diff --git a/src/content/tutorials/en/optimizing-webgl-sketches.mdx b/src/content/tutorials/en/optimizing-webgl-sketches.mdx
index 4ac1aafd50..08877c0cac 100644
--- a/src/content/tutorials/en/optimizing-webgl-sketches.mdx
+++ b/src/content/tutorials/en/optimizing-webgl-sketches.mdx
@@ -457,7 +457,7 @@ function draw() {
   loadPixels();
   for (let x = 0; x < width; x += 1) {
     for (let y = 0; y < height; y += 1) {
-      let idx = (y * height + x) * 4;
+      let idx = (y * width + x) * 4;
       let newValue;
       if (pixels[idx] + noise(x, y)*50 >= 220) {
         newValue = 255;