diff --git a/libweston/compositor.c b/libweston/compositor.c
index d806ebdc4..5ffecf23e 100644
--- a/libweston/compositor.c
+++ b/libweston/compositor.c
@@ -2392,34 +2392,6 @@ fixed_round_up_to_int(wl_fixed_t f)
 	return wl_fixed_to_int(wl_fixed_from_int(1) - 1 + f);
 }
 
-WESTON_EXPORT_FOR_TESTS void
-convert_size_by_transform_scale(int32_t *width_out, int32_t *height_out,
-				int32_t width, int32_t height,
-				uint32_t transform,
-				int32_t scale)
-{
-	assert(scale > 0);
-
-	switch (transform) {
-	case WL_OUTPUT_TRANSFORM_NORMAL:
-	case WL_OUTPUT_TRANSFORM_180:
-	case WL_OUTPUT_TRANSFORM_FLIPPED:
-	case WL_OUTPUT_TRANSFORM_FLIPPED_180:
-		*width_out = width / scale;
-		*height_out = height / scale;
-		break;
-	case WL_OUTPUT_TRANSFORM_90:
-	case WL_OUTPUT_TRANSFORM_270:
-	case WL_OUTPUT_TRANSFORM_FLIPPED_90:
-	case WL_OUTPUT_TRANSFORM_FLIPPED_270:
-		*width_out = height / scale;
-		*height_out = width / scale;
-		break;
-	default:
-		assert(0 && "invalid transform");
-	}
-}
-
 void
 weston_surface_update_size(struct weston_surface *surface)
 {
diff --git a/libweston/libweston-internal.h b/libweston/libweston-internal.h
index a1093aaac..e14c6fbf1 100644
--- a/libweston/libweston-internal.h
+++ b/libweston/libweston-internal.h
@@ -777,11 +777,33 @@ weston_surface_build_buffer_matrix(const struct weston_surface *surface,
 void
 weston_output_update_matrix(struct weston_output *output);
 
-void
+static inline void
 convert_size_by_transform_scale(int32_t *width_out, int32_t *height_out,
 				int32_t width, int32_t height,
 				uint32_t transform,
-				int32_t scale);
+				int32_t scale)
+{
+	assert(scale > 0);
+
+	switch (transform) {
+	case WL_OUTPUT_TRANSFORM_NORMAL:
+	case WL_OUTPUT_TRANSFORM_180:
+	case WL_OUTPUT_TRANSFORM_FLIPPED:
+	case WL_OUTPUT_TRANSFORM_FLIPPED_180:
+		*width_out = width / scale;
+		*height_out = height / scale;
+		break;
+	case WL_OUTPUT_TRANSFORM_90:
+	case WL_OUTPUT_TRANSFORM_270:
+	case WL_OUTPUT_TRANSFORM_FLIPPED_90:
+	case WL_OUTPUT_TRANSFORM_FLIPPED_270:
+		*width_out = height / scale;
+		*height_out = width / scale;
+		break;
+	default:
+		assert(0 && "invalid transform");
+	}
+}
 
 /* User authentication for remote backends */