metainfo/hash.go  | 8 +++++---
 metainfo/piece.go | 2 +-

diff --git a/metainfo/hash.go b/metainfo/hash.go
index d1c832b0bdd50d8b97446ca3b837b1c6428fef31..511647ed730b680ffe58c25ea48db38849fbf3cb 100644
--- a/metainfo/hash.go
+++ b/metainfo/hash.go
@@ -6,8 +6,10 @@ 	"encoding/hex"
 	"fmt"
 )
 
+const HashSize = 20
+
 // 20-byte SHA1 hash used for info and pieces.
-type Hash [20]byte
+type Hash [HashSize]byte
 
 func (h Hash) Bytes() []byte {
 	return h[:]
@@ -26,7 +28,7 @@ 	return fmt.Sprintf("%x", h[:])
 }
 
 func (h *Hash) FromHexString(s string) (err error) {
-	if len(s) != 40 {
+	if len(s) != 2*HashSize {
 		err = fmt.Errorf("hash hex string has bad length: %d", len(s))
 		return
 	}
@@ -34,7 +36,7 @@ 	n, err := hex.Decode(h[:], []byte(s))
 	if err != nil {
 		return
 	}
-	if n != 20 {
+	if n != HashSize {
 		panic(n)
 	}
 	return
diff --git a/metainfo/piece.go b/metainfo/piece.go
index ff753bd3c88d0f83d475fa91e5c354be1f06ae10..55cdc43c143e32d7e1b183e0250e00da076ac437 100644
--- a/metainfo/piece.go
+++ b/metainfo/piece.go
@@ -19,7 +19,7 @@ 	return int64(p.i) * p.Info.PieceLength
 }
 
 func (p Piece) Hash() (ret Hash) {
-	missinggo.CopyExact(&ret, p.Info.Pieces[p.i*20:(p.i+1)*20])
+	missinggo.CopyExact(&ret, p.Info.Pieces[p.i*HashSize:(p.i+1)*HashSize])
 	return
 }