Cash Flow Forecast Calculator — your results
Cash Flow Forecast Calculator
See month by month whether you'll have enough cash in the bank — not just whether you're profitable. Add your income and outgoings, tell it when the cash actually lands using your debtor and creditor days, and it'll show you where the tight months are before they happen. Built by Rule29, business accountants and advisers to SMEs across Hull and East Yorkshire.
Forecast settings
What's actually in the bank today.
Leave at £0 to flag any month you'd go overdrawn, or set it to an agreed overdraft limit or a cash buffer you want to protect.
Money coming in
"Days to get paid" is your debtor days — the gap between invoicing and the cash landing. Invoices are assumed to go out on the 1st of the month, so e.g. 45 days means invoiced the 1st, paid around the 15th of the next month.
Money going out
"Days to pay" is your creditor days — how long you take to actually pay a supplier or bill once it's due, on the same 1st-of-the-month assumption.
VAT payments
Then repeats every 3 months for the rest of the forecast.
One-off items
Things that happen once — a grant, a big purchase, a loan drawdown. Pick "money in" or "money out" below and you can still switch it afterwards.
Projected bank balance
Month by month
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Month by month
How this calculator works
A profit and loss forecast tells you whether the business makes money over a period. A cash flow forecast tells you something different and, day to day, more urgent: whether there's enough cash in the bank to pay the bills when they're actually due. The two can tell completely different stories in the same month — a profitable business can still run out of cash if customers pay slowly and suppliers need paying fast.
This calculator adds up everything you expect to invoice or incur each month, then shifts each amount to the month it actually lands in the bank using your debtor days (how long customers take to pay you) or creditor days (how long you take to pay out). Invoices and bills are assumed to be raised on the 1st of the month, so 45 debtor days means invoiced the 1st, cash landing around the 15th of the following month. Any debtor or creditor days greater than zero always lands in a later month than the one it was invoiced in — never the same month — so a steady £1,000/month line with, say, 30-day terms shows up as a steady £1,000/month one month later, rather than doubling up or skipping a month depending on how long a particular calendar month happens to be.
One-off items (a grant, a big purchase, a loan drawdown) and quarterly VAT payments are layered on top of the recurring lines. The running total is your projected closing balance for each month, and the chart and table show you exactly where it dips — including against a minimum safe balance you set yourself, whether that's a hard £0 or an agreed overdraft limit.
What a cash flow forecast doesn't tell you
This calculator is a planning tool, not a crystal ball. A few things worth keeping in mind:
- It's only as good as your assumptions. Debtor days, creditor days and growth rates are estimates — a customer who pays late just once can shift a tight month by weeks.
- It doesn't know about bad debts. Every invoice here is assumed to be paid in full, eventually. If a customer never pays, that income needs removing by hand.
- Cash owed beyond the end of the forecast window doesn't show up. A payment due to land in month 14 of a 12-month forecast simply isn't counted — lengthen the forecast if a late payment is close to the edge.
- It assumes a fixed invoicing date. Every recurring line is treated as if it's invoiced or incurred on the 1st of the month — a reasonable approximation for planning, but not a substitute for your actual invoice dates.
- Tax isn't fully modelled. VAT is included as a quarterly estimate you provide; corporation tax and PAYE/NI payment dates aren't built in and would need adding as one-off items.
Debtor days, creditor days, and why the gap matters
Debtor days measure how long it typically takes customers to pay you after you invoice them — sometimes called your receivables or debtor collection period. Retail and card-paid work might be close to 0 days; a business invoicing on 30- or 60-day terms, or one that chases payment slowly, can easily average 45-60 days or more.
Creditor days are the mirror image: how long you take to pay your own suppliers and bills once they're due. Some costs — wages, rent, direct debits — are effectively 0 days. Supplier accounts on 30-day terms give you a bit of breathing room.
