diff --git a/scripts/backup b/scripts/backup
index 5e07748..a39b788 100755
--- a/scripts/backup
+++ b/scripts/backup
@@ -53,7 +53,11 @@ my @backup_dirs = map {
 			my($bsize, $frsize, $blocks, $bfree, $bavail,
 			   $files, $ffree, $favail, $flag, $namemax)
 			    = statvfs($_);
-			[ $_, $bsize * $bavail ]
+			my $available_bytes = $bsize * $bavail;
+			my $avg_file_size = $bavail * ($blocks - $bfree) / ($files - $ffree);
+			my $available_bytes_by_files = $avg_file_size * $favail;
+			$available_bytes = $available_bytes_by_files if $available_bytes_by_files < $available_bytes;
+			[ $_, $available_bytes ]
                       } glob("/backup/*/active");
 my $sum_free = 0;
 $sum_free += $_->[1] for (@backup_dirs);
@@ -62,8 +66,10 @@ my $count_free = 0;
 my $backup_dir;
 for(@backup_dirs) {
     $count_free += $_->[1];
+    $ca->log(3, "considering base $_->[0] (est. $_->[1] bytes)\n");
     if ($count_free >= $rnd) {
 	$backup_dir = $_->[0];
+	$ca->log(3, "using base $_->[0]\n");
 	last;
     }
 }