How to forecast business cash flow — a 13-week method that fits a small business.
A repeatable, plain-English method for turning a bank CSV into a 91-day runway you update every Monday — plus a free CashWise forecast that runs the model in your browser.
Current Balance
$48,200
90-day
30-day Net Change
+$3,940
inflows − outflows
Inflows vs Outflows
14 in · 38 out
last 30 days
Daily net · last 12 weeks
USD · localStorage
Forecast · 90-day projection
+ $4,810History + next 90 days · runway $42,300
Most small businesses don't go broke from a bad month — they go broke from a bad quarter they didn't see coming. A 13-week cash flow forecast is the simplest way to widen the lens from "what does the bank balance look like today" to "what does the next quarter look like, week by week, with my real recurring items layered in." It is forward-looking cash visibility, not a categorised budget cap, and the gap between those two ideas is what catches owners off guard. Once a 91-day runway is on the page, the conversation stops being about cutting the next invoice and starts being about which week the cushion actually thins out.
The reason 13 weeks is the right horizon is structural. It covers a full payroll cycle plus rent, it covers one quarterly tax payment, and it covers a seasonal dip — long enough to actually do something about a shortfall, short enough that the inputs don't drift. A 30-day view hides quarterly obligations; a 52-week view loses its assumptions to drift by month three. 13 weeks sits in the band where the math is honest and the actions are still useful.
The good news is the inputs are the rows you have already paid. Payroll, rent, software/SaaS, the monthly bookkeeping fee, the quarterly tax escrow — these show up in your bank CSV every cycle, and a forecast just connects yesterday's recurring items to next quarter's calendar. You do not need a forecast built from guesses about future revenue; you need a forecast built from a bank export and a calendar.
CashWise AI runs that workflow locally in your browser. Drop in a CSV from Chase, Bank of America, Wells Fargo, or any generic Date / Description / Amount file; every row gets auto-tagged (Payroll, Rent, Software/SaaS, Revenue, Fees); the dashboard computes its KPIs locally and renders a 91-day runway with the low-water mark called out. The free tier stays free — no credit card, no trial countdown — and you can sign up free to bring your own CSV or open the seeded demo forecast to see the dashboard first.
The 13-week forecast is the same shape whether you are a solo founder, a twelve-person team, or the client list of a multi-entity bookkeeper. The inputs scale — more rows, more payroll lines, more quarterly tax escrows — but the four-to-eight-week lookout window stays the same. A forecast built from your own bank data beats a forecast built from a quarterly profit-and-loss snapshot every time, because the bank export is what actually moves money. The P&L is the retrospective. The forecast is the early warning, and a 91-day window is enough time to act on it before the cash actually leaves the account.
The 6-step method
How to forecast business cash flow in 13 weeks.
The same sequence works for a solo founder with one bank account and a 12-person shop with three. Each step has a single output you can hand to a bookkeeper or paste into a tool.
1. Pick the 13-week window that starts on a Monday.
Anchor the forecast to your payroll calendar. If payroll runs on the 1st and 15th, your 13-week window should start one week before the first payroll it includes so you see the balance before and after the outflow.
2. Pull a 90-day bank CSV — the rows you have already paid.
Chase, Bank of America, Wells Fargo, and most other US banks export a generic Date / Description / Amount CSV. The CSV is the input. The forecast engine just extends the recurring items forward by their cadence.
3. Classify every row into recurring or one-off.
Recurring rows are anything with a fixed cadence — payroll, rent, software, insurance, loan payments, the monthly bookkeeping fee. One-offs land in a separate column so they don't pollute the forward projection.
4. Stress-test three scenarios: flat, hiring, slowdown.
Lay a flat scenario on top of the actuals, then a hiring scenario (one new salary a quarter from now), then a slowdown scenario (one large invoice pays 30 days late). The shape of the runway tells you which lever is most exposed.
5. Set a Monday-morning cadence and an alert threshold.
Pick a day, pick a time, pick a threshold (e.g. "alert if any week in the next 8 dips below $5,000"). The cadence is more important than the model — a stale forecast is worse than no forecast.
6. Choose a tool only when the spreadsheet stops fitting in one tab.
A DIY spreadsheet can carry a 13-week forecast for a single account — see the comparison below for when the trade-off flips. When you outgrow it, open the CashWise demo to see the same method running in your browser.
DIY vs. tool
When a spreadsheet stops being the right tool.
