How to Forecast Cash Flow: A 13-Week Guide

What a 13-week cash-flow forecast is (and why 13 weeks)

A cash-flow forecast is simply a week-by-week estimate of the cash you expect to receive and the cash you expect to pay out. The goal isn't accounting precision — it's foresight. It tells you, in advance, which weeks are likely to be tight so you can act before a problem becomes a crisis.

Thirteen weeks is one calendar quarter. It's long enough to catch a slow patch, a big tax bill, or a seasonal dip coming down the road, but short enough that your estimates stay realistic. Anything further out tends to be guesswork; anything shorter doesn't give you room to react.

This is different from profit. A business can be profitable on paper and still run out of cash because customers pay late or stock is bought upfront. A cash-flow forecast tracks the actual timing of money, which is what keeps the doors open.

Before you start: what you'll need

Gather a few things so you're not guessing. The better your inputs, the more useful the forecast.

• Your current bank balance as of today — the single most important number.
• A list of customers who owe you money and when they're likely to pay.
• A list of bills and suppliers you owe, with due dates.
• Your regular fixed costs: rent, wages, loan repayments, subscriptions, insurance.
• Any known one-off items ahead — a tax payment, an equipment purchase, a deposit coming in.
• A simple spreadsheet with weeks as columns, or an accounting app that pulls these figures for you automatically.

Step by step: build the forecast

1. 1

   Set up your weeks

   Create a table with 13 columns, one per week, dated from this week forward. Down the side, make three blocks of rows: Money In, Money Out, and a Balance block at the bottom. Keep it simple — you can always add detail later.

2. 2

   Enter your opening balance

   In Week 1, put your actual bank balance as of today at the top of the Balance block. This is your starting point and the one figure that must be real, not estimated. Check it against your bank, not your memory.

3. 3

   List your expected money in

   For each week, add up the cash you genuinely expect to land in the bank that week: customer invoice payments, card and POS takings, online store payouts, and any other income. Place each amount in the week you realistically expect the money to arrive, not the week you invoiced.

4. 4

   List your expected money out

   For each week, list the payments leaving your account: supplier bills, wages and payroll, rent, loan repayments, subscriptions, and any tax payments. Put each one in the week it's actually due to be paid.

5. 5

   Calculate each week's net movement

   For every week, subtract total Money Out from total Money In. A positive number means cash grew that week; a negative number means it shrank. This is your net cash movement for the week.

6. 6

   Roll the closing balance forward

   Add each week's net movement to that week's opening balance to get its closing balance. Then carry that closing balance into the next week as its opening balance. Repeat across all 13 weeks so you have a running total — this chain is the heart of the forecast.

7. 7

   Scan for the danger weeks

   Read across the closing-balance row and highlight any week that dips close to zero or goes negative. Those are the weeks to plan around now, while you still have time to chase a payment, delay a purchase, or arrange a buffer.

How to read and act on what it shows

A forecast only earns its keep when it changes a decision. Once you can see a tight week coming, you have options — and weeks of notice to use them.

• Chase up invoices due before the danger week, and offer an early-payment nudge to your slowest payers.
• Ask a supplier to shift a large payment by a week or two, or move it to a stronger week yourself.
• Delay a non-urgent purchase — new equipment or a big stock order — until cash recovers.
• Set aside a buffer in strong weeks so the lean weeks have a cushion.
• If a real gap remains, arrange finance early rather than in a panic; lenders respond better to a plan than an emergency.

Keep it rolling — update it every week

A forecast built once and forgotten is worthless within a fortnight. Make it a rolling forecast: each week, replace your estimate for the week just gone with what actually happened, then add a fresh week 13 to the end so you're always looking a full quarter ahead.

This weekly habit does two things. It keeps the numbers trustworthy, and it sharpens your estimates over time as you see where your guesses were off. Fifteen minutes each Monday is usually enough once the template exists.

Compare your forecast against your actual bank balance regularly. When they drift apart, that gap is a lesson — maybe customers pay slower than you assumed, or a cost recurs that you forgot. Fold that insight back in.

Where an all-in-one app makes this easier

The hardest part of forecasting by hand is gathering accurate, up-to-date figures. When your sales, invoices, expenses, and bank activity live in separate places, you spend more time hunting for numbers than thinking about them.

Because Tallium is one app covering accounting, POS, inventory, and e-commerce under a single account, the raw inputs a forecast needs are already in one place: card and POS takings, online store sales, outstanding customer invoices, supplier bills, and scanned receipts. Its AI-automated receipt scanning and categorisation keep expenses current, and its reports and AI insights help you spot what's due and when — so your weekly update is a quick review rather than a data-hunting exercise.

Tallium is one simple, affordable flat USD price per unit, billed monthly, with unlimited users on every plan and no per-seat fees — so everyone who touches the numbers can be in the same system. There's no free trial; you pay from signup and can cancel anytime. The forecast itself still lives in your spreadsheet or planner, but the figures feeding it stay clean and close to hand.