diff --git a/benchmark/time_strftime.yml b/benchmark/time_strftime.yml
new file mode 100644
index 0000000000..28f62aec87
--- /dev/null
+++ b/benchmark/time_strftime.yml
@@ -0,0 +1,7 @@
+prelude: |
+  # frozen_string_literal: true
+  time = Time.now
+benchmark:
+  - time.strftime("%FT%T")     # 19B
+  - time.strftime("%FT%T.%3N") # 23B
+  - time.strftime("%FT%T.%6N") # 26B
diff --git a/strftime.c b/strftime.c
index 33e7d3fdb8..906e8360a9 100644
--- a/strftime.c
+++ b/strftime.c
@@ -171,7 +171,9 @@ resize_buffer(VALUE ftime, char *s, const char **start, const char **endp,
 	      ptrdiff_t n, size_t maxsize)
 {
 	size_t len = s - *start;
-	size_t nlen = len + n * 2;
+	size_t need = len + n * 2;
+	size_t nlen = rb_str_capacity(ftime);
+	while (nlen < need) nlen <<= 1;
 
 	if (nlen < len || nlen > maxsize) {
 		return 0;