CUERPO → editor del post (Markdown)
Yes, Excel is still relevant, and not out of nostalgia: an AI that writes formulas or edits your workbook produces something that has to be checked, and checking it takes someone who can read the formulas. This tutorial covers what AI does inside Excel today according to Microsoft's own pages, what Microsoft tells you to verify, and five checks I run on any formula an AI wrote. A free workbook has five AI-style answers with planted mistakes, so you can see each check catch its mistake.
What AI does inside Excel today
Copilot in Excel works in three modes, according to Microsoft's support pages: chat answers questions about your data, plan proposes steps, and edit changes the workbook directly. Edit mode can be stopped, its changes are visible to anyone with access to the saved file, including coauthors, and you can undo them or go back to a previous version. Editing only works when Calculation Options is set to Automatic.
It needs a license that includes it: a Microsoft 365 Personal or Family subscription with an AI credits plan, Microsoft 365 Premium, a Microsoft 365 Copilot subscription, or a business plan that is eligible for Copilot Chat. With a Copilot subscription or Microsoft 365 Premium you can choose between model families, such as GPT or Claude, for the session.
The older route, the COPILOT function that you typed into a cell, did not last: according to Microsoft's support page and message center it was retired starting September 14, 2026. If a workbook of yours uses it, check its status on that page and plan a replacement, because an AI feature in the cloud can be retired; a SUMIFS does not retire.
What Microsoft tells you to do with the result
Microsoft's own FAQ says Copilot's results can be inaccurate or misinterpret information, and tells you to review, edit and verify what it produces and to avoid using it for decisions in sensitive areas such as finance, legal or medical topics. That is the part that keeps the Excel skill relevant, for three reasons.
The output is an Excel object. A formula, a table, a PivotTable. Reading it, tracing it and judging whether it is right is Excel work.
A formula is reproducible. The same inputs give the same result, and you can follow it with Trace Precedents or Evaluate Formula. A reviewer or a client can follow a workpaper; they cannot follow a conversation.
The person who signs is still you. If the total is wrong, nobody asks which tool wrote it.
Five checks on a formula an AI wrote
The workbook has five formulas of the kind an AI answer can contain. I wrote them for this exercise; they do not come from any tool. Each has an independent result, a detector and a verdict. The data is a table of 24 fictitious invoices.
1. A fixed range that stops one row short
=SUM(Data!E9:E31)
The table has 24 rows and the range covers 23. The formula gives 41,755.65 and the real net total is 43,293.15. The check recomputes the total a different way, from Qty times Price, and compares how many rows the range covers with how many the table has:
=SUMPRODUCT(tblInvoices[Qty], tblInvoices[Price])
=ROWS(Data!E9:E31)&" of "&ROWS(tblInvoices[Net])&" rows"
The fix is to refer to the table column, which grows with the table:
=SUM(tblInvoices[Net])
2. A rate typed inside the formula
=SUM(tblInvoices[Net])*1.16
With the VAT rate at 16% in cell B8, the formula gives the right number, which is why this mistake survives. Change B8 to 10% and the independent result moves while the AI's formula does not: a difference of 2,597.59. A detector that reads the formula text finds it even when the numbers agree:
=ISNUMBER(SEARCH("1.16", FORMULATEXT(C15)))
The fix refers to the rate cell:
=SUM(tblInvoices[Net])*(1+$B$8)
3. A name with a trailing space
=SUMIF(tblInvoices[Customer], $B$9, tblInvoices[Net])
Two invoices of Alpha Supplies were typed with a space at the end, as they come from many exports, and SUMIF does not match them. The formula gives 17,256.76; the customer's real total is 21,017.10. Count the names that change when you trim them:
=SUMPRODUCT(--(tblInvoices[Customer]<>TRIM(tblInvoices[Customer])))
The fix is to trim inside the calculation, or to clean the column once with TRIM:
=SUMPRODUCT((TRIM(tblInvoices[Customer])=$B$9)*tblInvoices[Net])
4. Rounding the total instead of each line
=ROUND(SUM(tblInvoices[Net])*(1+$B$8),2)
An invoice rounds each line, and the formula rounds the sum. The formula gives 50,220.05; the invoices add up to 50,220.03. Two cents is small, and it is exactly the difference that makes a reconciliation not close. The check compares against the total of the documents, and a detector looks for ROUND wrapped around SUM:
=SUM(tblInvoices[Total])
=ISNUMBER(SEARCH("ROUND(SUM", FORMULATEXT(C17)))
The fix rounds each line before it adds:
=SUMPRODUCT(ROUND(tblInvoices[Net]*(1+$B$8),2))
The check against the documents is only valid at 16% VAT, which is the rate they were issued at.
5. Numbers stored as text
=SUM(tblInvoices[Net (CSV)])
Three amounts came from a CSV as text. SUM ignores them without a warning and gives 40,152.15 instead of 43,293.15. Count the text cells:
=SUMPRODUCT(--ISTEXT(tblInvoices[Net (CSV)]))
The fix converts them while it adds:
=SUMPRODUCT(--tblInvoices[Net (CSV)])
What the five checks have in common
| Mistake | Check that catches it |
|---|---|
| Fixed range | Recompute the same total a second way and compare the row counts |
| Rate typed in the formula | Change the input and see if the result moves; read the formula text |
| Trailing spaces | Count the values that change under TRIM |
| Rounding the total | Reconcile to the documents, not to another formula |
| Numbers as text | Count the text cells |
The common idea is an independent result: a second calculation that does not share the first one's assumptions. If the second calculation is the same formula typed again, it will repeat the same mistake.
Limits
- I did not test Copilot or any other AI tool for this post. The formulas are mine, written to look like common mistakes. I cannot tell you which tool makes which mistake.
- Microsoft's pages change. The license list, the modes and the status of the COPILOT function are as I read them in October 2026; check the pages before you rely on them.
- The detectors are simple. SEARCH finds only the literal you give it, and FORMULATEXT needs the formula to be in a cell. They are a first filter, not a linter.
- Converting text with a double minus depends on your regional settings. The sample uses whole numbers so it works the same everywhere; with decimals, check how your Excel reads them.
- An independent result has to be independent. The check in case 4 is the total of the documents; a second formula that rounds the same way would agree with the mistake.
Workbook
Download the formula-check workbook, with fictitious data, no sign-up and no email. It has the five cases with their formula text, independent result, detector and verdict, the invoice table, and eight guided tests with the expected result.
If you want your team's formulas and workpapers reviewed, whether a person or an AI wrote them, let's talk.