Budget vs actual analysis with AI: find the overspend and draft next quarter's budget
A small business is under budget overall, but three categories are over. A worked example in GPTExcel, from expenses export to a month-by-month chart, the subscriptions to cancel and a draft Q4 budget.
"Are we on budget?" sounds like a yes-or-no question, and the total usually says yes. The useful answer is underneath: which categories are over, since when, and why. This post works through a small agency's spending for January to September in GPTExcel, from the first question to a draft budget for next quarter.
The data
expenses_2026.xlsx has two sheets:
- Budget: each category's monthly budget, $58,800 a month in all.
- Expenses: every charge from January to September 2026, with the date, vendor, category, amount and how it was paid. The kind of list your card statement or accounting software exports.
1. Are we over budget?
Are we over budget this year?
GPTExcel multiplies each monthly budget by nine and compares it with what was actually spent:
| Category | Budget | Spent | Over / under |
|---|---|---|---|
| Software | $19,800 | $23,184 | +$3,384 (+17.1%) |
| Travel | $13,500 | $16,625 | +$3,125 (+23.1%) |
| Contractors | $45,000 | $47,147 | +$2,147 (+4.8%) |
| Payroll | $342,000 | $335,496 | −$6,504 |
| Marketing | $54,000 | $44,797 | −$9,203 |
Overall, the agency spent $520,916 against a budget of $529,200: $8,284 (1.6%) under. But that total hides three categories that are over. Marketing's underspend is what keeps the total under budget. Rent, Office and Utilities are on or under.
2. Month by month
Chart monthly spending against the budget
Plotting each month's spending against the monthly budget shows when it changed. The agency stayed under budget from January to May, then went over every month from June. June was the highest, at $61,581.
A bar chart of over or under budget works better here than two lines: the bars cross zero exactly where the problem starts.
3. What drove it?
What's behind the overspend?
Breaking the three categories down by month and vendor gives three separate stories:
- Software crept up, from $2,040 a month to $3,140 in July and August. Figma and Canva both started in April ($560 a month together), Notion in June, and Slack billed twice in July and August.
- Travel jumped in June: $6,930 that month, of which $6,410 was one conference in Lisbon (tickets, hotel and flights).
- Contractors have averaged $7,044 a month since July, against a $5,000 budget.
Each one needs a different response. The conference was a one-off. The software creep and the contractors will keep going unless someone decides otherwise.
4. What can we cancel?
Find duplicate charges and subscriptions we could cancel
| What | Cost | Suggestion |
|---|---|---|
| Slack, billed twice in July and August | $696 | Ask Slack to refund the two extra charges |
| Canva, since April | $260 a month | Adobe Creative Cloud and Figma are already paid for: three design tools. Cancel Canva |
| Notion, since June | $192 a month | Check who uses it: it overlaps with Google Workspace docs |
The refund and Canva alone save $696 now and $260 a month.
Duplicate and overlapping subscriptions are the easiest money to find in any expenses file, and the hardest to spot by scrolling: each charge looks normal on its own.
5. A draft budget for next quarter
Draft a Q4 budget based on this
GPTExcel builds the draft from the most recent quarter, July to September, adjusts for what was learned above, and
hands it back as budget_q4_2026.xlsx, with a note on each change:
| Category | Now | Proposed Q4 monthly |
|---|---|---|
| Payroll | $38,000 | $37,313 |
| Contractors | $5,000 | $7,044 |
| Marketing | $6,000 | $5,050 |
| Rent | $4,500 | $4,500 |
| Software | $2,200 | $2,532 |
| Travel | $1,500 | $1,500 |
| Office | $900 | $801 |
| Utilities | $700 | $667 |
| Total | $58,800 | $59,407 |
Software already assumes the Slack refund and no Canva. Travel stays at $1,500, because June's conference was a one-off. Contractors is set at the level the business is actually running at, so the budget reflects a decision rather than a hope.
Checking the numbers
Every figure comes from Python code run on the file, and you can open each step in the answer to see the code and its output. The Q4 file's totals are real formulas, so you can change a line and the total updates.
Try it with your own spending
Export your expenses (a card statement, your accounting software, or a spreadsheet you keep by hand), add a sheet with your monthly budget per category, and ask "Are we over budget this year?". See the AI Chat guide for attaching files and following up, and Analyze a sales report like a data analyst for the revenue side.