diff --git a/js/world.mjs b/js/world.mjs
index 0c6a1dc..9d0c719 100644
--- a/js/world.mjs
+++ b/js/world.mjs
@@ -1,7 +1,8 @@
 import { iter_2d, range } from "./utils/range.mjs";
 
-export const BOX_SIZE = 9;
-export const CENTER = Math.floor(BOX_SIZE / 2);
+const BOX_EXTENT_FROM_CENTER = 4;
+export const BOX_SIZE = BOX_EXTENT_FROM_CENTER * 2 + 1;
+export const CENTER = BOX_EXTENT_FROM_CENTER;