diff --git a/fmt/format.h b/fmt/format.h
index a42e29f8..788ec7cf 100644
--- a/fmt/format.h
+++ b/fmt/format.h
@@ -2832,7 +2832,8 @@ void BasicWriter<Char>::write_int(T value, Spec spec) {
   case 'n': {
     unsigned num_digits = internal::count_digits(abs_value);
     fmt::StringRef sep = std::localeconv()->thousands_sep;
-    std::size_t size = num_digits + sep.size() * (num_digits - 1) / 3;
+    unsigned size = static_cast<unsigned>(
+          num_digits + sep.size() * (num_digits - 1) / 3);
     CharPtr p = prepare_int_buffer(size, spec, prefix, prefix_size) + 1;
     internal::format_decimal(get(p), abs_value, 0, internal::ThousandsSep(sep));
     break;