diff --git a/cmd/restic/cmd_prune.go b/cmd/restic/cmd_prune.go
index 075edc769..6f3b7db12 100644
--- a/cmd/restic/cmd_prune.go
+++ b/cmd/restic/cmd_prune.go
@@ -8,7 +8,6 @@ import (
 
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
-	"github.com/restic/restic/internal/pack"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
 
@@ -233,7 +232,7 @@ func prune(opts PruneOptions, gopts GlobalOptions, repo restic.Repository, usedB
 	// iterate over all blobs in index to find out which blobs are duplicates
 	for blob := range repo.Index().Each(ctx) {
 		bh := blob.Handle()
-		size := uint64(pack.PackedSizeOfBlob(blob.Length))
+		size := uint64(blob.Length)
 		switch {
 		case usedBlobs.Has(bh): // used blob, move to keepBlobs
 			usedBlobs.Delete(bh)
@@ -261,19 +260,28 @@ func prune(opts PruneOptions, gopts GlobalOptions, repo restic.Repository, usedB
 
 	indexPack := make(map[restic.ID]packInfo)
 
+	// save computed pack header size
+	for pid, hdrSize := range repo.Index().PackSize(ctx, true) {
+		// initialize tpe with NumBlobTypes to indicate it's not set
+		indexPack[pid] = packInfo{tpe: restic.NumBlobTypes, usedSize: uint64(hdrSize)}
+	}
+
 	// iterate over all blobs in index to generate packInfo
 	for blob := range repo.Index().Each(ctx) {
-		ip, ok := indexPack[blob.PackID]
-		if !ok {
-			ip = packInfo{tpe: blob.Type, usedSize: pack.HeaderSize}
+		ip := indexPack[blob.PackID]
+
+		// Set blob type if not yet set
+		if ip.tpe == restic.NumBlobTypes {
+			ip.tpe = blob.Type
 		}
+
 		// mark mixed packs with "Invalid blob type"
 		if ip.tpe != blob.Type {
 			ip.tpe = restic.InvalidBlob
 		}
 
 		bh := blob.Handle()
-		size := uint64(pack.PackedSizeOfBlob(blob.Length))
+		size := uint64(blob.Length)
 		switch {
 		case duplicateBlobs.Has(bh): // duplicate blob
 			ip.usedSize += size
diff --git a/cmd/restic/cmd_rebuild_index.go b/cmd/restic/cmd_rebuild_index.go
index 850c03dd2..e1fdcd7e4 100644
--- a/cmd/restic/cmd_rebuild_index.go
+++ b/cmd/restic/cmd_rebuild_index.go
@@ -1,7 +1,6 @@
 package main
 
 import (
-	"github.com/restic/restic/internal/pack"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
 
@@ -91,17 +90,7 @@ func rebuildIndex(opts RebuildIndexOptions, gopts GlobalOptions, repo *repositor
 		}
 
 		Verbosef("getting pack files to read...\n")
-
-		// Compute size of each pack from index entries
-		packSizeFromIndex := make(map[restic.ID]int64)
-		for blob := range repo.Index().Each(ctx) {
-			size, ok := packSizeFromIndex[blob.PackID]
-			if !ok {
-				size = pack.HeaderSize
-			}
-			// update packSizeFromIndex
-			packSizeFromIndex[blob.PackID] = size + int64(pack.PackedSizeOfBlob(blob.Length))
-		}
+		packSizeFromIndex := repo.Index().PackSize(ctx, false)
 
 		err = repo.List(ctx, restic.PackFile, func(id restic.ID, packSize int64) error {
 			size, ok := packSizeFromIndex[id]
diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index 1e31ba986..133485497 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -178,13 +178,7 @@ func (c *Checker) LoadIndex(ctx context.Context) (hints []error, errs []error) {
 	c.masterIndex.MergeFinalIndexes()
 
