diff --git a/arguments.c b/arguments.c
index c94a3a29..485af2be 100644
--- a/arguments.c
+++ b/arguments.c
@@ -848,6 +848,41 @@ args_strtonum(struct args *args, u_char flag, long long minval,
 	return (ll);
 }
 
+/* Convert an argument value to a number, and expand formats. */
+long long
+args_strtonum_and_expand(struct args *args, u_char flag, long long minval,
+    long long maxval, struct cmdq_item *item, char **cause)
+{
+	const char		*errstr;
+	char			*formatted;
+	long long		 ll;
+	struct args_entry	*entry;
+	struct args_value	*value;
+
+	if ((entry = args_find(args, flag)) == NULL) {
+		*cause = xstrdup("missing");
+		return (0);
+	}
+	value = TAILQ_LAST(&entry->values, args_values);
+	if (value == NULL ||
+	    value->type != ARGS_STRING ||
+	    value->string == NULL) {
+		*cause = xstrdup("missing");
+		return (0);
+	}
+
+	formatted = format_single_from_target(item, value->string);
+	ll = strtonum(formatted, minval, maxval, &errstr);
+	free(formatted);
+	if (errstr != NULL) {
+		*cause = xstrdup(errstr);
+		return (0);
+	}
+
+	*cause = NULL;
+	return (ll);
+}
+
 /* Convert an argument to a number which may be a percentage. */
 long long
 args_percentage(struct args *args, u_char flag, long long minval,
@@ -904,3 +939,70 @@ args_string_percentage(const char *value, long long minval, long long maxval,
 	*cause = NULL;
 	return (ll);
 }
+
+/*
+ * Convert an argument to a number which may be a percentage, and expand
+ * formats.
+ */
+long long
+args_percentage_and_expand(struct args *args, u_char flag, long long minval,
+    long long maxval, long long curval, struct cmdq_item *item, char **cause)
+{
+	const char		*value;
+	struct args_entry	*entry;
+
+	if ((entry = args_find(args, flag)) == NULL) {
+		*cause = xstrdup("missing");
+		return (0);
+	}
+	value = TAILQ_LAST(&entry->values, args_values)->string;
+	return (args_string_percentage_and_expand(value, minval, maxval, curval,
+		    item, cause));
+}
+
+/*
+ * Convert a string to a number which may be a percentage, and expand formats.
+ */
+long long
+args_string_percentage_and_expand(const char *value, long long minval,
+    long long maxval, long long curval, struct cmdq_item *item, char **cause)
+{
+	const char	*errstr;
+	long long	 ll;
+	size_t		 valuelen = strlen(value);
+	char		*copy, *f;
+
+	if (value[valuelen - 1] == '%') {
+		copy = xstrdup(value);
+		copy[valuelen - 1] = '\0';
+
+		f = format_single_from_target(item, copy);
+		ll = strtonum(f, 0, 100, &errstr);
+		free(f);
+		free(copy);
+		if (errstr != NULL) {
+			*cause = xstrdup(errstr);
+			return (0);
+		}
+		ll = (curval * ll) / 100;
+		if (ll < minval) {
+			*cause = xstrdup("too small");
+			return (0);
+		}
+		if (ll > maxval) {
+			*cause = xstrdup("too large");
+			return (0);
+		}
+	} else {
+		f = format_single_from_target(item, value);
+		ll = strtonum(f, minval, maxval, &errstr);
+		free(f);
+		if (errstr != NULL) {
+			*cause = xstrdup(errstr);
+			return (0);
+		}
+	}
+
+	*cause = NULL;
+	return (ll);
+}
diff --git a/cmd-capture-pane.c b/cmd-capture-pane.c
index 964f831e..a3e84c46 100644
--- a/cmd-capture-pane.c
+++ b/cmd-capture-pane.c
@@ -133,7 +133,8 @@ cmd_capture_pane_history(struct args *args, struct cmdq_item *item,
 	if (Sflag != NULL && strcmp(Sflag, "-") == 0)
 		top = 0;
 	else {
-		n = args_strtonum(args, 'S', INT_MIN, SHRT_MAX, &cause);
+		n = args_strtonum_and_expand(args, 'S', INT_MIN, SHRT_MAX,
+			item, &cause);
 		if (cause != NULL) {
 			top = gd->hsize;
 			free(cause);
@@ -149,7 +150,8 @@ cmd_capture_pane_history(struct args *args, struct cmdq_item *item,
 	if (Eflag != NULL && strcmp(Eflag, "-") == 0)
 		bottom = gd->hsize + gd->sy - 1;
 	else {
-		n = args_strtonum(args, 'E', INT_MIN, SHRT_MAX, &cause);
+		n = args_strtonum_and_expand(args, 'E', INT_MIN, SHRT_MAX,
+			item, &cause);
 		if (cause != NULL) {
 			bottom = gd->hsize + gd->sy - 1;
 			free(cause);
diff --git a/cmd-join-pane.c b/cmd-join-pane.c
index f7f16a4e..fbe9eff2 100644
--- a/cmd-join-pane.c
+++ b/cmd-join-pane.c
@@ -72,10 +72,11 @@ cmd_join_pane_exec(struct cmd *self, struct cmdq_item *item)
 	struct window		*src_w, *dst_w;
 	struct window_pane	*src_wp, *dst_wp;
 	char			*cause = NULL;
-	int			 size, percentage, dst_idx;
+	int			 size, dst_idx;
 	int			 flags;
 	enum layout_type	 type;
 	struct layout_cell	*lc;
+	u_int			 curval = 0;
 