A DIY forecast is genuinely free. The cost shows up as the rows grow, the team gets a second laptop, and the model stops fitting in one tab. The table below compares four common paths so you can pick the one that matches your business today, not the one you hope it becomes in six months.
| How you forecast | DIY spreadsheet | Online Excel template gallery | Forecast spreadsheet tool | Cash flow forecast app |
|---|---|---|---|---|
| Setup time | ~2-3 hours | ~1-2 hours | ~30 minutes | Under 60 seconds — drop in a CSV |
| Ongoing maintenance | Full manual — every row, every week | Manual with template scaffolding | Mostly manual, formulas do the lifting | Auto-tagged from CSV; Monday digest email |
| Accuracy over time | Drifts as formulas break | Drifts unless you keep updating | Stable if formulas are kept current | Improves with each imported CSV — longer history, tighter runway |
| Multi-device sync | None — file lives on one laptop | Manual export / email | Cloud file or shared link | Browser sync on free; server-side on Pro |
| Scenario testing | Possible — copy sheet, edit assumptions | Possible — usually one scenario per template | Possible with structured inputs | Built-in: hiring, price hike, slowdown |
| Weekly digest email | No | No | Only if you build it | Yes — Monday at 7 AM local |
| Shortfall alerts | No | No | Conditional formatting only | Yes — when any week in the next 91 days dips below your threshold |
| Pricing | Free (your time) | Free / one-time | $10-40/month | Free forever — upgrade to Pro at $39/month |
The right answer depends on the size of the recurring-item list, not the size of the bank balance. A solo founder with five rows is fine on a spreadsheet; a 20-person team with 200 rows is not.
Built for the workflow
What the 13-week method looks like running.
Current balance
$48,200
Projected by end of week
$53,010
Next 4 weeks
Move to reserve
$1,420
Stripe payouts today cover payroll and rent with $1,420 of headroom. Move it to the reserve bucket so the Aug 15 quarterly tax estimate doesn't catch the client off guard.
YTD streak
8 of last 8 weeks swept — reserve buffer is locked in.
FAQ
Common "how to forecast" questions.
How far in advance should I forecast business cash flow?
Most small businesses that run a recurring 13-week cash flow forecast catch shortfalls 4-8 weeks before they would have surfaced from looking at the bank balance alone. A full quarter of forward visibility is enough runway to reschedule a payment, push an invoice, or pause a hire before cash actually gets tight.
How often should I update the forecast?
A weekly cadence is the sweet spot. Anything more frequent than that and you spend more time updating the model than acting on it; anything less and a new invoice or charge can move the low-water mark before you see it. Many owners update on Monday morning and email the digest to themselves and their bookkeeper so the numbers get a second pair of eyes.
What is a 13-week cash flow forecast and why 13 weeks?
A 13-week cash flow forecast is a forward-looking view of inflows vs. outflows across one quarter (roughly 90 days). 13 weeks is long enough to cover payroll and rent cycles, a full quarterly tax payment, and any seasonal dip, but short enough that the inputs — recurring items you have already paid — stay accurate without reassessing every assumption.
Do I need an accountant or bookkeeper to build a cash flow forecast?
No. A small business cash flow forecast can be assembled by the owner from one bank export and a list of recurring items. A bookkeeper adds value once you are stress-testing scenarios weekly and want a second set of eyes on the assumptions — but the first forecast should not wait on hiring one.
What is the difference between a forecast and a budget?
A budget says what you plan to spend by category; a cash flow forecast says when money actually moves in and out of the bank. A budget is a target. A cash flow forecast is a timing model that shows you the date your balance dips below zero, not the cap on a category.
Is a DIY spreadsheet forecast good enough?
For a single account and a handful of recurring items, yes — a simple spreadsheet can carry a 13-week forecast for a few months. The cost shows up as the recurring items grow, the team gets a second laptop, and the spreadsheet stops fitting in one tab. That is the moment most owners switch to a forecast tool, not to remove the discipline but to keep the same discipline as the data scales.
Can I upgrade to Pro later after forecasting on the free tier?
Yes. Free stays free forever. When you outgrow single-device local storage, upgrade to Pro ($39/month) for server-side sync across devices, scheduled CSV uploads, and ex-ante cash-flow insights. Team / multi-user accounts are on the roadmap.
See your 13-week cash runway in 60 seconds.
Open the seeded demo, drop in your own CSV, or sign up free — whichever step fits where you are in the method.