diff --git a/src/lapack_internal.hh b/src/lapack_internal.hh
new file mode 100644
index 00000000..700232d3
--- /dev/null
+++ b/src/lapack_internal.hh
@@ -0,0 +1,36 @@
+// Copyright (c) 2017-2023, University of Tennessee. All rights reserved.
+// SPDX-License-Identifier: BSD-3-Clause
+// This program is free software: you can redistribute it and/or modify it under
+// the terms of the BSD 3-Clause license. See the accompanying LICENSE file.
+
+#ifndef LAPACK_INTERNAL_HH
+#define LAPACK_INTERNAL_HH
+
+#include "lapack/util.hh"
+
+namespace lapack {
+
+//------------------------------------------------------------------------------
+/// @see to_lapack_int
+///
+inline lapack_int to_lapack_int_( int64_t x, const char* x_str )
+{
+    if constexpr (sizeof(int64_t) > sizeof(lapack_int)) {
+        lapack_error_if_msg( x > std::numeric_limits<lapack_int>::max(), "%s", x_str );
+    }
+    return lapack_int( x );
+}
+
+//----------------------------------------
+/// Convert int64_t to lapack_int.
+/// If lapack_int is 64-bit, this does nothing.
+/// If lapack_int is 32-bit, throws if x > INT_MAX, so conversion would overflow.
+///
+/// Note this is in src/lapack_internal.hh, so this macro won't pollute
+/// the namespace when apps #include <lapack.hh>.
+///
+#define to_lapack_int( x ) lapack::to_lapack_int_( x, #x )
+
+}  // namespace lapack
+
+#endif // LAPACK_INTERNAL_HH