window-total-width

window-total-width is a built-in function in `C source code'.

(window-total-width &optional WINDOW ROUND)

Return the total width of window WINDOW in columns.
WINDOW must be a valid window and defaults to the selected one.

The return value includes the widths of WINDOW's fringes, margins,
scroll bars and its right divider, if any. If WINDOW is an internal
window, the total width is the width of the screen areas spanned by its
children.

If WINDOW's pixel width is not an integral multiple of its frame's
character width, the number of lines occupied by WINDOW is rounded
internally. This is done in a way such that, if WINDOW is a parent
window, the sum of the total widths of all its children internally
equals the total width of WINDOW.

If the optional argument ROUND is `ceiling', return the smallest integer
larger than WINDOW's pixel width divided by the character width of
WINDOW's frame. ROUND `floor' means to return the largest integer
smaller than WINDOW's pixel width divided by the character width of
WINDOW's frame. Any other value of ROUND means to return the internal
total width of WINDOW.