diff --git a/core/types/block.go b/core/types/block.go
index 9ea3c9f128..59c1aa385f 100644
--- a/core/types/block.go
+++ b/core/types/block.go
@@ -19,6 +19,7 @@ package types
 
 import (
 	"encoding/binary"
+	"reflect"
 	"io"
 	"math/big"
 	"sort"
@@ -101,11 +102,12 @@ func (h *Header) Hash() common.Hash {
 	return rlpHash(h)
 }
 
+var headerSize = common.StorageSize(reflect.TypeOf(Header{}).Size())
+
 // Size returns the approximate memory used by all internal contents. It is used
 // to approximate and limit the memory consumption of various caches.
 func (h *Header) Size() common.StorageSize {
-	size := binary.Size(h)
-	return common.StorageSize(size) + common.StorageSize(len(h.Extra)+(h.Difficulty.BitLen()+h.Number.BitLen()+h.Time.BitLen())/8)
+	return headerSize + common.StorageSize(len(h.Extra)+(h.Difficulty.BitLen()+h.Number.BitLen()+h.Time.BitLen())/8)
 }
 
 func rlpHash(x interface{}) (h common.Hash) {