Center Emacs Windows

center-window.el

(defun balance-margins ()
  "This function balances the margins of all windows on the selected
   frame such that the first column and the fill column are the same
   distance from the left and right edge, respectively."
  (walk-windows
   (lambda (window)
     (let* ((total-width (window-total-width window))
            (fringe-width (apply '+ (butlast (window-fringes window))))
            (scroll-bar-width (window-scroll-bar-width window))
            (divider-width (window-right-divider-width window))
            (extras-width (+ fringe-width scroll-bar-width divider-width))
            (body-width (- total-width extras-width))
            (fill-column (buffer-local-value 'fill-column (window-buffer window)))
            (excess (max (- body-width fill-column) 0))
            (margin (floor (/ (float excess) 2.0))))
       (set-window-margins window margin margin)))))

;; Enable this hook to balance margins automatically when visiting files or adding/removing windows.
;; (add-hook 'window-configuration-change-hook 'balance-margins t)