Checking a formula the AI wrote before you rely on it

How can I check a formula I can't read?

Start by asking the AI to explain it. Per Microsoft's page, Copilot can also explain how each formula works when it creates new columns or rows.¹

Read the explanation against what you asked for. If you wanted "price times quantity" and the explanation mentions something else, stop there.

An explanation isn't proof, though. Google's help page for Gemini in Sheets carries a similar warning about inaccurate suggestions.² Microsoft's page also says to verify what Copilot creates.¹

What's the quickest test?

Pick three or four rows and work them out by hand or on a calculator. Compare. If they match, you've got some evidence. If one doesn't, you've found the problem in under a minute.

Choose rows that differ from each other: a small one, a large one, one at the end of the list.

What about the odd rows?

Formulas often fail on the rows nobody planned for. Test these on purpose:

• A blank cell.
• A zero, especially if the formula divides by it.
• A number stored as text.
• The very first and very last row in the range.
• A row with an unusually long name or a stray space.

If the answer looks wrong on any of them, tell the AI what happened and ask for a fix.

Can I watch the formula work step by step?

In Excel, yes. Evaluate Formula walks through a nested formula part by part, following the order Excel calculates it.³ A nested formula is one with functions inside other functions.

Select the cell, go to the Formulas tab, and choose Evaluate Formula in the Formula Auditing group. Each click of Evaluate shows the value of the underlined part. Only one cell can be evaluated at a time.³

One catch. Microsoft says some functions are recalculated each time the worksheet changes, and these can make the tool show results different from what's in the cell. They include RAND, OFFSET, CELL, INDIRECT, NOW, TODAY, RANDBETWEEN and INFO.³

Should I change the formula myself?

If you understand the change, yes. If you don't, ask the AI for a new version and run the same checks again.

Keep the old formula somewhere, such as a spare column, until the new one passes. Then delete the spare.

When is a check not enough?

When a wrong answer would cost real money, such as invoices, tax or pay. Have someone who knows the sheet look at it, and consider whether a plain formula you can read is the better tool. The page on exact jobs helps you decide.

How to ask for formulas in the first place is covered in the page on prompts for formulas.