Why 13 weeks: The mechanics of rolling cash visibility
Every business owner knows the feeling: the bank account looks fine on the 15th, and by the 28th the payroll check bounces. The problem isn't that the money disappeared — it's that nobody was looking at the right window.
A 13-week cash flow model fixes that by forcing you to look one full quarter ahead, week by week. Why 13 weeks and not 12 or 16? Because a quarter is the natural rhythm of business: rent cycles, supplier invoices, payroll runs, and customer payment terms all repeat on roughly a monthly or quarterly cadence. Thirteen weeks gives you enough runway to see a crunch coming and enough granularity to do something about it before it hits.
The "rolling" part is what makes it powerful. Each week, you drop the oldest week off the front and add a new week at the back. The model never sits still — it advances with you, always showing the next 13 weeks from today. This means the forecast is a living document, not a one-time exercise you do in January and forget by March.
Think of it as a windshield for your business. You can't drive safely looking only at the hood ornament. You need to see the road ahead — the curves, the potholes, the intersections — before you reach them. The 13-week window is your visibility range. When you see a dip in week 9, you have time to slow down, change lanes, or pull over before you hit it.
The mechanics are simple. Each week has four components: an opening balance, cash inflows, cash outflows, and a closing balance. The closing balance of week 1 becomes the opening balance of week 2. The closing balance of week 2 becomes the opening balance of week 3. And so on. The chain links forward, week after week, until you've built a 13-week picture of your cash position.
That picture is what lets you act early. If week 8 shows a negative closing balance, you have five weeks to fix it — defer a purchase, chase a late invoice, draw on a line of credit, or renegotiate terms. Without the model, you'd only find out when the bank calls.
Establishing the opening balance and cash baseline
Before you can project anything, you need to know what you're starting with. The opening balance of your 13-week model is not simply "what's in the bank account." It's the cash you can actually spend today.
Start with your bank balance as of the model's start date. Then adjust for three things:
Uncleared transactions. Checks you've deposited but haven't cleared, pending card settlements, or transfers in transit. These are real money, but they're not available yet. If you count them as available cash, your model will overstate your position by exactly the amount still in limbo.
Restricted funds. Money that's in your account but can't be spent. Security deposits, escrow balances, tax withholdings you're holding for the government, or customer prepayments tied to a specific obligation. This cash exists on your bank statement but it's not yours to deploy freely.
Committed disbursements already in flight. Checks you've written that haven't cleared, scheduled ACH payments, or automatic debits set to hit in the next few days. These are outflows that will happen regardless of what you do — they're already committed.
The formula for your true opening balance is:
$$Opening\ Balance = Bank\ Balance - Uncleared\ Items - Restricted\ Funds - Committed\ Disbursements$$
Let's walk through an example. Your bank statement shows $42,500. You have a $3,200 check deposited last Friday that hasn't cleared. You're holding a $5,000 security deposit for a client project. And you've written a $2,800 rent check that will clear on Monday.
$$42{,}500 - 3{,}200 - 5{,}000 - 2{,}800 = 31{,}500$$
Your true opening balance is $31,500 — not $42,500. The difference is $11,000 of money that either isn't available or is already spoken for. If you'd built your model on the raw bank balance, you'd be planning with cash you don't actually have.
Once you've established your true opening balance, write it down as the first line of your model. This is week 1's starting point. Every subsequent week's opening balance will be calculated from the previous week's closing balance — but the entire chain begins here, with this number.
Projecting cash inflows: AR timing and collection realism
Here's where most cash flow models go wrong: they treat revenue as if it arrives when it's earned. It doesn't. Cash arrives when the customer actually pays — and customers pay on their own schedule, not yours.
In a 13-week model, your inflows are not your sales. They're your collections. The distinction is critical. If you invoice $10,000 in week 1 with net-30 terms, that $10,000 is not a week 1 inflow. It's a week 5 or 6 inflow, at best — and only if the customer pays on time.
To project inflows realistically, you need to know three things about each customer or revenue stream:
The invoice date. When did you bill them?
The payment terms. Net-30? Net-45? Due on receipt?
The actual payment behavior. Do they pay on time, or do they run 10, 20, or 30 days late?
The third one is the killer. Your invoice says "due in 30 days," but your customer's accounts payable department might not cut the check until day 45. Or day 60. If you model collections at day 30, you'll be $10,000 short in the weeks when the money actually arrives — and you'll have planned around cash that isn't there.
Here's a practical way to think about it. You have a customer who owes you $8,000. You invoiced them on week 2. Their average payment lag is 38 days. That's roughly 5.5 weeks. So the cash lands in week 7 or 8 of your model — not week 5, when the invoice is technically due.
The same logic applies to recurring revenue. If you have a subscription business, your inflows are more predictable: customers pay on a fixed schedule, and you can model those receipts with reasonable confidence. But even here, be conservative. Some customers will churn, some will dispute charges, and some will pay late. Build a buffer into your inflow assumptions.
For a new business, the safest approach is to model collections based on your worst-case historical performance, not your best. If your average customer pays in 40 days but your slowest pays in 65, model at 45 days. You'd rather be pleasantly surprised by early cash than blindsided by late cash.
