diff --git a/src/process-in-chunks.test.ts b/src/process-in-chunks.test.ts
index bd9f8a8..93df451 100644
--- a/src/process-in-chunks.test.ts
+++ b/src/process-in-chunks.test.ts
@@ -208,6 +208,22 @@ describe("processInChunks", () => {
 
 describe("processInChunksByChunk", () => {
   describe("core behavior", () => {
+    it("passes correct chunk index to callback", async () => {
+      const items = [1, 2, 3, 4, 5, 6];
+      const receivedIndices: number[] = [];
+
+      await processInChunksByChunk(
+        items,
+        async (chunk, index) => {
+          receivedIndices.push(index);
+          return chunk.length;
+        },
+        { chunkSize: 2 },
+      );
+
+      expect(receivedIndices).toEqual([0, 1, 2]);
+    });
+
     it("passes entire chunk array to callback", async () => {
       const items = [1, 2, 3, 4, 5, 6];
       const receivedChunks: number[][] = [];
diff --git a/src/process-in-chunks.ts b/src/process-in-chunks.ts
index 850db28..6e90b8d 100644
--- a/src/process-in-chunks.ts
+++ b/src/process-in-chunks.ts
@@ -138,13 +138,11 @@ export async function processInChunksByChunk<T, R>(
   const errorMessagesSet = new Set<string>();
   const results: (R | undefined)[] = [];
 
-  let overallIndex = 0;
-
   for (const [index, items] of chunks.entries()) {
     logIfVerbose(`Processing chunk ${index + 1}/${chunks.length}`);
 
     try {
-      const processPromise = processFn(items, overallIndex);
+      const processPromise = processFn(items, index);
 
       /** Run throttle wait in parallel with processing if throttling is enabled */
       const result = await (throttleSeconds > 0
@@ -154,14 +152,12 @@ export async function processInChunksByChunk<T, R>(
         : processPromise);
 
       results.push(result);
-      overallIndex += items.length;
     } catch (err) {
       if (!noThrow) {
         throw err; // Rethrow original error immediately
       }
       errorMessagesSet.add(getErrorMessage(err));
       results.push(undefined);
-      overallIndex += items.length;
     }
   }