Key takeaways
What this article covers, in order:
- Why 13 weeks, and why weekly?
- What you need before you start
- How to build the forecast, step by step
- A worked example
- The weekly routine that keeps it honest
- Common mistakes
A 13-week cash flow forecast is a week-by-week list of the cash you expect to come in and go out over the next quarter, ending with a projected bank balance for each week. It's short enough to be accurate and long enough to spot a cash crunch while you can still do something about it.
Below you'll find how to build one from your books, a full worked example where every row adds up, and a 20-minute weekly routine that keeps it useful.
Why 13 weeks, and why weekly?
Monthly forecasts hide the problem weeks. Payroll might land on the 1st and the 15th, rent on the 1st, and your biggest customer might pay on the 28th. On a monthly view, it all nets out fine. On a weekly view, you can see that the second week of the month is $4,000 short.
Thirteen weeks is one quarter. Beyond that, guesses get fuzzy for most small businesses. Inside it, you usually know which invoices are out, which bills are due, and when payroll runs. That makes the forecast grounded in real data, not hope.
What you need before you start
Gather these first. If your books are reconciled, most of it is a few clicks away.
- Today's actual bank balance, from a reconciled account, not the number on the banking app that includes pending items
- Your AR aging report, so you know who owes what and how late they usually pay
- Your AP aging report, so you know which bills are due and when
- Payroll dates and amounts, using the total that actually leaves your account each run
- Fixed costs: rent, loan payments, insurance, software subscriptions
- Known one-offs: equipment purchases, annual renewals, holiday bonuses, owner draws
How to build the forecast, step by step
Step 1: Set the starting balance
Use the reconciled balance of your operating account as of the Monday you start. If you run several accounts, either combine them or forecast only the one that pays the bills.
Step 2: Forecast cash in, by week
Go invoice by invoice through your AR aging. Don't assume customers pay on the due date. Assume they pay when they usually pay. If a client always pays Net 30 invoices on day 41, put the cash in the week of day 41.
Then add expected new sales you'll invoice and collect inside the window. Be conservative. A forecast that's too optimistic is worse than none, because it makes you feel safe.
Step 3: Forecast cash out, by week
Payroll, rent, loan payments and subscriptions are easy because they repeat. Supplier bills come from your AP aging. Then add one-offs. Annual insurance, a vehicle registration, a holiday bonus. These are the ones that ambush people.
Step 4: Calculate net and closing balance
For each week: opening balance + cash in − cash out = closing balance. The closing balance becomes next week's opening balance.
Step 5: Draw a line at your minimum
Pick the lowest balance you're comfortable holding. Many owners use one payroll cycle plus rent. Any week that dips below that line needs a plan.
A worked example
Jasmine runs a four-person landscaping and property maintenance company outside Charlotte. She builds her forecast on Monday, Oct 12, 2026. Her reconciled checking balance that morning is $16,200. Payroll runs every other week at $6,800. Rent on her yard and equipment storage is $4,500, paid every fourth week. Her commercial insurance renews in the week of Dec 7, 2026, at $9,800 for the year.
Her minimum comfortable balance is $15,000.
| Week | Week of | Opening | Cash in | Payroll | Rent | Suppliers & other | One-off | Net | Closing |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Oct 12, 2026 | $16,200 | $9,400 | $6,800 | $0 | $3,100 | $0 | −$500 | $15,700 |
| 2 | Oct 19, 2026 | $15,700 | $7,800 | $0 | $4,500 | $2,700 | $0 | $600 | $16,300 |
| 3 | Oct 26, 2026 | $16,300 | $10,200 | $6,800 | $0 | $3,600 | $0 | −$200 | $16,100 |
| 4 | Nov 2, 2026 | $16,100 | $6,900 | $0 | $0 | $2,900 | $0 | $4,000 | $20,100 |
| 5 | Nov 9, 2026 | $20,100 | $8,800 | $6,800 | $0 | $3,200 | $0 | −$1,200 | $18,900 |
| 6 | Nov 16, 2026 | $18,900 | $6,100 | $0 | $4,500 | $2,800 | $0 | −$1,200 | $17,700 |
| 7 | Nov 23, 2026 | $17,700 | $11,300 | $6,800 | $0 | $3,400 | $0 | $1,100 | $18,800 |
| 8 | Nov 30, 2026 | $18,800 | $8,200 | $0 | $0 | $2,600 | $0 | $5,600 | $24,400 |
| 9 | Dec 7, 2026 | $24,400 | $6,400 | $6,800 | $0 | $3,100 | $9,800 | −$13,300 | $11,100 |
| 10 | Dec 14, 2026 | $11,100 | $9,700 | $0 | $4,500 | $2,900 | $0 | $2,300 | $13,400 |
| 11 | Dec 21, 2026 | $13,400 | $10,900 | $6,800 | $0 | $3,300 | $0 | $800 | $14,200 |
| 12 | Dec 28, 2026 | $14,200 | $8,600 | $0 | $0 | $2,700 | $0 | $5,900 | $20,100 |
| 13 | Jan 4, 2027 | $20,100 | $9,100 | $6,800 | $4,500 | $3,000 | $0 | −$5,200 | $14,900 |
| Total | $16,200 | $113,400 | $47,600 | $18,000 | $39,300 | $9,800 | −$1,300 | $14,900 |
Check the math: $16,200 + $113,400 − $47,600 − $18,000 − $39,300 − $9,800 = $14,900. That's the week 13 closing balance, so the table ties out.
