diff --git a/examples/layer-swipe.js b/examples/layer-swipe.js
index c9e4d0bd65..5d6a20b274 100644
--- a/examples/layer-swipe.js
+++ b/examples/layer-swipe.js
@@ -34,7 +34,8 @@ const swipe = document.getElementById('swipe');
 
 aerial.on('prerender', function(event) {
   const ctx = event.context;
-  const width = ctx.canvas.width * (swipe.value / 100);
+  const mapSize = map.getSize();
+  const width = mapSize[0] * (swipe.value / 100);
   const tl = getRenderPixel(event, [width, 0]);
   const tr = getRenderPixel(event, [mapSize[0], 0]);
   const bl = getRenderPixel(event, [width, mapSize[1]]);