diff --git a/whisper/whisperv6/doc.go b/whisper/whisperv6/doc.go
index 6ff378b23e..64925ba48b 100644
--- a/whisper/whisperv6/doc.go
+++ b/whisper/whisperv6/doc.go
@@ -67,6 +67,8 @@ const (
 
 	DefaultTTL     = 50 // seconds
 	SynchAllowance = 10 // seconds
+
+	EnvelopeHeaderLength = 20
 )
 
 type unknownVersionError uint64
diff --git a/whisper/whisperv6/envelope.go b/whisper/whisperv6/envelope.go
index e7d0f1f858..26d2c2af12 100644
--- a/whisper/whisperv6/envelope.go
+++ b/whisper/whisperv6/envelope.go
@@ -50,7 +50,7 @@ type Envelope struct {
 
 // size returns the size of envelope as it is sent (i.e. public fields only)
 func (e *Envelope) size() int {
-	return 20 + len(e.Version) + len(e.Data)
+	return EnvelopeHeaderLength + len(e.Version) + len(e.Data)
 }
 
 // rlpWithoutNonce returns the RLP encoded envelope contents, except the nonce.