diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index f7cd975b69d14..de88d8c8ba18b 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -18,7 +18,7 @@
 #include <ostream>
 #include <type_traits>
 
-#include "ostream.h"  // formatbuf
+#include "format.h"  // formatbuf
 
 FMT_BEGIN_NAMESPACE
 
@@ -329,6 +329,39 @@ inline auto localtime_s(...) -> null<> { return null<>(); }
 inline auto gmtime_r(...) -> null<> { return null<>(); }
 inline auto gmtime_s(...) -> null<> { return null<>(); }
 
+// It is defined here and not in ostream.h because the latter has expensive
+// includes.
+template <typename Streambuf> class formatbuf : public Streambuf {
+ private:
+  using char_type = typename Streambuf::char_type;
+  using streamsize = decltype(std::declval<Streambuf>().sputn(nullptr, 0));
+  using int_type = typename Streambuf::int_type;
+  using traits_type = typename Streambuf::traits_type;
+
+  buffer<char_type>& buffer_;
+
+ public:
+  explicit formatbuf(buffer<char_type>& buf) : buffer_(buf) {}
+
+ protected:
+  // The put area is always empty. This makes the implementation simpler and has
+  // the advantage that the streambuf and the buffer are always in sync and
+  // sputc never writes into uninitialized memory. A disadvantage is that each
+  // call to sputc always results in a (virtual) call to overflow. There is no
+  // disadvantage here for sputn since this always results in a call to xsputn.
+
+  auto overflow(int_type ch) -> int_type override {
+    if (!traits_type::eq_int_type(ch, traits_type::eof()))
+      buffer_.push_back(static_cast<char_type>(ch));
+    return ch;
+  }
+
+  auto xsputn(const char_type* s, streamsize count) -> streamsize override {
+    buffer_.append(s, s + count);
+    return count;
+  }
+};
+
 inline auto get_classic_locale() -> const std::locale& {
   static const auto& locale = std::locale::classic();
   return locale;
diff --git a/include/fmt/ostream.h b/include/fmt/ostream.h
index 2508e58d7d548..f9ba9d7b25b64 100644
--- a/include/fmt/ostream.h
+++ b/include/fmt/ostream.h
@@ -18,42 +18,11 @@
 #  include <io.h>
 #endif
 
-#include "format.h"
+#include "chrono.h"  // formatbuf
 
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
-template <typename Streambuf> class formatbuf : public Streambuf {
- private:
-  using char_type = typename Streambuf::char_type;
-  using streamsize = decltype(std::declval<Streambuf>().sputn(nullptr, 0));
-  using int_type = typename Streambuf::int_type;
-  using traits_type = typename Streambuf::traits_type;
-
-  buffer<char_type>& buffer_;
-
- public:
-  explicit formatbuf(buffer<char_type>& buf) : buffer_(buf) {}
-
- protected:
-  // The put area is always empty. This makes the implementation simpler and has
-  // the advantage that the streambuf and the buffer are always in sync and
-  // sputc never writes into uninitialized memory. A disadvantage is that each
-  // call to sputc always results in a (virtual) call to overflow. There is no
-  // disadvantage here for sputn since this always results in a call to xsputn.
-
-  auto overflow(int_type ch) -> int_type override {
-    if (!traits_type::eq_int_type(ch, traits_type::eof()))
-      buffer_.push_back(static_cast<char_type>(ch));
-    return ch;
-  }
-
-  auto xsputn(const char_type* s, streamsize count) -> streamsize override {
-    buffer_.append(s, s + count);
-    return count;
-  }
-};
-
 // Generate a unique explicit instantion in every translation unit using a tag
 // type in an anonymous namespace.
 namespace {