diff --git a/ChangeLog b/ChangeLog
index 869fd9535e..3b264b63e2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Jun 16 12:17:52 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (tr_trans): adjust buffer size by processed and rest
+	  lengths, instead of doubling repeatedly.
+
 Thu Jun 16 11:15:25 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* string.c (tr_trans): consider terminator length and fix heap
diff --git a/string.c b/string.c
index 7cb6af2727..73e742696a 100644
--- a/string.c
+++ b/string.c
@@ -6342,9 +6342,8 @@ tr_trans(VALUE str, VALUE src, VALUE repl, int sflag)
 		c = c0;
 		if (enc != e1) may_modify = 1;
 	    }
-	    while (t - buf + tlen >= max) {
-		offset = t - buf;
-		max *= 2;
+	    if ((offset = t - buf) + tlen > max) {
+		max = offset + tlen + (send - s);
 		REALLOC_N(buf, char, max + termlen);
 		t = buf + offset;
 	    }
@@ -6415,9 +6414,8 @@ tr_trans(VALUE str, VALUE src, VALUE repl, int sflag)
 		c = c0;
 		if (enc != e1) may_modify = 1;
 	    }
-	    while (t - buf + tlen >= max) {
-		offset = t - buf;
-		max *= 2;
+	    if ((offset = t - buf) + tlen > max) {
+		max = offset + tlen + (long)((send - s) * 1.2);
 		REALLOC_N(buf, char, max + termlen);
 		t = buf + offset;
 	    }