diff --git a/batch_consumer.go b/batch_consumer.go
index e97228a..e949e90 100644
--- a/batch_consumer.go
+++ b/batch_consumer.go
@@ -35,7 +35,7 @@ func newBatchConsumer(cfg *ConsumerConfig) (Consumer, error) {
 		return nil, err
 	}
 
-	messageGroupByteSizeLimit, err := ResolveUnionIntOrStringValue(cfg.BatchConfiguration.MessageGroupByteSizeLimit)
+	messageGroupByteSizeLimit, err := resolveUnionIntOrStringValue(cfg.BatchConfiguration.MessageGroupByteSizeLimit)
 	if err != nil {
 		return nil, err
 	}
@@ -112,14 +112,18 @@ func (b *batchConsumer) startBatch() {
 				return
 			}
 
-			if maximumMessageByteSizeLimit != 0 && messageByteSize+len(msg.message.Value) > maximumMessageByteSizeLimit {
+			msgSize := msg.message.TotalSize()
+
+			// Check if there is an enough byte in batch, if not flush it.
+			if maximumMessageByteSizeLimit != 0 && messageByteSize+msgSize > maximumMessageByteSizeLimit {
 				b.consume(&messages, &commitMessages, &messageByteSize)
 			}
 
 			messages = append(messages, msg.message)
 			commitMessages = append(commitMessages, *msg.kafkaMessage)
-			messageByteSize += len(msg.message.Value)
+			messageByteSize += msgSize
 
+			// Check if there is an enough size in batch, if not flush it.
 			if len(messages) == maximumMessageLimit {
 				b.consume(&messages, &commitMessages, &messageByteSize)
 			}
diff --git a/data_units.go b/data_units.go
index 99f86e9..3b7b7ee 100644
--- a/data_units.go
+++ b/data_units.go
@@ -6,7 +6,7 @@ import (
 	"strings"
 )
 
-func ResolveUnionIntOrStringValue(input any) (int, error) {
+func resolveUnionIntOrStringValue(input any) (int, error) {
 	switch value := input.(type) {
 	case int:
 		return value, nil
diff --git a/data_units_test.go b/data_units_test.go
index 7856095..a9c762e 100644
--- a/data_units_test.go
+++ b/data_units_test.go
@@ -36,7 +36,7 @@ func TestDcp_ResolveConnectionBufferSize(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			if got, _ := ResolveUnionIntOrStringValue(tt.input); got != tt.want {
+			if got, _ := resolveUnionIntOrStringValue(tt.input); got != tt.want {
 				t.Errorf("ResolveConnectionBufferSize() = %v, want %v", got, tt.want)
 			}
 		})
diff --git a/message.go b/message.go
index 58cb114..8f9f09c 100644
--- a/message.go
+++ b/message.go
@@ -39,6 +39,27 @@ type Message struct {
 	ErrDescription string
 }
 
+func (msg *Message) TotalSize() int {
+	return 14 + msg.keySize() + msg.valueSize() + msg.headerSize()
+}
+
+func (msg *Message) headerSize() int {
+	s := 0
+	for _, header := range msg.Headers {
+		s += sizeofString(header.Key)
+		s += len(header.Value)
+	}
+	return s
+}
+
+func (msg *Message) keySize() int {
+	return sizeofBytes(msg.Key)
+}
+
+func (msg *Message) valueSize() int {
+	return 4 + len(msg.Value)
+}
+
 type IncomingMessage struct {
 	kafkaMessage *kafka.Message
 	message      *Message
@@ -157,3 +178,11 @@ func (m *Message) RemoveHeader(header Header) {
 		}
 	}
 }
+
+func sizeofBytes(b []byte) int {
+	return 4 + len(b)
+}
+
+func sizeofString(s string) int {
+	return 2 + len(s)
+}