 	// compute pack size using index entries
-	for blob := range c.masterIndex.Each(ctx) {
-		size, ok := c.packs[blob.PackID]
-		if !ok {
-			size = pack.HeaderSize
-		}
-		c.packs[blob.PackID] = size + int64(pack.PackedSizeOfBlob(blob.Length))
-	}
+	c.packs = c.masterIndex.PackSize(ctx, false)
 
 	debug.Log("checking for duplicate packs")
 	for packID := range c.packs {
@@ -749,17 +743,17 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID, size int6
 		return errors.Errorf("Pack size does not match, want %v, got %v", size, realSize)
 	}
 
-	blobs, err := pack.List(r.Key(), packfile, size)
+	blobs, hdrSize, err := pack.List(r.Key(), packfile, size)
 	if err != nil {
 		return err
 	}
 
 	var errs []error
 	var buf []byte
-	sizeFromBlobs := int64(pack.HeaderSize) // pack size computed only from blob information
+	sizeFromBlobs := uint(hdrSize)
 	idx := r.Index()
 	for i, blob := range blobs {
-		sizeFromBlobs += int64(pack.PackedSizeOfBlob(blob.Length))
+		sizeFromBlobs += blob.Length
 		debug.Log("  check blob %d: %v", i, blob)
 
 		buf = buf[:cap(buf)]
@@ -809,7 +803,7 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID, size int6
 		}
 	}
 
-	if sizeFromBlobs != size {
+	if int64(sizeFromBlobs) != size {
 		debug.Log("Pack size does not match, want %v, got %v", size, sizeFromBlobs)
 		errs = append(errs, errors.Errorf("Pack size does not match, want %v, got %v", size, sizeFromBlobs))
 	}
diff --git a/internal/pack/pack.go b/internal/pack/pack.go
index 2c39009b1..0c26e6a75 100644
--- a/internal/pack/pack.go
+++ b/internal/pack/pack.go
@@ -46,7 +46,7 @@ func (p *Packer) Add(t restic.BlobType, id restic.ID, data []byte) (int, error)
 	return n, errors.Wrap(err, "Write")
 }
 
-var entrySize = uint(binary.Size(restic.BlobType(0)) + binary.Size(uint32(0)) + len(restic.ID{}))
+var EntrySize = uint(binary.Size(restic.BlobType(0)) + headerLengthSize + len(restic.ID{}))
 
 // headerEntry describes the format of header entries. It serves only as
 // documentation.
@@ -88,7 +88,7 @@ func (p *Packer) Finalize() (uint, error) {
 	bytesWritten += uint(hdrBytes)
 
 	// write length
-	err = binary.Write(p.wr, binary.LittleEndian, uint32(restic.CiphertextLength(len(p.blobs)*int(entrySize))))
+	err = binary.Write(p.wr, binary.LittleEndian, uint32(restic.CiphertextLength(len(p.blobs)*int(EntrySize))))
 	if err != nil {
 		return 0, errors.Wrap(err, "binary.Write")
 	}
@@ -100,7 +100,7 @@ func (p *Packer) Finalize() (uint, error) {
 
 // makeHeader constructs the header for p.
 func (p *Packer) makeHeader() ([]byte, error) {
-	buf := make([]byte, 0, len(p.blobs)*int(entrySize))
+	buf := make([]byte, 0, len(p.blobs)*int(EntrySize))
 
 	for _, b := range p.blobs {
 		switch b.Type {
@@ -151,7 +151,7 @@ func (p *Packer) String() string {
 
 var (
 	// we require at least one entry in the header, and one blob for a pack file
-	minFileSize = entrySize + crypto.Extension + uint(headerLengthSize)
+	minFileSize = EntrySize + crypto.Extension + uint(headerLengthSize)
 )
 
 const (
@@ -171,7 +171,7 @@ const (
 // the appropriate size.
 func readRecords(rd io.ReaderAt, size int64, max int) ([]byte, int, error) {
 	var bufsize int
-	bufsize += max * int(entrySize)
+	bufsize += max * int(EntrySize)
 	bufsize += crypto.Extension
 	bufsize += headerLengthSize
 
