diff --git a/server/src/main/java/org/elasticsearch/index/codec/vectors/cluster/HierarchicalKMeans.java b/server/src/main/java/org/elasticsearch/index/codec/vectors/cluster/HierarchicalKMeans.java
index 34d802e5f0aaf..43643e9a002d9 100644
--- a/server/src/main/java/org/elasticsearch/index/codec/vectors/cluster/HierarchicalKMeans.java
+++ b/server/src/main/java/org/elasticsearch/index/codec/vectors/cluster/HierarchicalKMeans.java
@@ -95,13 +95,11 @@ KMeansIntermediate clusterAndSplit(final FloatVectorValues vectors, final int ta
         // TODO: consider adding cluster size counts to the kmeans algo
         // handle assignment here so we can track distance and cluster size
         int[] centroidVectorCount = new int[centroids.length];
+        int effectiveK = 0;
         for (int assigment : assignments) {
             centroidVectorCount[assigment]++;
-        }
-
-        int effectiveK = 0;
-        for (int j : centroidVectorCount) {
-            if (j > 0) {
+            // this cluster has received an assignment, its now effective, but only count it once
+            if (centroidVectorCount[assigment] == 1) {
                 effectiveK++;
             }
         }