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Build a simple 12-month forecast with clear assumptions
Get a month-by-month income, costs and cash forecast laid out in a spreadsheet, with every assumption written down so it can be challenged.
When to use it
When planning the year or a big decision
Step by step
Gather the starting point
Export the last 12 months of monthly profit and loss and the current cash balance. Write down what you know is changing, such as a price rise, new hire or lost customer.
Agree the assumptions first
Ask the AI to list assumptions before any numbers, so you can correct them early.
Prompt to copyI'm building a 12-month forecast for [business] starting [month]. Attached are the last 12 months of monthly results and our current cash of [£]. Known changes: [list]. Step 1: list the assumptions you'd use for each line (growth, seasonality, timing of payments and receipts), in a table: Line | Assumption | Based on. Don't build the forecast yet; wait for me to confirm the assumptions.Build the forecast layout
Once you've corrected the assumptions, ask: 'Now lay out the forecast as a table with months across the top: income lines, cost lines, profit, then cash in, cash out and closing cash. Show the spreadsheet formulas I should use for each row so the figures update when I change an assumption.'
Put it in your spreadsheet
Copy the layout into Excel or Google Sheets with the assumptions on their own tab, and check that the formulas add up correctly.
Sense-check and share
Review the forecast with the business owner or manager, test a cautious and a hopeful version, and label it clearly as a forecast, not a promise.
What you get
Assumptions: sales grow 3% a month from March; July and August 20% lower (seasonal); new hire from April £2,800 a month; customers pay in 45 days. Closing cash: Jan £42k, Apr £31k (lowest), Dec £58k.
Tips
- Keep the assumptions tab front and centre; it's what people should argue about, not the formulas.
- Don't paste client personal data, full account numbers or payroll details into a public AI tool unless your organisation allows it. Use your work-approved assistant if you have one.