How to build a 13-week cash flow forecast
What it is, and what it is not
A 13-week cash flow forecast is a receipts-and-payments schedule. Every line is money actually moving in or out of a bank account, dated to the week it moves. That makes it a different document from anything in your accounts: a profit and loss account is accrual-based and tells you what you earned, and a budget is a plan and tells you what you intended.
| Document | Basis | Granularity | Question it answers |
| Profit and loss | Accrual — when earned or incurred | Month or year | Are we making money? |
| Budget | Accrual, planned | Year | Is the plan on track? |
| 13-week cash flow | Cash — when it moves | Week | Will there be money in the bank? |
The three coexist. A business can be profitable on the first, on budget on the second, and unable to make payroll in week seven on the third. That is the exact failure the weekly granularity exists to catch, and it is invisible at monthly granularity because a month with a shortfall in week two and a large receipt in week four nets out to a comfortable month.
Why 13 weeks
Thirteen weeks is a quarter. It is long enough that most of a business cycle appears in it — a full round of customer payment terms, a quarterly statutory payment, a seasonal dip — and short enough that the individual line items are things you actually know rather than things you are estimating. Beyond about a quarter, receipts become assumptions, and a forecast built from assumptions is a budget wearing a different name.
It is also the horizon over which you can still act. If week eleven is short, you have ten weeks to chase a debtor, delay a discretionary payment, draw on a facility, or bring a collection forward. Discovering the same shortfall in week ten leaves you with one option, and it is usually the expensive one.
Building it, line by line
Start with the opening balance
Week one opens with the actual cleared bank balance across all operating accounts. Not the book balance, and not including facilities you have not drawn. Every subsequent week opens with the previous week’s closing balance, which is what makes the shortfall visible when it arrives.
Receipts: work from the debtors ledger, not from revenue
Take the aged receivables list and place each open invoice in the week you expect it to be paid, based on how that customer has actually paid before rather than on the terms printed on the invoice. A customer with 30-day terms who consistently pays on day 52 goes in week eight. This is the single most common place a forecast goes wrong, and it goes wrong optimistically.
- Open invoices — by expected payment date, using observed behaviour.
- New sales expected to be invoiced and collected inside the window — only where the collection genuinely falls within 13 weeks.
- Other receipts — GST refunds, interest, asset sales, loan drawdowns, promoter infusions.
Payments: everything, on the date it leaves
- Payroll, on its actual date, including any bonus or arrears month.
- Statutory dues — GST, TDS, PF and ESI on their filing dates, advance tax in the quarter it falls. These have fixed dates and no flexibility, which is precisely why they cause the sharp weeks.
- Supplier payments, from the creditors ledger by due date.
- Rent, utilities, subscriptions, insurance — the fixed base.
- Loan EMIs and interest, on the debit date.
- Capital expenditure already committed.
Keep one line labelled clearly as discretionary — marketing spend, new hires not yet made, optional equipment. It is the lever you pull when a week goes short, and separating it from the committed lines is what makes the decision quick rather than agonising.
Reading the result
Read the closing balance row, not the totals. The forecast has told you something useful when you can name the tightest week and say what is causing it.
- A negative closing balance in any week is the finding. It does not matter that the quarter ends comfortably.
- A balance that stays low for several consecutive weeks is more dangerous than one sharp dip, because it removes your capacity to absorb anything unexpected.
- A balance that climbs steadily is worth examining too. Cash accumulating beyond a sensible buffer is cash not being deployed.
When a week goes short, the levers are ordered by how cheap they are: chase a specific overdue invoice, offer a small early-payment discount on a large one, delay a discretionary payment, negotiate a supplier date, draw on an existing facility, and only then look at new borrowing. The forecast tells you how much you need and by when, which is what turns that list from a panic into a sequence.
Rolling it forward, and keeping it honest
Every week, do two things. Replace week one’s forecast with what actually happened, and add a new week thirteen at the far end. The forecast stays 13 weeks long forever, which is why it is called a rolling forecast.
The comparison of forecast against actual is not an audit exercise; it is how the forecast gets good. After six or eight weeks you will know that your collections forecast runs about a week optimistic, or that a particular customer always slips, and you can correct for it. A forecast nobody compares to reality drifts within a month and gets quietly abandoned within two.
Build it in a spreadsheet, with weeks across the columns and line items down the rows. It should fit on one screen. If it needs a second screen, the line items are too detailed — this is a cash tool, not a general ledger.
Frequently asked questions
Is a 13-week forecast the same as a runway calculation?
No. A runway calculation divides available cash by average monthly burn and gives one number of months. A 13-week forecast schedules the actual dates, which is what reveals a shortfall in a specific week that the average hides entirely. Runway is the summary; the forecast is the detail that lets you act.
How accurate should it be?
Weeks one to four should be close to exact, because almost every item is already known. Weeks five to eight carry real estimation error on collections. Weeks nine to thirteen are directional. That decay is expected, and it is the reason the forecast is rolled weekly rather than built quarterly.
Should I include GST in the receipts and payments?
Yes — the forecast is about money moving, and the tax component moves with the invoice. Show the net GST payment on its own line on the filing date, because that is a real cash outflow on a fixed date and it is the item most often forgotten.
What if my business has very predictable cash flow?
Then it will take twenty minutes a week and it will confirm what you expected, which is a fine outcome. The value is asymmetric: the weeks it tells you nothing cost you almost nothing, and the one week it catches a payroll shortfall pays for all of them.
Where to go next on this site
- How Many Months of Runway Do I Have — The summary number this forecast breaks into weeks.
- Profitable But No Cash in the Bank — The diagnosis for the gap between the P&L and the balance.
- Can I Afford to Hire — A committed payment added to every future week — test it here first.
- Cash Flow Calculator — Operating, investing and financing cash flows.
- Working Capital Calculator — The receivables and payables position behind the weekly timing.
- Lesson 13: Cash Flow Statement — The accounting statement, as distinct from this operating schedule.
Sources
- Reserve Bank of India — Working capital facilities and current rates, where a shortfall is being funded
- GST portal — Filing dates that fix several of the payment lines