diff --git a/archiver.go b/archiver.go
index 7e68f30d..f89410e2 100644
--- a/archiver.go
+++ b/archiver.go
@@ -264,6 +264,26 @@ func isSymlink(info fs.FileInfo) bool {
 	return info.Mode()&os.ModeSymlink != 0
 }
 
+// streamSizeBySeeking determines the size of the stream by
+// seeking to the end, then back again, so the resulting
+// seek position upon returning is the same as when called
+// (assuming no errors).
+func streamSizeBySeeking(s io.Seeker) (int64, error) {
+	currentPosition, err := s.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return 0, fmt.Errorf("getting current offset: %w", err)
+	}
+	maxPosition, err := s.Seek(0, io.SeekEnd)
+	if err != nil {
+		return 0, fmt.Errorf("fast-forwarding to end: %w", err)
+	}
+	_, err = s.Seek(currentPosition, io.SeekStart)
+	if err != nil {
+		return 0, fmt.Errorf("returning to prior offset %d: %w", currentPosition, err)
+	}
+	return maxPosition, nil
+}
+
 // skipList keeps a list of non-intersecting paths
 // as long as its add method is used. Identical
 // elements are rejected, more specific paths are
diff --git a/fs.go b/fs.go
index 560727b5..a2c7510a 100644
--- a/fs.go
+++ b/fs.go
@@ -98,14 +98,10 @@ func FileSystem(ctx context.Context, filename string, stream ReaderAtSeeker) (fs
 
 		// determine size -- we know that the stream value we get back from
 		// Identify is the same type as what we input because it is a Seeker
-		size, err := stream.Seek(0, io.SeekEnd)
+		size, err := streamSizeBySeeking(stream)
 		if err != nil {
 			return nil, fmt.Errorf("seeking for size: %w", err)
 		}
-		_, err = stream.Seek(0, io.SeekStart)
-		if err != nil {
-			return nil, fmt.Errorf("seeking back to beginning: %w", err)
-		}
 
 		sr := io.NewSectionReader(stream, 0, size)
 
diff --git a/zip.go b/zip.go
index c012c080..73361d8b 100644
--- a/zip.go
+++ b/zip.go
@@ -337,22 +337,6 @@ type seekReaderAt interface {
 	io.Seeker
 }
 
-func streamSizeBySeeking(s io.Seeker) (int64, error) {
-	currentPosition, err := s.Seek(0, io.SeekCurrent)
-	if err != nil {
-		return 0, fmt.Errorf("getting current offset: %w", err)
-	}
-	maxPosition, err := s.Seek(0, io.SeekEnd)
-	if err != nil {
-		return 0, fmt.Errorf("fast-forwarding to end: %w", err)
-	}
-	_, err = s.Seek(currentPosition, io.SeekStart)
-	if err != nil {
-		return 0, fmt.Errorf("returning to prior offset %d: %w", currentPosition, err)
-	}
-	return maxPosition, nil
-}
-
 // Additional compression methods not offered by archive/zip.
 // See https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT section 4.4.5.
 const (