Most owners check the bank balance, see a number that looks fine, and find out three weeks later that payroll and a big vendor bill land in the same week. A 13-week cash flow forecast fixes that. It takes about an afternoon, and you don't need to be good with spreadsheets.
What a 13-week forecast is, and why 13
It's a week-by-week list of the cash you expect to come in and go out over the next quarter. Not profit. Not what your accountant's reports say. Just cash, on the week it actually moves.
Thirteen weeks is about one quarter. That's far enough out to see trouble coming and close enough that your guesses are still decent. A yearly budget tells you where you want to go. This tells you whether you can make payroll on the 14th.
You will be wrong in places. That's fine. The goal is not a perfect prediction. The goal is to see tight weeks early enough to do something about them.
Step 1: Gather what you need (30 to 45 minutes)
Pull these together before you open a spreadsheet:
- Your current bank balance, across all operating accounts
- The last two or three months of bank and card statements
- Open invoices you've sent but haven't been paid on, with due dates
- Bills you owe, with due dates
- Payroll schedule and typical amount, including the employer-side costs your payroll provider shows
- Recurring payments: rent, insurance, loan or lease payments, software, utilities, equipment
- Any known one-offs: a tax payment, an annual insurance premium, a delivery of inventory, a seasonal slowdown
For tax dates and payroll tax deposit schedules, ask your accountant or payroll provider when yours fall. Don't guess those.
Step 2: Set up the sheet (20 minutes)
Open a blank spreadsheet. Put the weeks across the top: Week 1 through Week 13. Under each, write the date the week starts, such as the Monday.
Down the left side, make these rows:
- Beginning cash
- Total cash in
- Total cash out
- Ending cash
Below those, add detail rows. Under cash in, list your sources: customer payments, card or online sales deposits, deposits on jobs, anything else. Under cash out, list payroll, rent, inventory or materials, vendor bills, taxes, insurance, loan or lease payments, owner pay, and an "other" line.
Now add two formulas. Say your totals sit in rows 2, 3 and 4, with beginning cash in row 1 and ending cash in row 5. Then:
- Ending cash = beginning cash + total in − total out
- Next week's beginning cash = this week's ending cash
Set Week 1 beginning cash to today's real bank balance. Copy the formulas across all 13 weeks. Each total row should sum the detail rows beneath it.
Step 3: Fill in cash coming in (45 minutes)
This is where most first forecasts go wrong. Owners enter sales on the day they make them. Put cash in on the day you expect to receive it.
- Invoices you've already sent. Place each one in the week you realistically expect payment, not the due date. If a customer always pays ten days late, use that.
- Card and online sales. Processors usually deposit a day or two after the sale, so a Friday-through-Sunday rush may hit your bank the following week. Check your own deposit pattern on your statements.
- Future sales. Look at the same weeks in recent months. For a steady business, an average week is fine. For a seasonal one, use your real pattern.
When unsure, be a little pessimistic. Say you normally bring in $40,000 a month. Forecasting $9,000 a week feels right, but if the last three months show $8,000 to $9,500, use $8,500 and see how it looks.
Step 4: Fill in cash going out (45 minutes)
Start with fixed items, which are the easy ones. Rent goes in the week it's due. Payroll goes in each pay week, including the extra weeks in months with a third pay date if you pay every other week. Insurance, software and equipment payments go on their due dates.
Then the variable ones. Look at what you spent on inventory, materials or supplies over the last few months and spread it across the weeks. If a supplier requires payment up front for an order you know is coming, put it in the week you'll pay.
Then the ones people forget:
- Quarterly or annual bills, such as insurance premiums and licenses
- Sales tax, payroll tax deposits and estimated tax payments, on the dates your accountant gives you
- Owner pay or draws
- Repairs you know are coming
- Card payments if you carry a balance
A useful check: add up your 13 weeks of cash out and compare it to the last 13 weeks on your statements. If your forecast is far lower, you've probably missed something.
Step 5: Read it, and decide what "negative" means
Look at the ending cash row. Where does it dip? How low?
A week that goes below zero is the obvious alarm. But set a second line: a cash floor. Pick a number you never want to drop under, such as one payroll plus your rent. Say yours is $8,000. A week that ends at $3,000 isn't negative, but it breaks your floor and deserves attention.
Here's a tiny example. Say you start with $12,000. Week 1: $9,000 in, $10,500 out, so you end at $10,500. Week 2: $7,500 in, $14,000 out because payroll and a supplier bill land together, so you end at $4,000. That's under the $8,000 floor, and you know it today, not on the day the payroll runs.
What to do when a week goes negative
Work through these in order, from cheapest to most involved.
1. Check the forecast
Is a bill in the wrong week? Did you double count? Did you assume a customer would pay on a day they never do? Fix errors first.
2. Pull cash in sooner
- Send invoices the day work is done, not at month end
- Call the customers with the largest open balances before the tight week
- Ask for deposits or progress payments on bigger jobs
- Offer an easy way to pay, such as card or bank transfer
3. Push cash out later, by asking
Call vendors before a bill is due, not after. A simple script: "I'm planning my cash for the next few weeks and I'd like to pay this on the 20th instead of the 10th. Can we do that, and is there anything you need from me?" Many vendors would rather agree to a date than chase a late payment. Not all will, so get it confirmed in writing.
4. Trim or move spending
Can an equipment purchase wait two weeks? Can an inventory order be split? Can owner pay shift for one cycle? Small moves across several lines often close a gap.
5. Only then, look at outside funding
If the gap is real and the first four steps don't cover it, a short-term funding option may be worth a look. Structures differ: some are loans, some are purchases of future sales, and repayment can be fixed, daily, weekly or tied to sales. Ask for the total cost in dollars, how and how often it's repaid, and what it does to your weekly cash. Then put that repayment into your forecast and see whether the gap really closes or just moves. A Full Port advisor can walk through options with you at no cost to you.
Keep it alive: 15 minutes every week
A forecast you build once and forget is a snapshot. Make it a Monday habit:
- Replace last week's forecast with what actually happened
- Update your bank balance in the beginning cash cell
- Add a new Week 13 so you always see a full quarter ahead
- Adjust any weeks that have changed
- Check the lowest ending cash and ask whether it's under your floor
After a month or two, you'll see where your guesses run off, like customers who pay later than you think. Your forecast gets better each time you correct it.
The short version
- A 13-week forecast tracks cash on the day it moves, not when you earn or owe it.
- Build it in four rows: beginning cash, cash in, cash out, ending cash, with each week starting where the last one ended.
- Be realistic about when customers actually pay, and don't forget tax, insurance and owner pay.
- If a week dips under zero or under your floor, fix errors, speed up collections, ask vendors to shift dates, and trim spending before looking at funding.
- Update it for 15 minutes every week so it always shows the next 13.