/options.h b/src/include/options.h
index ecf0b23..3997895 100644
--- a/src/include/options.h
+++ b/src/include/options.h
@@ -93,6 +93,7 @@ struct opts_s {
 	bool rate;                     /* rate counter flag */
 	bool average_rate;             /* average rate counter flag */
 	bool bytes;                    /* bytes transferred flag */
+	bool ratio;                    /* in:out ratio flag */
 	bool bits;                     /* report transfer size in bits */
 	bool decimal_units;            /* decimal prefix flag */
 	bool bufpercent;               /* transfer buffer percentage flag */
diff --git a/src/main/options.c b/src/main/options.c
index cf85a6f..f8877e7 100644
--- a/src/main/options.c
+++ b/src/main/options.c
@@ -1290,7 +1290,8 @@ opts_t opts_parse(unsigned int argc, char **argv)
 		opts->eta = true;
 		opts->rate = true;
 		opts->bytes = true;
-		/* TODO: add ratio format if "-M both" */
+		if (PV_SIDE_BOTH == opts->side)
+			opts->ratio = true;
 	}
 
 	/* If -Z was given but not -E, pretend one -E was given too. */
diff --git a/src/pv/state.c b/src/pv/state.c
index 38f4e9e..298cdd2 100644
--- a/src/pv/state.c
+++ b/src/pv/state.c
@@ -412,6 +412,8 @@ void pv_state_free(pvstate_t state)
 
 /*
  * Set the formatting string, given a set of old-style formatting options.
+ *
+ * TODO: allow opts->ratio to be passed here as well.
  */
 void pv_state_set_format(pvstate_t state, bool progress, bool timer, bool eta, bool fineta, bool rate, bool average_rate, bool bytes, bool bufpercent, size_t lastwritten,	/*@null@ */
 			 const char *name)