diff --git a/format.c b/format.c
index 10ac613e..497b5b52 100644
--- a/format.c
+++ b/format.c
@@ -401,10 +401,8 @@ format_session(struct format_tree *ft, struct session *s)
 	*strchr(tim, '\n') = '\0';
 	format_add(ft, "session_created_string", "%s", tim);
 
-	if (s->flags & SESSION_UNATTACHED)
-		format_add(ft, "session_attached", "%d", 0);
-	else
-		format_add(ft, "session_attached", "%d", 1);
+	format_add(ft, "session_attached", "%u", s->attached);
+	format_add(ft, "session_many_attached", "%u", s->attached > 1);
 }
 
 /* Set default format keys for a client. */
diff --git a/resize.c b/resize.c
index 8d0bd275..b3b031c0 100644
--- a/resize.c
+++ b/resize.c
@@ -55,6 +55,7 @@ recalculate_sizes(void)
 	RB_FOREACH(s, sessions, &sessions) {
 		has_status = options_get_number(&s->options, "status");
 
+		s->attached = 0;
 		ssx = ssy = UINT_MAX;
 		for (j = 0; j < ARRAY_LENGTH(&clients); j++) {
 			c = ARRAY_ITEM(&clients, j);
@@ -69,6 +70,7 @@ recalculate_sizes(void)
 					ssy = c->tty.sy - 1;
 				else if (c->tty.sy < ssy)
 					ssy = c->tty.sy;
+				s->attached++;
 			}
 		}
 		if (ssx == UINT_MAX || ssy == UINT_MAX) {
diff --git a/tmux.1 b/tmux.1
index 51d927a5..c9e5815a 100644
--- a/tmux.1
+++ b/tmux.1
@@ -3141,13 +3141,14 @@ The following variables are available, where appropriate:
 .It Li "saved_cursor_y" Ta "" Ta "Saved cursor Y in pane"
 .It Li "scroll_region_lower" Ta "" Ta "Bottom of scroll region in pane"
 .It Li "scroll_region_upper" Ta "" Ta "Top of scroll region in pane"
-.It Li "session_attached" Ta "" Ta "1 if session attached"
+.It Li "session_attached" Ta "" Ta "Number of clients session is attached to"
 .It Li "session_created" Ta "" Ta "Integer time session created"
 .It Li "session_created_string" Ta "" Ta "String time session created"
 .It Li "session_group" Ta "" Ta "Number of session group"
 .It Li "session_grouped" Ta "" Ta "1 if session in a group"
 .It Li "session_height" Ta "" Ta "Height of session"
 .It Li "session_id" Ta "" Ta "Unique session ID"
+.It Li "session_many_attached" Ta "" Ta "1 if multiple clients attached"
 .It Li "session_name" Ta "#S" Ta "Name of session"
 .It Li "session_width" Ta "" Ta "Width of session"
 .It Li "session_windows" Ta "" Ta "Number of windows in session"
diff --git a/tmux.h b/tmux.h
index 0a6ca4e4..fb2fb940 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1083,6 +1083,8 @@ struct session {
 #define SESSION_UNATTACHED 0x1	/* not attached to any clients */
 	int		 flags;
 
+	u_int            attached;
+
 	struct termios	*tio;
 
 	struct environ	 environ;