SUMIFS across 40 monthly sheets or one long table with a month column Formula Help

Consolidate, and do it in Power Query so the day of work turns into a refresh button. Point it at the workbook, append all the sheets, add the sheet name as a month column, load to a table, pivot from there. Cross-sheet SUMIFS with INDIRECT is possible and genuinely awful: slow, volatile, and it fails silently when someone renames a sheet or inserts a row. You will spend the same day debugging it in six months. The monthly sheets can stay exactly as they are for the humans who like them — the query reads them, it does not replace them.

One more argument for consolidating: INDIRECT is volatile, so 40 sheets of SUMIFS built on it will recalculate on every keystroke. On a workbook this size you will feel it, and it gets worse every month you add.

Three and a half years is 42 months and you said 40 sheets. Worth finding out which two are missing or named differently before you build anything on top.

If you genuinely cannot consolidate, at least build a named list of sheet names and run SUMPRODUCT over INDIRECT against that list rather than writing 40 separate references. Still ugly, but a new month costs you one row instead of an edit in every formula.