diff --git a/src/common/compat.h b/src/common/compat.h
index a29a3242b7..6ac958c01f 100644
--- a/src/common/compat.h
+++ b/src/common/compat.h
@@ -10,9 +10,6 @@
 #ifdef _WIN32
 #include <winsock2.h>
 #include <ws2tcpip.h>
-#ifndef SIO_IDEAL_SEND_BACKLOG_QUERY
-#define SIO_IDEAL_SEND_BACKLOG_QUERY 0x4004747b
-#endif
 #endif
 #include "lib/cc/torint.h"
 #include "lib/testsupport/testsupport.h"
diff --git a/src/lib/net/socket.h b/src/lib/net/socket.h
index 99b0de4ff9..cb0ccbe817 100644
--- a/src/lib/net/socket.h
+++ b/src/lib/net/socket.h
@@ -113,4 +113,8 @@ STATIC int tor_ersatz_socketpair(int family, int type, int protocol,
 #endif
 #endif /* defined(COMPAT_PRIVATE) */
 
+#if defined(_WIN32) && !defined(SIO_IDEAL_SEND_BACKLOG_QUERY)
+#define SIO_IDEAL_SEND_BACKLOG_QUERY 0x4004747b
+#endif
+
 #endif