Hi,

Difficult to comment without seeing the problem in context, but intuitively I feel that the formula is becoming inefficient and too complex.

Here are a couple of ideas.

1. Rather than rely on Excel functions, get a macro to do it. This could automatically open each of the contributory workbooks, grab the data required and summarise it as it goes along, concluding with an instruction that would add the summary totals to your master workbook.

2. To sum cells across multi tabs in a single workbook provided each cell means the same thing in each tab, the standard solution is to add an additional sheet before the first sheet you want to summarise, and an additional sheet after the last sheet you want to summarise. Name these sheets 'Start' and 'End' and hide them.

Now on your summary sheet use formulas like the following.

=SUM(Start:End!aA1
This will sum all the cells in A1 in all the sheets that are placed between the Start/End sheets.

Hope this gives you a couple of ideas.

Regards