diff --git a/fuzz_test.go b/fuzz_test.go
index 272d491..f797fd1 100644
--- a/fuzz_test.go
+++ b/fuzz_test.go
@@ -408,7 +408,7 @@ func TestCrashers_OCF_e2e(t *testing.T) {
 			if err != nil {
 				t.Fatalf("GOT: %v; WANT: %v", err, nil)
 			}
-			if err := ocfw.Append(datums); err != nil {
+			if _, err := ocfw.Append(datums); err != nil {
 				t.Fatalf("GOT: %v; WANT: %v", err, nil)
 			}
 		})
diff --git a/ocf_test.go b/ocf_test.go
index 3d4d4bd..1e47b98 100644
--- a/ocf_test.go
+++ b/ocf_test.go
@@ -39,7 +39,7 @@ func testOCFRoundTripWithHeaders(t *testing.T, compressionName string, headers m
 
 	valuesToWrite := []int64{13, 42, -12, -1234}
 
-	if err = ocfw.Append(valuesToWrite); err != nil {
+	if _, err = ocfw.Append(valuesToWrite); err != nil {
 		t.Fatal(err)
 	}
 
diff --git a/ocf_writer.go b/ocf_writer.go
index 820af5c..c72a90c 100644
--- a/ocf_writer.go
+++ b/ocf_writer.go
@@ -158,30 +158,30 @@ func (ocfw *OCFWriter) quickScanToTail(ior io.Reader) error {
 // more data items in the slice than MaxBlockCount allows, the data slice will
 // be chunked into multiple blocks, each not having more than MaxBlockCount
 // items.
-func (ocfw *OCFWriter) Append(data interface{}) error {
+func (ocfw *OCFWriter) Append(data interface{}) (int, error) {
 	arrayValues, err := convertArray(data)
 	if err != nil {
-		return err
+		return 0, err
 	}
 
 	// Chunk data so no block has more than MaxBlockCount items.
 	for int64(len(arrayValues)) > MaxBlockCount {
-		if err := ocfw.appendDataIntoBlock(arrayValues[:MaxBlockCount]); err != nil {
-			return err
+		if _, err := ocfw.appendDataIntoBlock(arrayValues[:MaxBlockCount]); err != nil {
+			return 0, err
 		}
 		arrayValues = arrayValues[MaxBlockCount:]
 	}
 	return ocfw.appendDataIntoBlock(arrayValues)
 }
 
-func (ocfw *OCFWriter) appendDataIntoBlock(data []interface{}) error {
+func (ocfw *OCFWriter) appendDataIntoBlock(data []interface{}) (int, error) {
 	var block []byte // working buffer for encoding data values
 	var err error
 
 	// Encode and concatenate each data item into the block
 	for _, datum := range data {
 		if block, err = ocfw.header.codec.BinaryFromNative(block, datum); err != nil {
-			return fmt.Errorf("cannot translate datum to binary: %v; %s", datum, err)
+			return 0, fmt.Errorf("cannot translate datum to binary: %v; %s", datum, err)
 		}
 	}
 
@@ -196,10 +196,10 @@ func (ocfw *OCFWriter) appendDataIntoBlock(data []interface{}) error {
 		cw, _ := flate.NewWriter(bb, flate.DefaultCompression)
 		// writing bytes to cw will compress bytes and send to bb.
 		if _, err := cw.Write(block); err != nil {
-			return err
+			return 0, err
 		}
 		if err := cw.Close(); err != nil {
-			return err
+			return 0, err
 		}
 		block = bb.Bytes()
 
@@ -213,7 +213,7 @@ func (ocfw *OCFWriter) appendDataIntoBlock(data []interface{}) error {
 		block = compressed
 
 	default:
-		return fmt.Errorf("should not get here: cannot compress block using unrecognized compression: %d", ocfw.header.compressionID)
+		return 0, fmt.Errorf("should not get here: cannot compress block using unrecognized compression: %d", ocfw.header.compressionID)
 
 	}
 
@@ -224,8 +224,8 @@ func (ocfw *OCFWriter) appendDataIntoBlock(data []interface{}) error {
 	buf = append(buf, block...)                      // serialized objects
 	buf = append(buf, ocfw.header.syncMarker[:]...)  // sync marker
 
-	_, err = ocfw.iow.Write(buf)
-	return err
+	n, err := ocfw.iow.Write(buf)
+	return n, err
 }
 
 // Codec returns the codec used by OCFWriter. This function provided because
diff --git a/ocf_writer_test.go b/ocf_writer_test.go
index 1d334b4..194b466 100644
--- a/ocf_writer_test.go
+++ b/ocf_writer_test.go
@@ -164,7 +164,7 @@ func TestOCFWriterAppendWhenCannotWrite(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	err = ocfw.Append([]interface{}{13, 42})
+	_, err = ocfw.Append([]interface{}{13, 42})
 	ensureError(t, err, testPathname)
 }
 
@@ -186,7 +186,8 @@ func TestOCFWriterAppendSomeItemsToNothing(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if err = ocfw.Append([]interface{}{13, 42}); err != nil {
+	bytesWritten, err := ocfw.Append([]interface{}{13, 42})
+	if err != nil {
 		t.Fatal(err)
 	}
 
@@ -227,6 +228,9 @@ func TestOCFWriterAppendSomeItemsToNothing(t *testing.T) {
 	if actual, expected := values[1], int64(42); actual != expected {
 		t.Errorf("GOT: %v; WANT: %v", actual, expected)
 	}
+	if actual, expected := bytesWritten, 20; actual != expected {
+		t.Errorf("GOT: %v; WANT: %v", actual, expected)
+	}
 }
 
 func TestOCFWriterAppendSomeItemsToSomeItems(t *testing.T) {
@@ -247,7 +251,8 @@ func TestOCFWriterAppendSomeItemsToSomeItems(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if err = ocfw.Append([]interface{}{-10, -100}); err != nil {
+	bytesWritten, err := ocfw.Append([]interface{}{-10, -100})
+	if err != nil {
 		t.Fatal(err)
 	}
 
@@ -294,4 +299,7 @@ func TestOCFWriterAppendSomeItemsToSomeItems(t *testing.T) {
 	if actual, expected := values[3], int64(-100); actual != expected {
 		t.Errorf("GOT: %v; WANT: %v", actual, expected)
 	}
+	if actual, expected := bytesWritten, 21; actual != expected {
+		t.Errorf("GOT: %v; WANT: %v", actual, expected)
+	}
 }