diff --git a/libstdc++-v3/include/bits/fstream.tcc b/libstdc++-v3/include/bits/fstream.tcc
index 7ccc887b882..2e936962837 100644
--- a/libstdc++-v3/include/bits/fstream.tcc
+++ b/libstdc++-v3/include/bits/fstream.tcc
@@ -757,23 +757,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       streamsize __ret = 0;
       // Optimization in the always_noconv() case, to be generalized in the
-      // future: when __n is sufficiently large we write directly instead of
-      // using the buffer.
+      // future: when __n is larger than the available capacity we write
+      // directly instead of using the buffer.
       const bool __testout = (_M_mode & ios_base::out
 			      || _M_mode & ios_base::app);
       if (__check_facet(_M_codecvt).always_noconv()
 	  && __testout && !_M_reading)
 	{
-	  // Measurement would reveal the best choice.
-	  const streamsize __chunk = 1ul << 10;
 	  streamsize __bufavail = this->epptr() - this->pptr();
 
 	  // Don't mistake 'uncommitted' mode buffered with unbuffered.
 	  if (!_M_writing && _M_buf_size > 1)
 	    __bufavail = _M_buf_size - 1;
 
-	  const streamsize __limit = std::min(__chunk, __bufavail);
-	  if (__n >= __limit)
+	  if (__n >= __bufavail)
 	    {
 	      const streamsize __buffill = this->pptr() - this->pbase();
 	      const char* __buf = reinterpret_cast<const char*>(this->pbase());
diff --git a/libstdc++-v3/testsuite/27_io/basic_filebuf/sputn/char/63746.cc b/libstdc++-v3/testsuite/27_io/basic_filebuf/sputn/char/63746.cc
new file mode 100644
index 00000000000..baab9340734
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_filebuf/sputn/char/63746.cc
@@ -0,0 +1,38 @@
+// { dg-require-fileio "" }
+
+#include <fstream>
+#include <testsuite_hooks.h>
+
+class testbuf : public std::filebuf {
+public:
+  char_type* pub_pprt() const
+  {
+    return this->pptr();
+  }
+
+  char_type* pub_pbase() const
+  {
+    return this->pbase();
+  }
+};
+
+void test01()
+{
+  using namespace std;
+
+  // Leave capacity to avoid flush.
+  const streamsize chunk_size = BUFSIZ - 1 - 1;
+  const char data[chunk_size] = {};
+
+  testbuf a_f;
+  VERIFY( a_f.open("tmp_63746_sputn", ios_base::out) );
+  VERIFY( chunk_size == a_f.sputn(data, chunk_size) );
+  VERIFY( (a_f.pub_pprt() - a_f.pub_pbase()) == chunk_size );
+  VERIFY( a_f.close() );
+}
+
+int main()
+{
+  test01();
+  return 0;
+}