 	dst_s = target->s;
 	dst_wl = target->wl;
@@ -98,23 +99,30 @@ cmd_join_pane_exec(struct cmd *self, struct cmdq_item *item)
 	if (args_has(args, 'h'))
 		type = LAYOUT_LEFTRIGHT;
 
+	/* If the 'p' flag is dropped then this bit can be moved into 'l'. */
+	if (args_has(args, 'l') || args_has(args, 'p')) {
+		if (args_has(args, 'f')) {
+			if (type == LAYOUT_TOPBOTTOM)
+				curval = dst_w->sy;
+			else
+				curval = dst_w->sx;
+		} else {
+			if (type == LAYOUT_TOPBOTTOM)
+				curval = dst_wp->sy;
+			else
+				curval = dst_wp->sx;
+		}
+	}
+
 	size = -1;
 	if (args_has(args, 'l')) {
-		if (type == LAYOUT_TOPBOTTOM) {
-			size = args_percentage(args, 'l', 0, INT_MAX,
-			    dst_wp->sy, &cause);
-		} else {
-			size = args_percentage(args, 'l', 0, INT_MAX,
-			    dst_wp->sx, &cause);
-		}
+		size = args_percentage_and_expand(args, 'l', 0, INT_MAX, curval,
+			   item, &cause);
 	} else if (args_has(args, 'p')) {
-		percentage = args_strtonum(args, 'p', 0, 100, &cause);
-		if (cause == NULL) {
-			if (type == LAYOUT_TOPBOTTOM)
-				size = (dst_wp->sy * percentage) / 100;
-			else
-				size = (dst_wp->sx * percentage) / 100;
-		}
+		size = args_strtonum_and_expand(args, 'l', 0, 100, item,
+			   &cause);
+		if (cause == NULL)
+			size = curval * size / 100;
 	}
 	if (cause != NULL) {
 		cmdq_error(item, "size %s", cause);
diff --git a/cmd-send-keys.c b/cmd-send-keys.c
index d6a95434..e22d94a6 100644
--- a/cmd-send-keys.c
+++ b/cmd-send-keys.c
@@ -151,7 +151,8 @@ cmd_send_keys_exec(struct cmd *self, struct cmdq_item *item)
 	char				*cause = NULL;
 
 	if (args_has(args, 'N')) {
-		np = args_strtonum(args, 'N', 1, UINT_MAX, &cause);
+		np = args_strtonum_and_expand(args, 'N', 1, UINT_MAX, item,
+			 &cause);
 		if (cause != NULL) {
 			cmdq_error(item, "repeat count %s", cause);
 			free(cause);
diff --git a/cmd-split-window.c b/cmd-split-window.c
index 4c8054ec..18df715e 100644
--- a/cmd-split-window.c
+++ b/cmd-split-window.c
@@ -66,67 +66,46 @@ cmd_split_window_exec(struct cmd *self, struct cmdq_item *item)
 	enum layout_type	 type;
 	struct layout_cell	*lc;
 	struct cmd_find_state	 fs;
-	int			 size, percentage, flags, input;
-	const char		*template, *errstr, *p;
-	char			*cause, *cp, *copy;
-	size_t			 plen;
+	int			 size, flags, input;
+	const char		*template;
+	char			*cause = NULL, *cp;
 	struct args_value	*av;
-	u_int			 count = args_count(args);
+	u_int			 count = args_count(args), curval = 0;
 
