How to Build a 13-Week Cash Flow Forecast (Step by Step)
A 13-week cash flow forecast is the single most important financial tool for any company between $5M and $100M. It answers the only question that actually kills companies: will we run out of cash?
Why 13 Weeks?
Thirteen weeks is one quarter. It’s long enough to see trends and short enough to be accurate. Weekly granularity catches timing issues that monthly forecasts miss, like the week when payroll, rent, and a vendor payment all hit simultaneously.
Building the Forecast
Step 1: Start with your bank balance
Today’s actual cash balance. Not projected, not adjusted, the real number.
Step 2: Map your cash inflows by week
Expected customer payments (use your AR aging), any committed new deals, recurring revenue collections, and other income. Be conservative, use historical collection patterns, not invoice dates.
Step 3: Map your cash outflows by week
Payroll (including taxes), rent, vendor payments (use your AP aging), loan payments, and any committed capital expenditures. Don’t forget quarterly payments like insurance, taxes, and annual subscriptions.
Step 4: Calculate weekly ending balance
Starting cash + inflows − outflows = ending cash. Each week’s ending balance becomes the next week’s starting balance.
Step 5: Set your floor
Define the minimum cash balance you’re comfortable with. Two months of operating expenses is a reasonable floor. If the forecast shows you dropping below this level, you have lead time to act, cut costs, accelerate collections, or arrange financing.
A Worked Example (Illustration)
Illustration only. The company and every number below are made up, in round figures, to show how the five steps fit together. This is not a client and not a result.
The example is an online retailer that sells through its own store, one marketplace and a few wholesale accounts. Amounts are in thousands of dollars ($K).
Step 1: starting cash. The bank balance on the Monday of week 1 is $700K.
Step 2: inflows, in the week the cash arrives.
- Online store: card payments are paid out within days, so each week’s sales arrive that week. $150K a week, rising to $180K from week 10.
- Marketplace: pays out every two weeks. $100K in weeks 2, 4, 6, 8, 10 and 12.
- Wholesale: three invoices, each placed in the week that customer has usually paid, not on the invoice date. $70K in week 4, $90K in week 8 and $70K in week 12.
Step 3: outflows, in the week the cash leaves.
- Inventory: supplier payments of $300K in week 2, $550K in week 6, $300K in week 10 and $300K in week 12.
- Freight and duties: $40K in week 7, $30K in week 10 and $20K in week 13.
- Payroll, including payroll taxes: $60K every two weeks (weeks 1, 3, 5, 7, 9, 11 and 13).
- Rent: $30K in the first week of each month (weeks 1, 5 and 9).
- Marketing: $30K every week.
- Software and overhead: $30K a month (weeks 2, 6 and 10).
- Loan payment: $20K a month (weeks 4, 8 and 12).
- Sales tax: $25K a month (weeks 3, 7 and 11).
- Quarterly items: insurance $40K in week 5, estimated income tax $60K in week 11.
Step 4: weekly ending balance. Starting cash + inflows − outflows = ending cash, and each week’s ending balance is the next week’s starting balance.
| Month 1: weeks 1–4 (illustration, $K) | Wk 1 | Wk 2 | Wk 3 | Wk 4 |
|---|---|---|---|---|
| Starting cash | 700 | 730 | 620 | 655 |
| Online store | 150 | 150 | 150 | 150 |
| Marketplace payouts | – | 100 | – | 100 |
| Wholesale payments | – | – | – | 70 |
| Total inflows | 150 | 250 | 150 | 320 |
| Inventory | – | 300 | – | – |
| Freight and duties | – | – | – | – |
| Payroll and payroll taxes | 60 | – | 60 | – |
| Rent | 30 | – | – | – |
| Marketing | 30 | 30 | 30 | 30 |
| Software and overhead | – | 30 | – | – |
| Loan payment | – | – | – | 20 |
| Sales tax | – | – | 25 | – |
| Insurance (quarterly) | – | – | – | – |
| Estimated income tax (quarterly) | – | – | – | – |
| Total outflows | 120 | 360 | 115 | 50 |
| Net cash flow | 30 | (110) | 35 | 270 |
| Ending cash | 730 | 620 | 655 | 925 |
| Above / (below) the $600K floor | 130 | 20 | 55 | 325 |
| Month 2: weeks 5–8 (illustration, $K) | Wk 5 | Wk 6 | Wk 7 | Wk 8 |
|---|---|---|---|---|
| