diff --git a/event.c b/event.c
index 4fb13810e..f84b75745 100644
--- a/event.c
+++ b/event.c
@@ -842,10 +842,14 @@ event_handle_randr_screen_change_notify(xcb_randr_screen_change_notify_event_t *
     /* Do (part of) what XRRUpdateConfiguration() would do (update our state) */
     if (ev->rotation & (XCB_RANDR_ROTATION_ROTATE_90 | XCB_RANDR_ROTATION_ROTATE_270)) {
         globalconf.screen->width_in_pixels = ev->height;
+        globalconf.screen->width_in_millimeters = ev->mheight;
         globalconf.screen->height_in_pixels = ev->width;
+        globalconf.screen->height_in_millimeters = ev->mwidth;
     } else {
         globalconf.screen->width_in_pixels = ev->width;
+        globalconf.screen->width_in_millimeters = ev->mwidth;
         globalconf.screen->height_in_pixels = ev->height;
+        globalconf.screen->height_in_millimeters = ev->mheight;;
     }
 
     globalconf.screen_need_refresh = true;
diff --git a/lib/beautiful/xresources.lua b/lib/beautiful/xresources.lua
index 58c48d109..085fd082d 100644
--- a/lib/beautiful/xresources.lua
+++ b/lib/beautiful/xresources.lua
@@ -83,6 +83,22 @@ function xresources.get_dpi(s)
         if awesome and awesome.xrdb_get_value then
             xresources.dpi = tonumber(awesome.xrdb_get_value("", "Xft.dpi"))
         end
+        -- Following Keith Packard's whitepaper on Xft,
+        -- https://keithp.com/~keithp/talks/xtc2001/paper/xft.html#sec-editing
+        -- the proper fallback for Xft.dpi is the vertical DPI reported by
+        -- the X server. This will generally be 96 on Xorg, unless the user
+        -- has configured it differently
+        if not xresources.dpi then
+            if root then
+                local mm_to_inch = 25.4
+                _, h = root.size()
+                _, hmm = root.size_mm()
+                if hmm ~= 0 then
+                    xresources.dpi = round(h*mm_to_inch/hmm)
+                end
+            end
+        end
+        -- ultimate fallback
         if not xresources.dpi then
             xresources.dpi = 96
         end
diff --git a/root.c b/root.c
index 52c149bc0..7ae953ffb 100644
--- a/root.c
+++ b/root.c
@@ -448,6 +448,20 @@ luaA_root_size(lua_State *L)
     return 2;
 }
 
+/** Get the physical size of the root window, in millimeter.
+ *
+ * @return Width of the root window, in millimeters.
+ * @return height of the root window, in millimeters.
+ * @function size_mm
+ */
+static int
+luaA_root_size_mm(lua_State *L)
+{
+    lua_pushinteger(L, globalconf.screen->width_in_millimeters);
+    lua_pushinteger(L, globalconf.screen->height_in_millimeters);
+    return 2;
+}
+
 /** Get the attached tags.
  * @return A table with all tags.
  * @function tags
@@ -474,6 +488,7 @@ const struct luaL_Reg awesome_root_lib[] =
     { "drawins", luaA_root_drawins },
     { "wallpaper", luaA_root_wallpaper },
     { "size", luaA_root_size },
+    { "size_mm", luaA_root_size_mm },
     { "tags", luaA_root_tags },
     { "__index", luaA_default_index },
     { "__newindex", luaA_default_newindex },