When I need to change the font size I evaluate the following region:

  (set-frame-font "JetBrains Mono-14")

This is far from optimal since it involves a lot of typing.

I'm looking for an interactive function that would ask me for the new font size and modify the numeric value passed to set-frame-font.

As a workaround I wrote the following function:

(defun change-font-size (new-size)
  "Change the font size to the given value"
  (interactive "sNew font size: ")
  (set-frame-font (concat (face-attribute 'default :family) "-" new-size)

(bind-key "C-x f" #'change-font-size)

But I wonder if there's a better way of doing this.

The best solution for me would be to change the font size of emacs based in the monitor in which it is being displayed, but I could not find any satisfactory solution. I do not want to do busy waiting on my emacs, instead I'd like emacs listening to notifications of the windows manager. However I do not know whether this is possible.

EDIT using the suggestion in one of the answers I wrote this hook to resize the window based on the display emacs is in.

(defun my/adjust-font-size-based-on-display ()
  (let ((display-width (nth 3 (assq 'geometry (frame-monitor-attributes))))
     (cond ((<= display-width 1920) 18) ;; HD
           ((<= display-width 2560) 11) ;; UWHD
           ((<= display-width 4096) 14) ;; 4K

(add-hook 'window-size-change-functions (lambda (frame) (my/adjust-font-size-based-on-display)))
(add-hook 'focus-in-hook 'my/adjust-font-size-based-on-display)

Suggestions on how to improve the hook above are welcome!

  • The Emacs default C-x C-+ and/or C-x C-- for increase/decrease font size does not work?
    – Ian
    Sep 16, 2020 at 13:13
  • No, they only change the size for the current buffer and not globally. Sep 16, 2020 at 16:12

2 Answers 2


The :height attribute of a face is ten times the font size. You can directly set the height attribute:

(defun change-font-size (new-size)
  "Change the font size to the given value"
  (interactive "nNew font size: ")
  (set-face-attribute 'default nil :height (* 10 new-size)))

Note that it needs a number, not a string, so I changed your interactive s to n.

For the second part, where you have it depend on monitor size,

(defun my/check-monitor ()
   (if (<= (display-pixel-width) 1440) 16 20)))

This will set the font size to 16 if the monitor width is up to 1440px and 20 if it's a wider monitor. You can extend this to include more logic depending on width and height.

I am not sure if emacs has any built in hooks to detect a monitor change, but if not, either find a way to send a command to emacs (e.g. run emacsclient -e "(my/check-monitor)" from a shell script), or have it check the monitor width occasionally in a timer, maybe with run-with-idle-timer.

  • That works much better! Thank you! And I didn't think about sending a command to emacs from the window manager. That sounds like a good alternative. I have to check whether gnome WM supports these kind of hooks. Sep 17, 2020 at 7:47
  • Regarding the hook, it seems focus-in-hook is what I need. Now I only need to figure out how to get emacs to correctly report the size of the display it is in: display-pixel-width reports always the pixel width of the external monitor. Sep 17, 2020 at 8:25
  • I edited my question with a hook that achieves the automagically resizing. Has some caveats, but works for me :) Sep 17, 2020 at 11:00
  • Great! I think (display-pixel-width (car (frame-list))) will tell you the pixel width of the monitor emacs is on, but emacs could have a frame on each monitor, so that would end up being ambiguous.
    – amitp
    Sep 17, 2020 at 15:41

I don't know if that will appeal to you but what I do is use text-scale-increase and text-scale-decrease. Putting them in a hydra like this:

(require 'hydra)

(defhydra hydra-zoom (global-map "<f2>")
  ("g" text-scale-increase "in")
  ("l" text-scale-decrease "out"))

then allows me to increase it a couple of times by typing <F2> g g ..., repeating the g (g for greater but suit yourself :-) ).

Although this is not a global change, I generally find that I don't need to change the font size globally: I set the font (including its size) once and for all in my init file to whatever I need for most of my work and then just use the above to change the size of the font in a buffer e.g. when presenting my screen in a meeting to make it readable.

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