diff --git a/src/input_handler.cc b/src/input_handler.cc
index 1dbb63bf..f3f6f710 100644
--- a/src/input_handler.cc
+++ b/src/input_handler.cc
@@ -420,7 +420,7 @@ public:
 
     void insert(Codepoint cp)
     {
-        m_line = m_line.substr(0, m_cursor_pos) + codepoint_to_str(cp)
+        m_line = m_line.substr(0, m_cursor_pos) + String{cp}
                + m_line.substr(m_cursor_pos);
         ++m_cursor_pos;
     }
@@ -1187,7 +1187,7 @@ private:
 
     void insert(Codepoint key)
     {
-        auto str = codepoint_to_str(key);
+        String str{key};
         context().selections().insert(str, InsertMode::InsertCursor);
         context().hooks().run_hook("InsertChar", str, context());
     }
diff --git a/src/keys.cc b/src/keys.cc
index 3e0277e0..f98a5dc0 100644
--- a/src/keys.cc
+++ b/src/keys.cc
@@ -131,7 +131,7 @@ String key_to_str(Key key)
         res = "F" + to_string((int)(key.key - Key::F1 + 1));
     }
     else
-        res = codepoint_to_str(key.key);
+        res = String{key.key};
 
     switch (key.modifiers)
     {
diff --git a/src/string.hh b/src/string.hh
index 5b62bf12..638f30f9 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -102,6 +102,7 @@ public:
     String(const char* content, ByteCount len) : m_data(content, (size_t)(int)len) {}
     explicit String(Codepoint cp, CharCount count = 1)
     {
+        reserve(utf8::codepoint_size(cp) * (int)count);
         while (count-- > 0)
             utf8::dump(std::back_inserter(*this), cp);
     }
@@ -252,13 +253,6 @@ inline String operator"" _str(const char* str, size_t)
     return String(str);
 }
 
-inline String codepoint_to_str(Codepoint cp)
-{
-    String str;
-    utf8::dump(std::back_inserter(str), cp);
-    return str;
-}
-
 int str_to_int(StringView str); // throws on error
 Optional<int> str_to_int_ifp(StringView str);
 
diff --git a/src/utf8.hh b/src/utf8.hh
index f38327eb..648c6011 100644
--- a/src/utf8.hh
+++ b/src/utf8.hh
@@ -178,6 +178,20 @@ ByteCount codepoint_size(char byte)
 
 struct invalid_codepoint{};
 
+inline ByteCount codepoint_size(Codepoint cp)
+{
+    if (cp <= 0x7F)
+        return 1;
+    else if (cp <= 0x7FF)
+        return 2;
+    else if (cp <= 0xFFFF)
+        return 3;
+    else if (cp <= 0x10FFFF)
+        return 4;
+    else
+        throw invalid_codepoint{};
+}
+
 template<typename OutputIterator>
 void dump(OutputIterator&& it, Codepoint cp)
 {