The gap between the two is what drives a cash squeeze that a profit and loss account won't show you. A business that invoices on 45-day terms but pays its own suppliers in 14 is financing that 31-day gap out of its own cash reserves for every sale it makes — and the faster it grows, the bigger that financing gap gets, even while it stays profitable on paper. Shortening debtor days (deposits, quicker invoicing, chasing overdue accounts) or lengthening creditor days (negotiating supplier terms) are two of the most direct levers a business has over its own cash position, and this calculator is built to let you test both.
Supporting businesses across Hull and East Yorkshire
Rule29 works with SMEs and entrepreneurs across Hull and East Yorkshire on the numbers behind cash, pricing and growth. This calculator is a starting point; for a cash flow forecast built around your actual bank statements and payment history, we're happy to talk it through.
Worried about a squeeze coming up?
Get in touch with Rule29 for advice tailored to your business — visit rule29.co.uk or speak to your usual Rule29 contact.
Cash flow forecast calculator FAQs
What is a cash flow forecast?
A month-by-month projection of the cash actually moving in and out of your bank account — as opposed to a profit and loss forecast, which shows whether the business is profitable over a period regardless of when the cash lands. A business can be profitable and still run out of cash if customers pay slowly.
What are debtor days?
The average number of days between raising an invoice and actually receiving payment for it — also called your receivables period. This calculator uses it to work out which month a sale's cash actually lands in, not just which month it was earned.
What are creditor days?
The average number of days you take to pay a bill or supplier invoice once it's due. Longer creditor days keep cash in your business for longer; shorter ones (or upfront costs like wages) use it up sooner.
How does this calculator work out which month a payment lands in?
Every recurring income or outgoing is assumed to be invoiced or incurred on the 1st of its month. Your debtor or creditor days are then added to that date using real calendar-day arithmetic — so 45 days from the 1st typically lands in the middle of the following month — and the full amount is counted in whichever month the payment date falls in. Any debtor or creditor days greater than zero are guaranteed to land at least one month later than the invoice month, so a steady monthly amount always shows up as a steady monthly amount, just shifted — it never doubles up in one month and skips the next.
Why does my forecast show a dip even though I'm profitable overall?
Because profit and cash timing are different things. A month with high sales but slow-paying customers can still be a cash low point if your outgoings — wages, rent, a supplier bill, a VAT payment — fall due before that cash lands. That's exactly the kind of month this calculator is built to surface.
What does the "minimum safe balance" do?
It's the line below which you want a warning. Leave it at £0 to be flagged the moment you'd go overdrawn, or set it to an agreed overdraft limit or a cash buffer you never want to dip below — any month projected to fall under that level is highlighted on the chart, the table, and in a banner above your results.
Does this calculator account for VAT?
Yes, as an estimated net VAT payment every quarter, on whichever month you specify. It's a single figure you provide rather than a full VAT return calculation, so it's worth basing it on a recent quarter's actual liability.
Can I model a loan, grant or one-off cost?
Yes — add it as a one-off item with the month it lands in and whether it's money in or out. Anything that happens once rather than every month (a piece of equipment, a grant, a loan drawdown or repayment lump sum) belongs here rather than in the recurring income or outgoing lines.
How accurate is this forecast?
It's only as accurate as the assumptions you put in — growth rates, debtor and creditor days, and the amounts themselves are all estimates. Treat it as a planning tool for spotting the shape and timing of a potential squeeze, not a substitute for a forecast built around your actual bank statements and aged debtor/creditor reports; that's exactly what Rule29 can help with.
What happens if a customer pays late or doesn't pay at all?
This calculator assumes every invoice is eventually paid in full on your stated debtor days. It doesn't model bad debts or partial payments — if you're forecasting around a customer you're genuinely worried about, the safest approach is to remove or delay that income line by hand and see how the forecast changes.
How is this different from your breakeven calculator?
The breakeven calculator answers "how much do I need to sell to cover my costs?" using annual or monthly totals, without worrying about timing. This calculator takes the timing itself as the whole point — it answers "will there actually be enough cash in the bank on the day I need it?", which can be a very different, and often more urgent, question.