How to Build a 13-Week Cash Flow Forecast

Set Up Your Structure

Your forecast needs one column for each of the 13 weeks, plus a row structure that captures every significant cash movement in your business. Keep it simple. The most common mistake is building a model so complex it never gets maintained — which defeats the entire purpose.

Columns

Label each column with the week ending date — for example, "w/e 12 Jul", "w/e 19 Jul", and so on for 13 weeks. Add a "Actuals" column to the left of week 1, where you will record what actually happened as each week closes. This is how the forecast rolls forward.

Rows — Inflows

List every source of cash coming into the business. For most SMEs this means: customer receipts (broken down by major customer or invoice if possible), any government grants or subsidies expected, loan drawdowns, and any other income. The more specific the better — a row for "Customer A receipts" is more useful than a single "Total receipts" line, because it tells you which customer to chase if a payment does not arrive.

Rows — Outflows

List every cash payment going out: supplier payments (by major supplier), payroll, rent, loan repayments, tax payments (VAT, corporate tax, social contributions), utilities, and any significant one-off expenditure. Again, specificity matters. "VAT payment — €12,400 due 14 July" is actionable. "Taxes" is not.

Net cash movement and closing balance

At the bottom of each week, calculate: Total Inflows minus Total Outflows equals Net Movement. Then add Net Movement to the Opening Balance (last week's Closing Balance) to get your Closing Balance. This running balance is the most important number in the entire model — it tells you whether you will have enough cash at the end of each week to meet your obligations.

Populate Your Inflows

This is where most people start too broadly and end up with a forecast that is too vague to be useful. The goal is to forecast cash receipts — not revenue — which means accounting for the timing gap between when you invoice and when you actually receive payment.

Start with your accounts receivable

Pull your list of outstanding invoices. For each one, identify the due date and — based on that customer's payment history — estimate the realistic collection date. A customer who consistently pays 10 days late should have their receipt forecast 10 days after the due date, not on it. This single adjustment makes your inflow forecast dramatically more accurate.

Use payment term knowledge, not wishful thinking

If a customer is on Net 60 terms and you invoiced them three weeks ago, do not forecast the receipt in the next two weeks. Forecast it in week five or six. If a customer has a pattern of paying in 45 days despite 30-day terms, use 45 days. Your forecast should reflect how your customers actually behave, not how you wish they would.

Flag concentration risk

If one or two customers represent a large share of your inflows, make their receipts visible as separate rows. If Customer A does not pay on time in week three, you need to see the impact on your closing balance immediately — not discover it when you check the bank account.

Populate Your Outflows

Outflows are generally more predictable than inflows — most of your costs recur on a known schedule. This is an advantage: it means you can populate the outflow side of your forecast with high confidence even several weeks out.

Fixed recurring costs

Payroll, rent, loan repayments, insurance premiums — enter these on the exact date they leave your account, every week they occur across all 13 weeks. These are non-negotiable and should always be visible in the forecast, with no exceptions.

Variable supplier payments

For each significant supplier, enter the expected payment date based on invoice terms. If you have invoices received but not yet due, enter them in the week they fall due. If you have flexibility on timing — suppliers where you are paying early — enter the latest date you could pay while staying within terms.

Tax and regulatory payments

VAT, corporate tax, social contributions — these tend to be large, infrequent, and easy to forget until the week before they are due. Enter all known tax payment dates for the full 13-week horizon at the start of every forecast cycle. A €15,000 VAT bill that appears in week three with no warning is a cash crisis. The same bill that has been visible in the forecast for six weeks is just a scheduled outflow.

One-off and discretionary items

Any significant planned expenditure that does not recur — equipment purchases, planned marketing spend, travel — should be entered in the relevant week. These are also the items most easily deferred if your closing balance is projected to fall below your minimum threshold.

Set a Minimum Cash Threshold

Your forecast needs a reference line — a minimum closing balance below which you trigger action. Without this, the forecast is just a number; with it, it becomes a decision-making tool.

For most SMEs, a sensible minimum is between two and four weeks of total operating costs. If your weekly outflows average €25,000, a minimum threshold of €50,000–€100,000 gives you a buffer against unexpected timing shifts. The right number depends on your business — seasonal businesses, businesses with lumpy revenue, and businesses with large single customers should maintain a higher buffer than those with predictable, diversified income.

Mark this threshold visibly in your forecast — a coloured row or a conditional format that highlights any week where the closing balance falls below it. When that week appears, that is your signal to act: chase a specific receivable earlier, defer a discretionary payment, or draw on a credit facility. The forecast has done its job — it has given you time.

Roll It Forward Every Week

A cash flow forecast that is not updated weekly is not a forecast — it is a historical document. The rolling update is what makes this tool genuinely useful, and it takes far less time than most people expect: typically 30–60 minutes per week once the model is set up.

Replace the oldest week with actuals

At the start of each week, take last week's forecast column and replace the numbers with what actually happened — what cash actually came in, what actually went out. This is your reconciliation step. Variances between forecast and actual are not failures — they are information. A customer who was forecast to pay in week one but paid in week two tells you something about that customer's payment behaviour that should be reflected in all future forecasts.

Add a new week 13

With the oldest week converted to actuals, add a new 13th week to the right of the forecast. Populate it with expected inflows and outflows using the same logic as before. Your forecast now covers weeks two through fourteen — the horizon has rolled forward by one week.

Review the closing balances

Before you close the model, review the closing balance row for every week. Has anything fallen below your minimum threshold? Has a large outflow appeared in the new week 13 that you had not previously seen? Has a customer receipt shifted out by a week, tightening an already-close balance in week four? These are the questions the weekly review is designed to surface — and to surface early enough to act on them.

Build in Scenarios

A single-scenario forecast answers "what do we expect to happen?" A scenario-enhanced forecast also answers "what happens if things go wrong?" — which is the question that matters most when you are managing cash.

You do not need a complex model to do this. For each forecast cycle, consider two simple stress tests alongside your base case:

Late payment scenario

What happens to your closing balances if your three largest customers each pay 14 days late? This is the most common cash flow shock for SMEs and the easiest to model. Shift the relevant inflow rows forward by two weeks and see what the closing balance looks like. If it falls below your minimum threshold, you have advance warning — and can pre-emptively chase those customers or arrange a facility before the gap materialises.

Downside revenue scenario

What happens if next month's new business is 30% lower than expected? Reduce the inflows in weeks five through nine accordingly and review the closing balance. If the model shows a problem at 30% downside, you know the business needs either a stronger sales pipeline or a larger cash buffer to absorb that risk comfortably.

These scenarios do not need to be separate models — a simple toggle or a copy of your base forecast with adjusted inputs is enough. The discipline of running them once a month ensures that you are never surprised by outcomes that were, in retrospect, entirely foreseeable.