@@ -195,7 +195,7 @@ func readRecords(rd io.ReaderAt, size int64, max int) ([]byte, int, error) {
 		err = InvalidFileError{Message: "header length is zero"}
 	case hlen < crypto.Extension:
 		err = InvalidFileError{Message: "header length is too small"}
-	case (hlen-crypto.Extension)%uint32(entrySize) != 0:
+	case (hlen-crypto.Extension)%uint32(EntrySize) != 0:
 		err = InvalidFileError{Message: "header length is invalid"}
 	case int64(hlen) > size-int64(headerLengthSize):
 		err = InvalidFileError{Message: "header is larger than file"}
@@ -206,7 +206,7 @@ func readRecords(rd io.ReaderAt, size int64, max int) ([]byte, int, error) {
 		return nil, 0, errors.Wrap(err, "readHeader")
 	}
 
-	total := (int(hlen) - crypto.Extension) / int(entrySize)
+	total := (int(hlen) - crypto.Extension) / int(EntrySize)
 	if total < max {
 		// truncate to the beginning of the pack header
 		b = b[len(b)-int(hlen):]
@@ -252,52 +252,55 @@ func (e InvalidFileError) Error() string {
 	return e.Message
 }
 
-// List returns the list of entries found in a pack file.
-func List(k *crypto.Key, rd io.ReaderAt, size int64) (entries []restic.Blob, err error) {
+// List returns the list of entries found in a pack file and the length of the
+// header (including header size and crypto overhead)
+func List(k *crypto.Key, rd io.ReaderAt, size int64) (entries []restic.Blob, hdrSize uint32, err error) {
 	buf, err := readHeader(rd, size)
 	if err != nil {
-		return nil, err
+		return nil, 0, err
 	}
 
 	if len(buf) < k.NonceSize()+k.Overhead() {
-		return nil, errors.New("invalid header, too small")
+		return nil, 0, errors.New("invalid header, too small")
 	}
 
+	hdrSize = headerLengthSize + uint32(len(buf))
+
 	nonce, buf := buf[:k.NonceSize()], buf[k.NonceSize():]
 	buf, err = k.Open(buf[:0], nonce, buf, nil)
 	if err != nil {
-		return nil, err
+		return nil, 0, err
 	}
 
-	entries = make([]restic.Blob, 0, uint(len(buf))/entrySize)
+	entries = make([]restic.Blob, 0, uint(len(buf))/EntrySize)
 
 	pos := uint(0)
 	for len(buf) > 0 {
 		entry, err := parseHeaderEntry(buf)
 		if err != nil {
-			return nil, err
+			return nil, 0, err
 		}
 		entry.Offset = pos
 
 		entries = append(entries, entry)
 		pos += entry.Length
-		buf = buf[entrySize:]
+		buf = buf[EntrySize:]
 	}
 
-	return entries, nil
+	return entries, hdrSize, nil
 }
 
 // PackedSizeOfBlob returns the size a blob actually uses when saved in a pack
 func PackedSizeOfBlob(blobLength uint) uint {
-	return blobLength + entrySize
+	return blobLength + EntrySize
 }
 
 func parseHeaderEntry(p []byte) (b restic.Blob, err error) {
-	if uint(len(p)) < entrySize {
+	if uint(len(p)) < EntrySize {
 		err = errors.Errorf("parseHeaderEntry: buffer of size %d too short", len(p))
 		return b, err
 	}
-	p = p[:entrySize]
+	p = p[:EntrySize]
 
 	switch p[0] {
 	case 0:
diff --git a/internal/pack/pack_internal_test.go b/internal/pack/pack_internal_test.go
index 84b03adc2..6502e4a35 100644
--- a/internal/pack/pack_internal_test.go
+++ b/internal/pack/pack_internal_test.go
@@ -41,7 +41,7 @@ func TestParseHeaderEntry(t *testing.T) {
 	buf.Reset()
 	_ = binary.Write(buf, binary.LittleEndian, &h)
 
-	b, err = parseHeaderEntry(buf.Bytes()[:entrySize-1])
+	b, err = parseHeaderEntry(buf.Bytes()[:EntrySize-1])
 	rtest.Assert(t, err != nil, "no error for short input")
 }
 
