diff --git a/Documentation/btrfs-filesystem.rst b/Documentation/btrfs-filesystem.rst
index 5b4fdbfa..207f7b02 100644
--- a/Documentation/btrfs-filesystem.rst
+++ b/Documentation/btrfs-filesystem.rst
@@ -251,11 +251,20 @@ resize [options] [<devid>:][+/-]<size>[kKmMgGtTpPeE]|[<devid>:]max <path>
         The resize command does not manipulate the size of underlying
         partition.  If you wish to enlarge/reduce a filesystem, you must make sure you
         expand the partition before enlarging the filesystem and shrink the
-        partition after reducing the size of the filesystem.  This can done using
+        partition after reducing the size of the filesystem.  This can be done using
         :manref:`fdisk(8)` or :manref:`parted(8)` to delete the existing partition and recreate
         it with the new desired size.  When recreating the partition make sure to use
         the same starting partition offset as before.
 
+        The size of the portion that the filesystem uses of an underlying device can be
+        determined via the :command:`btrfs filesystem show --raw` command on the
+        filesystem’s mount point (where it’s given for each *devid* after the string
+        `size` or via the :command:`btrfs inspect-internal dump-super` command on the
+        specific device (where it’s given as the value of `dev_item.total_bytes`, which
+        is not to be confused with `total_bytes`).
+        The value is also the address of the first byte not used by the
+        filesystem.
+
         Growing is usually instant as it only updates the size. However, shrinking could
         take a long time if there are data in the device area that's beyond the new
         end. Relocation of the data takes time.