How to forecast cash flow

Most small businesses that fail are profitable on paper — they run out of cash. Profit and cash flow are different numbers, and the gap between them kills more companies than any other single cause. A proper cash flow forecast shows, month by month, whether you can pay bills, meet payroll, and fund growth. This guide walks through building a 13-week forecast, the categories that matter most, and the handful of signals that tell you cash is about to be tight.

Cash flow vs profit — why they diverge

Profit is the income statement: revenue minus expenses, recognized when earned.

Cash flow is the bank account: money in, money out, on the dates it actually moves.

An agency invoices $50k in January (revenue recorded). Client pays in March. Rent, payroll, and SaaS still hit in January and February. On paper January was profitable; in reality the bank account dropped $70k before any cash arrived.

The three accounting things that cause the gap:

Accounts receivable (AR). Revenue billed but not yet collected. Money you’re owed is not cash.

Inventory. Stock you paid for but haven’t sold. Cash out, no revenue yet.

Capital expenditures. Equipment, leasehold improvements — expensed over years on the P&L, but the cash went out in month one.

The 13-week rolling forecast (the standard)

13 weeks is the CFO-standard horizon: long enough to see the next payroll cycle, quarterly tax, and big AR collections; short enough that predictions are still realistic.

Structure:

Row 1: Opening cash balance. Closing cash from the prior week.

Rows 2-N: Cash inflows. AR collections from specific customers, new sales forecast by week, other income (refunds, grants, tax return).

Rows N-M: Cash outflows. Payroll dates, rent, recurring software (by date due), AP payments to vendors, quarterly taxes, debt service.

Closing cash balance. Opening + inflows − outflows. This becomes next week’s opening cash.

Update every Monday. Takes 30 minutes once built. The discipline of updating is the value — forces you to notice when forecasts drift.

Categorize cash flows the right way

Accounting standards split cash flow into three categories — helpful for spotting what’s really happening:

Operating cash flow. The day-to-day business: customer payments in, operating expenses out. Healthy business = positive operating cash flow.

Investing cash flow. Equipment purchases, acquisitions, sales of assets. Usually negative for growing companies (spending on future capacity).

Financing cash flow. Loans in/out, equity raised, dividends or distributions paid.

A business with positive operating cash flow but collapsing total cash is funding growth from the balance sheet. A business with negative operating cash flow and positive financing cash flow is living on the last round — dangerous.

Step 1 — forecast AR collections realistically

The #1 source of cash forecast error is assuming invoices are paid on the due date. They’re often not.

Track your actual DSO (days sales outstanding) = (AR balance / revenue per day). If DSO is 45 days but you invoice on Net-30, your AR forecast should assume payment 15 days late, not on day 30.

Customer-level adjustment: big customers with long AP cycles pay in 60–90 days. Assume that. Small customers often pay faster. Build an aging profile into the forecast, not an average.

Step 2 — commit to payment timing, don’t estimate

Most outflows are known with certainty once you have the data:

Payroll — fixed dates and amounts. Include employer taxes (add ~8–12% to gross payroll).

Rent — 1st or last of the month, fixed.

Software subscriptions — renewal dates are knowable; pull them from your credit card statement and list by date.

Quarterly estimated taxes — April 15, June 15, September 15, January 15. Easy to forget; massive impact when they hit.

Annual costs — insurance, domain renewals, accountant fee for year-end. List them in the month due.

Step 3 — add a sales forecast with honesty discount

Revenue forecasts are where optimism creeps in. Discount discipline:

Signed contracts with clear delivery date: 100% of expected value in the right week.

Verbal commitment or LOI: 60% of value, pushed a month out from when the customer said.

Strong pipeline opportunity: 30% of value, 2 months out.

Top-of-funnel inquiries: don’t include in the forecast at all.

Every founder overestimates conversion speed. The honest discount on sales forecasts is usually 30–50%, not the 10% we want to apply.

Signal #1: The minimum balance watchtower

What’s the minimum cash balance across all 13 weeks? If it dips below 30 days of operating expenses, you’re entering stress.

Below 2 weeks of operating expenses: emergency mode. Start accelerating collections (call AR aging report customers), delay non-critical payments, open the line of credit if you have one.

Target: maintain a minimum cash buffer of at least 60 days of operating expenses. 3–6 months is comfortable. <30 days is a warning state.

Signal #2: Growing AR aging buckets

Pull the AR aging report monthly. It splits what you’re owed by how overdue:

Current — not yet due.

1–30 days late — usually normal.

31–60 days late — investigate. Call customer.

61–90 days late — escalate, consider collection calls or holding deliveries.

90+ days late — unlikely to collect, write off or send to collections.

If the >60-day bucket is growing as a percentage of total AR, your working capital is deteriorating even if top-line revenue is fine.

Signal #3: AP terms being stretched

The inverse of AR: if you’re paying vendors later than your terms (stretching Net-30 to Net-45 or 60), you’re using vendor credit to fund operations — a rough early indicator of tight cash. Unsustainable and relationship-damaging.

If you find yourself doing this regularly, the cash problem is structural and needs a fix (raise, cost reduction, faster collections, terms renegotiation) — not just better timing.

What to cut when cash is tight

Order of cuts, from easiest to hardest:

(1) Discretionary — marketing experiments, travel, catered lunches, new software, hiring pauses.

(2) Deferrable — negotiate longer payment terms with vendors, delay capital purchases, downgrade software tiers.

(3) Headcount reductions — last resort but highest leverage. Better done once decisively than gradually.

Don’t cut things that drive cash in the door (sales people, top performers, existing customer support). Those are your revenue. Cut what doesn’t drive collections.

Run the numbers

Enter monthly inflows and outflows into the cash flow calculator to see your position across months and quarters. Pair with the startup runway calculator for months-to-zero view, and the budget calculator for the same view on personal household cash flow.