@@ -58,7 +58,7 @@ func (rd *countingReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 func TestReadHeaderEagerLoad(t *testing.T) {
 
 	testReadHeader := func(dataSize, entryCount, expectedReadInvocationCount int) {
-		expectedHeader := rtest.Random(0, entryCount*int(entrySize)+crypto.Extension)
+		expectedHeader := rtest.Random(0, entryCount*int(EntrySize)+crypto.Extension)
 
 		buf := &bytes.Buffer{}
 		buf.Write(rtest.Random(0, dataSize))                                // pack blobs data
@@ -83,8 +83,8 @@ func TestReadHeaderEagerLoad(t *testing.T) {
 	testReadHeader(100, eagerEntries+1, 2)
 
 	// file size == eager header load size
-	eagerLoadSize := int((eagerEntries * entrySize) + crypto.Extension)
-	headerSize := int(1*entrySize) + crypto.Extension
+	eagerLoadSize := int((eagerEntries * EntrySize) + crypto.Extension)
+	headerSize := int(1*EntrySize) + crypto.Extension
 	dataSize := eagerLoadSize - headerSize - binary.Size(uint32(0))
 	testReadHeader(dataSize-1, 1, 1)
 	testReadHeader(dataSize, 1, 1)
@@ -96,8 +96,8 @@ func TestReadHeaderEagerLoad(t *testing.T) {
 
 func TestReadRecords(t *testing.T) {
 	testReadRecords := func(dataSize, entryCount, totalRecords int) {
-		totalHeader := rtest.Random(0, totalRecords*int(entrySize)+crypto.Extension)
-		off := len(totalHeader) - (entryCount*int(entrySize) + crypto.Extension)
+		totalHeader := rtest.Random(0, totalRecords*int(EntrySize)+crypto.Extension)
+		off := len(totalHeader) - (entryCount*int(EntrySize) + crypto.Extension)
 		if off < 0 {
 			off = 0
 		}
@@ -127,8 +127,8 @@ func TestReadRecords(t *testing.T) {
 	testReadRecords(100, eagerEntries, eagerEntries+1)
 
 	// file size == eager header load size
-	eagerLoadSize := int((eagerEntries * entrySize) + crypto.Extension)
-	headerSize := int(1*entrySize) + crypto.Extension
+	eagerLoadSize := int((eagerEntries * EntrySize) + crypto.Extension)
+	headerSize := int(1*EntrySize) + crypto.Extension
 	dataSize := eagerLoadSize - headerSize - binary.Size(uint32(0))
 	testReadRecords(dataSize-1, 1, 1)
 	testReadRecords(dataSize, 1, 1)
diff --git a/internal/pack/pack_test.go b/internal/pack/pack_test.go
index 99755c36f..471e901c3 100644
--- a/internal/pack/pack_test.go
+++ b/internal/pack/pack_test.go
@@ -53,19 +53,18 @@ func verifyBlobs(t testing.TB, bufs []Buf, k *crypto.Key, rd io.ReaderAt, packSi
 	for _, buf := range bufs {
 		written += len(buf.data)
 	}
-	// header length
-	written += binary.Size(uint32(0))
-	// header + header crypto
-	headerSize := len(bufs) * (binary.Size(restic.BlobType(0)) + binary.Size(uint32(0)) + len(restic.ID{}))
-	written += restic.CiphertextLength(headerSize)
+	// header length + header + header crypto
+	headerSize := binary.Size(uint32(0)) + restic.CiphertextLength(len(bufs)*int(pack.EntrySize))
+	written += headerSize
 
 	// check length
 	rtest.Equals(t, uint(written), packSize)
 
 	// read and parse it again
-	entries, err := pack.List(k, rd, int64(packSize))
+	entries, hdrSize, err := pack.List(k, rd, int64(packSize))
 	rtest.OK(t, err)
 	rtest.Equals(t, len(entries), len(bufs))
+	rtest.Equals(t, headerSize, int(hdrSize))
 
 	var buf []byte
 	for i, b := range bufs {
diff --git a/internal/repository/master_index.go b/internal/repository/master_index.go
index 041bbc669..a23296a7e 100644
--- a/internal/repository/master_index.go
+++ b/internal/repository/master_index.go
@@ -5,6 +5,7 @@ import (
 	"sync"
 
