diff --git a/src/calibre/srv/metadata.py b/src/calibre/srv/metadata.py
index 23eb18e9d8..569071cdfe 100644
--- a/src/calibre/srv/metadata.py
+++ b/src/calibre/srv/metadata.py
@@ -66,7 +66,15 @@ def add_field(field, db, book_id, ans, field_metadata):
 def book_as_json(db, book_id):
     db = db.new_api
     with db.safe_read_lock:
-        ans = {'formats':db._formats(book_id)}
+        fmts = db._formats(book_id, verify_formats=False)
+        ans = []
+        fm = {}
+        for fmt in fmts:
+            m = db.format_metadata(book_id, fmt)
+            if m and m.get('size', 0) > 0:
+                ans.append(fmt)
+                fm[fmt] = m['size']
+        ans = {'formats': ans, 'format_sizes': fm}
         if not ans['formats'] and not db.has_id(book_id):
             return None
         fm = db.field_metadata
diff --git a/src/pyj/book_list/book_details.pyj b/src/pyj/book_list/book_details.pyj
index 915465df0e..a6876b6160 100644
--- a/src/pyj/book_list/book_details.pyj
+++ b/src/pyj/book_list/book_details.pyj
@@ -7,13 +7,13 @@ from elementmaker import E
 from gettext import gettext as _
 
 from ajax import ajax, encode_query_component
+from book_list.globals import get_session_data
 from book_list.item_list import create_item, create_item_list
 from book_list.library_data import (
     book_metadata, cover_url, current_library_id, current_virtual_library,
     download_url, library_data, load_status, set_book_metadata
 )
 from book_list.router import back, home, open_book
-from book_list.globals import get_session_data
 from book_list.theme import get_color, get_font_size
 from book_list.top_bar import add_button, create_top_bar, set_title
 from book_list.ui import query_as_href, set_panel_handler, show_panel
@@ -23,8 +23,8 @@ from dom import add_extra_css, build_rule, clear, svgicon
 from modals import create_custom_dialog, error_dialog
 from session import get_interface_data
 from utils import (
-    conditional_timeout, debounce, fmt_sidx, parse_url_params, safe_set_inner_html,
-    sandboxed_html
+    conditional_timeout, debounce, fmt_sidx, human_readable, parse_url_params,
+    safe_set_inner_html, sandboxed_html
 )
 from widgets import create_button, create_spinner
 
@@ -74,7 +74,7 @@ def href_for_search(name, val):
 def on_fmt_click(ev):
     fmt = ev.currentTarget.dataset.format
     book_id = int(ev.currentTarget.dataset.bookId)
-    title = this
+    title, sz = this
 
     create_custom_dialog(title, def(parent, close_modal):
 
@@ -87,7 +87,8 @@ def on_fmt_click(ev):
             E.div(class_='button-box',
                 create_button(_('Read'), 'book', action.bind(None, read_format)),
                 '\xa0',
-                create_button(_('Download'), 'cloud-download', action.bind(None, download_format)),
+                create_button(_('Download'), 'cloud-download', action.bind(None, download_format),
+                              tooltip=_('File size: {}').format(human_readable(sz))),
             )
         ))
     )
@@ -194,7 +195,8 @@ def render_metadata(mi, table, book_id):  # {{{
                 td.appendChild(E.a(
                     fmt, class_='simple-link', href='javascript:void(0)',
                     title=_('Read or download this book in the {} format').format(fmt),
-                    onclick=on_fmt_click.bind(mi.title), data_format=fmt, data_book_id='' + book_id))
+                    onclick=on_fmt_click.bind(v'[mi.title, mi.format_sizes[fmt] || 0]'),
+                    data_format=fmt, data_book_id='' + book_id))
                 if fmt is not val[-1]:
                     td.appendChild(document.createTextNode(', '))
 
@@ -424,7 +426,8 @@ def render_book(container_id, book_id):
     read_button = create_button(_('Read'), 'book', read_book.bind(None, book_id), _('Read this book'))
     fmt = preferred_format(book_id)
     download_button = create_button(_('Download'), 'cloud-download', download_url(book_id, fmt),
-        _('Download this book in the {} format').format(fmt), download_filename=f'{metadata.title}.{fmt.toLowerCase()}')
+        _('Download this book in the {0} format ({1})').format(fmt, human_readable(metadata.format_sizes[fmt] or 0)),
+        download_filename=f'{metadata.title}.{fmt.toLowerCase()}')
     row = E.div(read_button, '\xa0\xa0\xa0', download_button, style='margin-bottom: 1ex')
     if not metadata.formats or not metadata.formats.length:
         row.style.display = 'none'