Thanks Mike. I haven't tried this yet, but here's the reason I need it.

I have a number of spreadsheets that need to look good. One, for example,
is a booking form for a holiday venue. The form is divided into sections
which are outlined with borders in a grid format. To accommodate the
different types of data in the various sections, the form is made up of
several columns of varying widths, with different combinations of cells
merged on different rows.

Some of the sections are full page width and others are half width (two
sections side by side). It's obviously going to look much better if both
sides are of equal width, and I've built it like that, but sometimes I need
to tweak column widths to make the data fit, and then I have to get both
sides equal again. At the moment I check all the column widths and add them
up, but I'm looking for a quicker and less tedious way.

It would be great if I could merge all the columns on the left side of the
page, and all the columns on the right, on a row outside the print area, and
check the width of the merged cells whenever I needed to, but it doesn't work
like that: it only gives you the unmerged width of the first column in each
merged group.

I'll try your formula later and let you know if I have any problems with it.