diff --git a/pdu/pdu.go b/pdu/pdu.go
index 9a51c00..0f4c15b 100644
--- a/pdu/pdu.go
+++ b/pdu/pdu.go
@@ -10,6 +10,7 @@ type (
 		EncodeInto(*[]uint8)
 		Encode() []uint8
 		Decode([]uint8)
+		// Size in bytes
 		Size() uint32
 	}
 
diff --git a/pdu/submit.go b/pdu/submit.go
index a205cfd..8e572ef 100644
--- a/pdu/submit.go
+++ b/pdu/submit.go
@@ -66,5 +66,25 @@ func (p *SUBMIT_SM) EncodeInto(buf *[]byte) {
 func (p *SUBMIT_SM) Decode(data []byte) {
 }
 func (p *SUBMIT_SM) Size() uint32 {
-	return 0
+	var size uint32
+	size += 16 // header
+	size += uint32(len(p.service_type) * 1)
+	size += 1 // source_addr_ton
+	size += 1 // source_addr_npi
+	size += uint32(len(p.source_addr) * 1)
+	size += 1 // dest_addr_ton
+	size += 1 // dest_addr_npi
+	size += uint32(len(p.destination_addr) * 1)
+	size += 1 // esm_class
+	size += 1 // protocol_id
+	size += 1 // priority_flag
+	size += uint32(len(p.schedule_delivery_time) * 1)
+	size += uint32(len(p.validity_period) * 1)
+	size += 1 // registered_delivery
+	size += 1 // replace_if_present
+	size += 1 // data_coding
+	size += 1 // sm_default_msg_id
+	size += 1 // sm_length
+	size += uint32(len(p.short_message) * 1)
+	return size
 }