Below is an updated version of the patch with entries in etc/NEWS and a
fix for a typo in the doc-string of `default-font-height'.  The old
ChangeLog entry can be reused:

Implement functions for measuring fonts and max chars per line

* simple.el (default-font-width): new function.
* (default-font-height): elaborate doc-string
* window.el (window-font-width): new function
(window-font-height): new function
(window-max-chars-per-line): new function

diff --git a/etc/NEWS b/etc/NEWS
index 14a9176..4adcb3d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -163,6 +163,24 @@ possible inaccuracies in the end position.
 In particular, it now returns the average width of the font's
 characters, which can be used for geometry-related calculations.
 
+** A new function `default-font-width' returns the average width of a
+character in the current buffer's default font.  If the default face
+is remapped (see `face-remapping-alist'), the value for the remapped
+face is returned.  This function complements the existing function
+`default-font-height'.
+
+** New functions `window-font-height' and `window-font-width return
+the height and average width of characters in a specified face and
+window.  If FACE is remapped (see `face-remapping-alist'), the
+function returns the information for the remapped face.
+
+** A new function `window-max-chars-per-line' returns the maximal
+number of characters that can be displayed on one line.  If a face
+and/or window are provided, these values are used for the
+calculation.  This function is different from `window-body-width' in
+that it accounts for (i) continuation glyphs, (ii) the size of the
+font, and (iii) the specified window.
+
 
 * Editing Changes in Emacs 25.1
 
diff --git a/lisp/simple.el b/lisp/simple.el
index 0fcd5db..b700c68 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -5377,7 +5377,10 @@ lines."
 (declare-function font-info "font.c" (name &optional frame))
 
 (defun default-font-height ()
-  "Return the height in pixels of the current buffer's default face font."
+  "Return the height in pixels of the current buffer's default face font.
+
+If the default font is remapped (see `face-remapping-alist'), the
+function returns the height of the remapped face."
   (let ((default-font (face-font 'default)))
     (cond
      ((and (display-multi-font-p)
@@ -5388,6 +5391,25 @@ lines."
       (aref (font-info default-font) 3))
      (t (frame-char-height)))))
 
+(defun default-font-width ()
+  "Return the width in pixels of the current buffer's default face font.
+
+If the default font is remapped (see `face-remapping-alist'), the
+function returns the width of the remapped face."
+  (let ((default-font (face-font 'default)))
+    (cond
+     ((and (display-multi-font-p)
+	   ;; Avoid calling font-info if the frame's default font was
+	   ;; not changed since the frame was created.  That's because
+	   ;; font-info is expensive for some fonts, see bug #14838.
+	   (not (string= (frame-parameter nil 'font) default-font)))
+      (let* ((info (font-info (face-font 'default)))
+	     (width (aref info 11)))
+	(if (> width 0)
+	    width
+	  (aref info 10))))
+     (t (frame-char-width)))))
+
 (defun default-line-height ()
   "Return the pixel height of current buffer's default-face text line.
 
diff --git a/lisp/window.el b/lisp/window.el
index c95b0d6..990f953 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -1830,6 +1830,57 @@ optional argument PIXELWISE is passed to the functions."
       (window-body-width window pixelwise)
     (window-body-height window pixelwise)))
 
+(defun window-font-width (&optional window face)
+   "Return average character width for the font of FACE used in WINDOW.
+WINDOW must be a live window and defaults to the selected one.
+
+If FACE is nil or omitted, the default face is used.  If FACE is
+remapped (see `face-remapping-alist'), the function returns the
+information for the remapped face."
+   (with-selected-window (window-normalize-window window t)
+     (let* ((face (if face face 'default))
+	    (info (font-info (face-font face)))
+	    (width (aref info 11)))
+       (if (> width 0)
+	  width
+	 (aref info 10)))))
+
+(defun window-font-height (&optional window face)
+   "Return character height for the font of FACE used in WINDOW.
+WINDOW must be a live window and defaults to the selected one.
+
+If FACE is nil or omitted, the default face is used.  If FACE is
+remapped (see `face-remapping-alist'), the function returns the
+information for the remapped face."
+   (with-selected-window (window-normalize-window window t)
+     (let* ((face (if face face 'default))
+	    (info (font-info (face-font face))))
+       (aref info 3))))
+
+(defun window-max-chars-per-line (&optional window face)
+  "Return the number of characters that can be displayed on one line in WINDOW.
+WINDOW must be a live window and defaults to the selected one.
+
+The character width of FACE is used for the calculation.  If FACE
+is nil or omitted, the default face is used.  If FACE is
+remapped (see `face-remapping-alist'), the function uses the
+remapped face.
+
+This function is different from `window-body-width' in two
+ways.  First, it accounts for the portions of the line reserved
+for the continuation glyph.  Second, it accounts for the size of
+the font."
+  (with-selected-window (window-normalize-window window t)
+    (let* ((window-width (window-body-width window t))
+	   (font-width (window-font-width window face))
+	   (ncols (/ window-width font-width)))
+      (if (and (display-graphic-p)
+	       overflow-newline-into-fringe
+	       (/= (frame-parameter nil 'left-fringe) 0)
+	       (/= (frame-parameter nil 'right-fringe) 0))
+	  ncols
+	(1- ncols)))))
+
 (defun window-current-scroll-bars (&optional window)
   "Return the current scroll bar types for WINDOW.
 WINDOW must be a live window and defaults to the selected one.