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Specifically, I'm interested in the width of characters in the default font assuming that the default font is mono-spaced. From reading the documentation, I got the impression that this should give me the correct value:

(aref (font-info (face-font 'default)) 2)

However, these values are consistently too big. For example, my current default font is

-unknown-DejaVu Sans Mono-normal-normal-normal-*-13-*-*-*-m-0-iso10646-1

and the code above gives me 13. However, when I make a screenshot and measure the characters their width is only 8 pixels. frame-char-widthgives me 8 pixels but if I change the default font size (using text-scale-increase) it continues to say 8, so that isn't useful.

I'm working with Emacs 25.0.50.2 on Ubuntu 14.04.

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  • From the doc of font-info, the resulting array contains MAX-WIDTH, SPACE-WIDTH, AVERAGE-WIDTH, which should be same for a mono font. Does (aref (font-info ...) 7) gives you the correct result?
    – amdyes
    Commented Oct 28, 2019 at 6:56

2 Answers 2

Reset to default
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The second element of the font-info is the font's pixelsize, which is (roughly speaking) its height. Getting the width of any particular glyph in the font is a little more work:

(aref (aref (font-get-glyphs (font-at (point)) 65 66) 0) 4)
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  • While this answers my question (thank you!), I realize that my question wasn't precise enough. Given a face, I'd like to know how much horizontal space a character would take if I'd insert it in the buffer. Using your approach, I could insert the character in the buffer, measure it, and remove it. However, I'm looking for a solution that doesn't require any modifications in the buffer. Is that possible?
    – tmalsburg
    Commented Dec 18, 2014 at 1:39
  • I could be wrong, but I doubt it's possible in general -- I suspect the font hasn't been instantiated before it is displayed.
    – jch
    Commented Dec 18, 2014 at 1:50
  • @tmalsburg use a temp buffer. But I'm not sure how they interact with fonts.
    – Malabarba
    Commented Dec 18, 2014 at 2:31
  • @jch do you have a link or something explaining where this information can be found. The solution is fine, but it's a little cryptic on its own.
    – Malabarba
    Commented Dec 18, 2014 at 2:32
  • @Malabarba, the only source I'm aware of is the docstring of font-get-glyphs.
    – jch
    Commented Dec 18, 2014 at 2:47
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Here is the complete solution for my original problem. Thanks to @jch and @Malabarba for contributing the crucial pieces.

(defun default-font-width () 
  "Return the width in pixels of a character in the current
window's default font.  More precisely, this returns the
width of the letter ‘m’.  If the font is mono-spaced, this
will also be the width of all other printable characters."
  (let ((window (selected-window))
        (remapping face-remapping-alist))
    (with-temp-buffer
      (make-local-variable 'face-remapping-alist)
      (setq face-remapping-alist remapping)
      (set-window-buffer window (current-buffer))
      (insert "m")
      (aref (aref (font-get-glyphs (font-at 1) 1 2) 0) 4))))

Update: This discussion led to the addition of three new functions in Emacs 25: default-font-width, window-font-width, and window-font-height (default-font-height was already part of emacs). window-font-width and window-font-height can be used to calculate the width and height or arbitrary faces.

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