One night at the start of the month, I had my invoice ledger open and noticed I could call Claude from a side panel inside Excel.
The ledger lists client names, amounts, and a contact email on every row. My hand stayed on the mouse for a while. Was it really fine to hand this over as it was?
I think some decisions are better made before you read the how-to. Here is what I settled on after rereading the official notes and holding them up against my own ledger. If you can use Excel, you don't need a terminal for any of it.
First, check what the docs say it cannot do
Claude for Excel is an add-in. It answers questions about the workbook you have open, changes values while keeping formula relationships intact, and traces errors back to their source. Answers come with cell-level citations you can click to jump to the cell. It is available on Pro, Max, Team, and Enterprise plans.
The part I almost skipped was the limitations. In my own words:
- Final deliverables without human review, audit-critical calculations without verification, and highly sensitive or regulated data without proper controls are not recommended uses.
- Macros, VBA, and data tables are unsupported.
- It does not run on Excel 2016 or 2019 (perpetual licenses), Excel on iPad, or Excel on Android. Excel on the web, Microsoft 365 on Windows, and Excel on Mac are supported.
There is one more warning I would not skip: files from outside can carry hidden instructions meant to steer the add-in. Downloaded templates and vendor spreadsheets are the classic case. Risky actions ask for your confirmation, but a confirmation you click through in a hurry protects nothing.
Three lines I drew before showing it anything
I sorted the ledger's columns like this.
| Kind of column | What I do | Why |
|---|---|---|
| Amounts, dates, categories used for totals | Let Claude read them | If it gets one wrong, a check can catch it and I can roll back |
| Names, emails, addresses | Delete from the working copy | The question doesn't need them, and what isn't shared can't leak |
| Files that arrived from someone else | Don't open them as they are | I can't see a hidden instruction by eye |
The middle row matters most. If all I want to know is which category grew over last month, neither the client names nor the emails are needed.
I only let it read what I could undo if it were wrong. I stuck that sentence on the first row of the ledger.
Making the working copy
Four steps:
- Close the original, then use Save As to make a copy such as
ledger_claude_work.xlsx. Claude only ever touches this copy. - In the copy, delete the name, email, and address columns. The original stays untouched.
- Confirm row one is a clear header and there are no merged cells. If the headers are vague, I can't tell which column Claude is pointing at.
- In the add-in's Settings, use the Instructions field for formatting you want every time. The docs' own examples, such as thousand separators and bold headers, are plenty. These instructions apply to Excel only, separately from PowerPoint or Word.
I close the original before opening the copy. Two workbooks with near-identical names open side by side make it surprisingly easy to lose track of which one just changed.
Make the first request read-only
On the first run, I don't let it change anything. It reads, and I check the answer.
❌ "Tidy up this ledger."
✅ "The total of column E doesn't match the total of column C. Find the rows where they differ and tell me the cell locations. Don't change any values."
The official best practices say to be specific about what you want changed. The first prompt sets no boundary, so I can't tell how much it may rewrite. The second only searches, and changes nothing.
When the answer comes back, open the cells it cites yourself. Skip that, and half the point of read-only is gone.
Only from the next request do I allow edits. The docs say Claude warns before overwriting existing data, but I don't rely on the warning. I take one extra copy first, every time.
Keep one check outside Claude
You can't verify Claude's answer by asking Claude. I keep a single check cell outside the table.
' Put in a spare cell of the working copy (never in the original)
' Column C = invoiced amount, column E = amount received. Zero means they match.
=SUM(C2:C200)-SUM(E2:E200)
After any change, I look at this number. If it isn't what I expected, I close the copy without saving. Since I never touched the original, there is nothing to restore.
Knowing in advance how I'd undo things made it easier to widen what I ask for, a little at a time.
Today's step: count the columns that identify a person
The first step was simply counting the columns in the open workbook that can identify a person. If the count is zero, go ahead and try. If it is one or more, make a copy and delete those columns first.
That's the order in which I first showed Claude a slice of my ledger. I plan to widen it only after the check formula has never once let me down. The idea of treating files from outside with suspicion has something in common with the lines I drew for Claude in Chrome's permission modes.