 	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/pack"
 	"github.com/restic/restic/internal/restic"
 	"github.com/restic/restic/internal/ui/progress"
 	"golang.org/x/sync/errgroup"
@@ -111,6 +112,27 @@ func (mi *MasterIndex) Packs() restic.IDSet {
 	return packs
 }
 
+// PackSize returns the size of all packs computed by index information.
+// If onlyHdr is set to true, only the size of the header is returned
+// Note that this function only gives correct sizes, if there are no
+// duplicates in the index.
+func (mi *MasterIndex) PackSize(ctx context.Context, onlyHdr bool) map[restic.ID]int64 {
+	packSize := make(map[restic.ID]int64)
+
+	for blob := range mi.Each(ctx) {
+		size, ok := packSize[blob.PackID]
+		if !ok {
+			size = pack.HeaderSize
+		}
+		if !onlyHdr {
+			size += int64(blob.Length)
+		}
+		packSize[blob.PackID] = size + int64(pack.EntrySize)
+	}
+
+	return packSize
+}
+
 // Count returns the number of blobs of type t in the index.
 func (mi *MasterIndex) Count(t restic.BlobType) (n uint) {
 	mi.idxMutex.RLock()
diff --git a/internal/repository/repack.go b/internal/repository/repack.go
index 9304204e1..423f3c831 100644
--- a/internal/repository/repack.go
+++ b/internal/repository/repack.go
@@ -92,7 +92,7 @@ func Repack(ctx context.Context, repo restic.Repository, packs restic.IDSet, kee
 		for job := range processQueue {
 			tempfile, packID, packLength := job.tempfile, job.hash, job.packLength
 
-			blobs, err := pack.List(repo.Key(), tempfile, packLength)
+			blobs, _, err := pack.List(repo.Key(), tempfile, packLength)
 			if err != nil {
 				return err
 			}
diff --git a/internal/repository/repository.go b/internal/repository/repository.go
index f40ce9097..ea79829c6 100644
--- a/internal/repository/repository.go
+++ b/internal/repository/repository.go
@@ -740,16 +740,11 @@ func (r *Repository) List(ctx context.Context, t restic.FileType, fn func(restic
 }
 
 // ListPack returns the list of blobs saved in the pack id and the length of
-// the file as stored in the backend.
-func (r *Repository) ListPack(ctx context.Context, id restic.ID, size int64) ([]restic.Blob, int64, error) {
+// the the pack header.
+func (r *Repository) ListPack(ctx context.Context, id restic.ID, size int64) ([]restic.Blob, uint32, error) {
 	h := restic.Handle{Type: restic.PackFile, Name: id.String()}
 
-	blobs, err := pack.List(r.Key(), restic.ReaderAt(ctx, r.Backend(), h), size)
-	if err != nil {
-		return nil, 0, err
-	}
-
-	return blobs, size, nil
+	return pack.List(r.Key(), restic.ReaderAt(ctx, r.Backend(), h), size)
 }
 
 // Delete calls backend.Delete() if implemented, and returns an error
diff --git a/internal/restic/repository.go b/internal/restic/repository.go
index 5efdfbc03..b9ba77171 100644
--- a/internal/restic/repository.go
+++ b/internal/restic/repository.go
@@ -32,7 +32,10 @@ type Repository interface {
 	//
 	// The function fn is called in the same Goroutine List() was called from.
 	List(ctx context.Context, t FileType, fn func(ID, int64) error) error
-	ListPack(context.Context, ID, int64) ([]Blob, int64, error)
+
+	// ListPack returns the list of blobs saved in the pack id and the length of
+	// the the pack header.
+	ListPack(context.Context, ID, int64) ([]Blob, uint32, error)
 
 	Flush(context.Context) error
 
@@ -63,6 +66,7 @@ type MasterIndex interface {
 	Lookup(ID, BlobType) []PackedBlob
 	Count(BlobType) uint
 	Packs() IDSet
+	PackSize(ctx context.Context, onlyHdr bool) map[ID]int64
 
 	// Each returns a channel that yields all blobs known to the index. When
 	// the context is cancelled, the background goroutine terminates. This