diff --git a/index.html b/index.html
index f7a3ec9..58ad16f 100644
--- a/index.html
+++ b/index.html
@@ -447,7 +447,10 @@ function save() {
 function blockMean(blockId) {
   const graded = S.courses.filter(c => c.blockId === blockId && c.grade != null && c.grade !== '');
   if (!graded.length) return null;
-  return graded.reduce((s, c) => s + +c.grade, 0) / graded.length;
+  const sumWeighted = graded.reduce((s, c) => s + +c.grade * (+c.credits || 0), 0);
+  const sumCoef     = graded.reduce((s, c) => s + (+c.credits || 0), 0);
+  if (sumCoef === 0) return null;
+  return sumWeighted / sumCoef;
 }
 
 // status: 'pass' | 'fail' | 'prog'