How do I make a cash flow forecast for a US small business?

GuideUSChecked 30 October 20267 primary sources7 min readUK version

The short answer

For a US small business, build a 13-week cash flow forecast: start from today's bank balance, put each customer payment in the week it will really arrive, and list every payment out, including payroll, payroll tax deposits, sales tax returns and quarterly estimated tax. Update it weekly, and act early on the week cash runs lowest.

What is a 13-week cash flow forecast?

A week-by-week view of the next quarter's cash, rolled forward every week. It shows the lowest cash point and the week it falls, which a profit and loss report cannot.

Profitable businesses run short because the timing is out of step: payroll goes out every week or two and suppliers want paying on their terms, while customers on net 30 or net 60 terms pay a month or two after the invoice, and construction retainage waits for completion. The forecast puts those dates side by side. Set a floor you will not go below, such as one payroll.

What goes in it?

Every cash movement, on the date it happens, not the date it is earned or billed:

  • Opening cash: the bank balance today.
  • Receipts: each invoice in the week you expect the money, based on how that customer actually pays, plus deposits and card payouts. Deposits and gift cards are cash now with work still owed.
  • Payroll on each pay date, and payroll tax deposits: monthly depositors pay by the 15th of the next month; semiweekly depositors on a Wednesday or Friday after each payday (IRS Publication 15).
  • Sales tax collected, paid over on your state's schedule, such as the 20th of the next month for monthly Texas filers. It was never your income.
  • Quarterly estimated tax for the owners: April 15, June 15 and September 15, 2026, and January 15, 2027 (IRC sec. 6654).
  • Everything else: rent, suppliers, insurance installments and the workers' comp audit, loan payments, owner draws, and wage rises on the dates they take effect. As of Oct 2026.

How do I build it from my books?

Start from QuickBooks or Xero, not from memory:

  1. Take the bank balance and the aged receivables report.
  2. Place each open invoice in the week that customer's payment history says it will clear. Days sales outstanding (receivables divided by annual revenue, times 365) tells you if customers are slowing.
  3. Add bills from payables, the payroll register and your tax calendar.
  4. Each week, compare actuals with the forecast, add a new week thirteen, and ask why any line was wrong.

Keep a separate account for tax money, and ask your CPA to set the estimated tax figures from your last return.

What does one look like?

An example Texas service business with $30,000 in the bank on Monday, November 2, 2026. Assumed, indicative and As of Oct 2026: $9,500 a week out for net payroll ($5,000) and other costs ($4,500); payroll tax deposits of $7,000 a month as a monthly depositor, because it reported $50,000 or less in its lookback period, before it grew; Texas sales tax of $2,400 a month as a monthly filer; and a $6,000 fourth estimated tax payment on January 15, 2027.

WeekCash inCash outClosing cashWhat lands
1$14,000$9,500$34,500
2$11,000$9,500$36,000
3$9,000$18,900$26,100October payroll tax deposit and sales tax
4$12,000$9,500$28,600
5$9,000$9,500$28,100
6$10,000$9,500$28,600
7$8,000$16,500$20,100November payroll tax deposit
8$7,000$11,900$15,200November sales tax
9$4,000$9,500$9,700Holiday week, slow payers
10$8,000$9,500$8,200
11$9,000$22,500-$5,300December payroll tax deposit and estimated tax
12$16,000$11,900-$1,200December sales tax
13$28,000$9,500$17,300A large invoice is paid

The quarter ends $12,700 down but still positive, at $17,300. Week 11 is the problem: two tax payments land in the same week as slow post-holiday receipts, and the account goes $5,300 overdrawn. Holding a floor of one week's net payroll ($5,000) needs $10,300 more by week 11, and the forecast shows it ten weeks ahead.

What are the warning signs?

  • The low point falls below your floor, or moves closer every week.
  • Receivables grow faster than sales, so days sales outstanding rises.
  • One or two customers make up most of the receipts.
  • Deposits or gift card money have been spent before the work is done.
  • You are tempted to hold back a payroll tax deposit. Never do: withheld taxes are trust funds, and the IRS can assess 100% against any responsible person who willfully fails to pay them (IRC sec. 6672). As of Oct 2026.

What do I do when the forecast shows a gap?

Fix the timing first, then borrow if you must, and borrow before the gap rather than in it.

  1. Collect: chase the invoices due before the low week (see how to collect an unpaid invoice).
  2. Change terms: shorter payment terms, progress billing or billing in advance, and deposits where the law allows. California caps a home improvement down payment at 10% of the price or $1,000, whichever is less (Bus. and Prof. Code sec. 7159.5(a)(3)). As of Oct 2026.
  3. Move spending that can wait until after the low week.
  4. Finance: a line of credit, often with a personal guarantee and priced from prime or SOFR plus a margin; a borrowing base on receivables, or factoring for slow commercial payers; equipment financing; or an SBA 7(a) loan of up to $5 million. As of Oct 2026.

Compare the cost of quick money first: a $30,000 merchant cash advance at a factor of 1.35 costs $10,500 for about 7 months' money (indicative, As of Oct 2026). Have your CPA check the tax lines and read the lender's term sheet before you sign.

Greymill

How Greymill helps

Cash & Profit covers cash, profit, prices and forecasts: whether the numbers work. Connect QuickBooks or Xero and it reads your books and shows the gap before it bites. Ask it to build a cash flow forecast and you get the work itself, such as a 13-week cash flow spreadsheet, drafted for you to check; every plan includes spreadsheet models. One of the example suggested tasks is cash for the next 13 weeks, before the tax bill lands.

The specialists share one memory: tell Daily Operations about a new supplier and Cash & Profit has it in your cash flow. Loans & Funding compares your finance options and gets you ready to talk to a lender, and Writing drafts the chase emails, waiting for you to send.

Greymill only changes your books when you press Approve and never makes a payment, nothing is sent unless you press Send, and conversations are not used to train AI. It owes no professional duty and says when your CPA must sign something off. Try it free for 7 days on full Scale access, no card needed; plans from $130 a month plus tax. See pricing and how it works.

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Questions owners ask

Why 13 weeks?

Thirteen weeks is one quarter: long enough to show three months of payroll tax deposits and sales tax returns, and short enough to forecast each invoice by the week it will be paid. Roll it forward a week at a time.

Should my cash flow forecast include sales tax?

Yes. Sales tax you collect belongs to the state and goes out on your state's schedule, such as the 20th of the next month for monthly Texas filers. Showing it as cash you can spend overstates your cash. As of Oct 2026.

How often should I update a cash flow forecast?

Every week. Replace last week's forecast with actuals, add a new week at the end, and check why any line was wrong, especially customer receipts.

Can I skip a payroll tax deposit to cover a short week?

No. Withheld payroll taxes are trust funds, and the IRS can assess 100% of them against any responsible person who willfully fails to pay (IRC sec. 6672). Late deposits also carry penalties. Find the cash elsewhere. As of Oct 2026.

Sources

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