+	type = LAYOUT_TOPBOTTOM;
 	if (args_has(args, 'h'))
 		type = LAYOUT_LEFTRIGHT;
-	else
-		type = LAYOUT_TOPBOTTOM;
-	if ((p = args_get(args, 'l')) != NULL) {
-		plen = strlen(p);
-		if (p[plen - 1] == '%') {
-			copy = xstrdup(p);
-			copy[plen - 1] = '\0';
-			percentage = strtonum(copy, 0, INT_MAX, &errstr);
-			free(copy);
-			if (errstr != NULL) {
-				cmdq_error(item, "percentage %s", errstr);
-				return (CMD_RETURN_ERROR);
-			}
-			if (args_has(args, 'f')) {
-				if (type == LAYOUT_TOPBOTTOM)
-					size = (w->sy * percentage) / 100;
-				else
-					size = (w->sx * percentage) / 100;
-			} else {
-				if (type == LAYOUT_TOPBOTTOM)
-					size = (wp->sy * percentage) / 100;
-				else
-					size = (wp->sx * percentage) / 100;
-			}
-		} else {
-			size = args_strtonum(args, 'l', 0, INT_MAX, &cause);
-			if (cause != NULL) {
-				cmdq_error(item, "lines %s", cause);
-				free(cause);
-				return (CMD_RETURN_ERROR);
-			}
-		}
-	} else if (args_has(args, 'p')) {
-		percentage = args_strtonum(args, 'p', 0, INT_MAX, &cause);
-		if (cause != NULL) {
-			cmdq_error(item, "create pane failed: -p %s", cause);
-			free(cause);
-			return (CMD_RETURN_ERROR);
-		}
+
+	/* If the 'p' flag is dropped then this bit can be moved into 'l'. */
+	if (args_has(args, 'l') || args_has(args, 'p')) {
 		if (args_has(args, 'f')) {
 			if (type == LAYOUT_TOPBOTTOM)
-				size = (w->sy * percentage) / 100;
+				curval = w->sy;
 			else
-				size = (w->sx * percentage) / 100;
+				curval = w->sx;
 		} else {
 			if (type == LAYOUT_TOPBOTTOM)
-				size = (wp->sy * percentage) / 100;
+				curval = wp->sy;
 			else
-				size = (wp->sx * percentage) / 100;
+				curval = wp->sx;
 		}
-	} else
-		size = -1;
+	}
+
+	size = -1;
+	if (args_has(args, 'l')) {
+		size = args_percentage_and_expand(args, 'l', 0, INT_MAX, curval,
+			   item, &cause);
+	} else if (args_has(args, 'p')) {
+		size = args_strtonum_and_expand(args, 'l', 0, 100, item,
+			    &cause);
+		if (cause == NULL)
+			size = curval * size / 100;
+	}
+	if (cause != NULL) {
+		cmdq_error(item, "size %s", cause);
+		free(cause);
+		return (CMD_RETURN_ERROR);
+	}
 
 	window_push_zoom(wp->window, 1, args_has(args, 'Z'));
 	input = (args_has(args, 'I') && count == 0);
diff --git a/tmux.h b/tmux.h
index 33b72d0b..811a4220 100644
--- a/tmux.h
+++ b/tmux.h
@@ -2383,10 +2383,16 @@ struct args_value *args_first_value(struct args *, u_char);
 struct args_value *args_next_value(struct args_value *);
 long long	 args_strtonum(struct args *, u_char, long long, long long,
 		     char **);
+long long	 args_strtonum_and_expand(struct args *, u_char, long long,
+		     long long, struct cmdq_item *, char **);
 long long	 args_percentage(struct args *, u_char, long long,
 		     long long, long long, char **);
 long long	 args_string_percentage(const char *, long long, long long,
 		     long long, char **);
+long long	 args_percentage_and_expand(struct args *, u_char, long long,
+		     long long, long long, struct cmdq_item *, char **);
+long long	 args_string_percentage_and_expand(const char *, long long,
+		     long long, long long, struct cmdq_item *, char **);
 
 /* cmd-find.c */
 int		 cmd_find_target(struct cmd_find_state *, struct cmdq_item *,