What Jasmine learns from it
Over the quarter, she's roughly break-even on cash. That alone is useful. The real news is week 9. The insurance renewal lands in a payroll week, and her balance drops to $11,100, which is $3,900 below her floor. She stays under the line through week 11 and slips under again in week 13.
Without the forecast, she'd find out on Dec 8, 2026, when the insurance draft cleared. With it, she has eight weeks to fix it. Her options:
- Ask the insurer about monthly or quarterly installments instead of one annual payment
- Chase the two property managers whose invoices usually run 45 days, so their Nov 2026 payments land before week 9
- Push a planned $2,000 mower purchase into January 2027
- Line up a small credit line now, while the numbers look healthy, rather than in a panic
Any one of these closes most of the gap. Two of them close all of it.
The weekly routine that keeps it honest
A forecast you build once and never touch is a nice spreadsheet. A forecast you update every Monday is a management tool. Here's a 20-minute routine:
- Reconcile last week. Match the bank feed to your books so the opening balance is real.
- Replace forecast with actuals. Swap last week's guesses for what really happened.
- Note the misses. If a customer was late, push their cash to a later week and adjust your assumptions about them.
- Add week 14. Drop the week that just ended and add a new one at the end. That's what makes it "rolling".
- Check the floor. Any week below your minimum gets an action and an owner.
Over a few months, you'll notice patterns. Maybe your forecast always overestimates collections by 10%. Good. Now you know to trim.
Common mistakes
| Mistake | Why it hurts | Fix |
|---|---|---|
| Starting from an unreconciled balance | Every week is off by the same unknown amount | Reconcile first, forecast second |
| Using invoice due dates for cash in | Customers rarely pay exactly on time | Use each customer's real payment habit |
| Forgetting annual and quarterly costs | They cause the sharpest dips | List them once and keep the list |
| Mixing profit and cash | Depreciation and accruals aren't cash | Only include money that moves |
| Leaving out owner draws and loan principal | They're real cash out | Add them as their own lines |
| Building it once | It's stale within two weeks | Update every Monday |
How HelloBooks helps
HelloBooks doesn't build the 13-week forecast for you, but it supplies the inputs that make one accurate. Bank feeds keep your actual balance current (you can connect most US banks and credit cards, or import a CSV statement), and bank reconciliation makes sure the starting number is one you can trust: the reconcile screen suggests a match for each statement line, and the reconciliation report shows opening balance, cleared items, outstanding items and closing balance, exportable as PDF or CSV. The AR aging and AP aging reports, included on Free, give you the invoice-by-invoice and bill-by-bill detail for cash in and cash out.
On Starter, you can export reports to Excel, which saves retyping the aging lists into your forecast sheet. On Pro, AI Analysis runs on every report, and bills & approvals give you a clearer view of what's committed. You can also see your history in the Cash Flow report to check how close last quarter's forecast came. More on the cash side is on the cash flow management page.
FAQs
What's the difference between a 13-week forecast and a budget?
A budget is a plan for revenue and expenses, usually monthly and on an accrual basis. A 13-week forecast tracks only cash, week by week, and changes every week as reality comes in. You can have both. They answer different questions.
Should I include sales I haven't made yet?
Yes, but carefully. Include likely sales from regular customers and contracts you've signed. Leave out wishful pipeline. If you're unsure, run a second version with no new sales to see your worst case.
How accurate should a 13-week forecast be?
The first few weeks should be close, within a few percent, because they're mostly known invoices and bills. Weeks 10 to 13 will be rougher. That's fine; you'll update them as they get closer.
Can I use a spreadsheet for this?
Yes. Most small businesses do. Keep it simple: one row per week, the columns in the example above, and a formula that carries each closing balance to the next opening balance.
What should I do if the forecast shows a shortfall?
Look at the levers in order: speed up collections, delay non-urgent purchases, ask suppliers or insurers for installment terms, then consider financing. If the gap is large or keeps coming back, sit down with your bookkeeper or CPA to look at pricing and margins.
Who should own the forecast?
One person. In a small business that's often the owner or the bookkeeper. Shared ownership usually means nobody updates it.
Thirteen rows, one Monday habit, and you'll rarely be surprised by your bank balance again.
Start free, no credit card. Try HelloBooks Free
