Fix a broken spreadsheet formula with ChatGPT (including the ones you inherited)

Every office has one: a spreadsheet built by someone who left, full of nested formulas nobody dares touch. Mine had a 400-cell monster that broke every January. The fix that finally worked wasn't learning more Excel — it was a specific way of asking ChatGPT that turns the mystery into a diagnosis.

The prompt structure that works: paste the exact formula, then two sentences — what the sheet is supposed to produce, and what it's producing instead. 'This should sum sales for Q1 by region, but it returns zero for two regions' gives the model the failure symptom, which is the missing half of most cry-for-help questions. Then ask it to explain what the formula actually does, not just to fix it.

The explanation step is the one people skip and shouldn't. When ChatGPT explained that the inherited formula used absolute references pointing at a helper row that gets deleted every year, the fix became obvious — and so did the reason it broke every January. You're not just getting a repaired formula; you're getting the story of why it was fragile, which is what lets you decide between patching and rebuilding.

For the common failures, it's overkill in a good way: date formats that are text pretending to be numbers, merged cells eating ranges, locale commas versus semicolons. Paste, describe the symptom, get the fix with a version that works in your Excel flavor — say 'Google Sheets' if that's where you live, because the syntax differs and the model knows.

One caution from experience: retype the fixed formula by hand rather than pasting if your sheet has macros or linked files, and sanity-check one cell against a manual calculation before trusting the whole column. The model fixes logic; you own the verification. Ten minutes of checking saved my January this year, for the first time in four.