diff --git a/source.list b/source.list
index db128d271b..45c14de4ce 100644
--- a/source.list
+++ b/source.list
@@ -393,6 +393,7 @@ widget_type.h
 os/windows/win32.h
 music/win32_m.h
 sound/win32_s.h
+unit_conversion.h
 video/win32_v.h
 window_func.h
 window_gui.h
diff --git a/src/strings.cpp b/src/strings.cpp
index d3b04b9afb..77af0a4ab8 100644
--- a/src/strings.cpp
+++ b/src/strings.cpp
@@ -34,6 +34,7 @@
 #include "smallmap_gui.h"
 #include "window_func.h"
 #include "debug.h"
+#include "unit_conversion.h"
 #include "game/game_text.hpp"
 #include "network/network_content_gui.h"
 #include <stack>
diff --git a/src/tracerestrict_gui.cpp b/src/tracerestrict_gui.cpp
index 5e5e5e902a..f1dce0f2e2 100644
--- a/src/tracerestrict_gui.cpp
+++ b/src/tracerestrict_gui.cpp
@@ -36,17 +36,9 @@
 #include "cargotype.h"
 #include "sortlist_type.h"
 #include "group.h"
+#include "unit_conversion.h"
 #include "table/sprites.h"
 
-extern uint ConvertSpeedToDisplaySpeed(uint speed);
-extern uint ConvertDisplaySpeedToSpeed(uint speed);
-extern uint ConvertWeightToDisplayWeight(uint weight);
-extern uint ConvertDisplayWeightToWeight(uint weight);
-extern uint ConvertPowerToDisplayPower(uint power);
-extern uint ConvertDisplayPowerToPower(uint power);
-extern uint ConvertForceToDisplayForce(uint force);
-extern uint ConvertDisplayForceToForce(uint force);
-
 /** Widget IDs */
 enum TraceRestrictWindowWidgets {
 	TR_WIDGET_CAPTION,
diff --git a/src/unit_conversion.h b/src/unit_conversion.h
new file mode 100644
index 0000000000..9dadc35872
--- /dev/null
+++ b/src/unit_conversion.h
@@ -0,0 +1,19 @@
+/* $Id$ */
+
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file unit_conversion.h Functions related to unit conversion. */
+
+uint ConvertSpeedToDisplaySpeed(uint speed);
+uint ConvertDisplaySpeedToSpeed(uint speed);
+uint ConvertWeightToDisplayWeight(uint weight);
+uint ConvertDisplayWeightToWeight(uint weight);
+uint ConvertPowerToDisplayPower(uint power);
+uint ConvertDisplayPowerToPower(uint power);
+uint ConvertForceToDisplayForce(uint force);
+uint ConvertDisplayForceToForce(uint force);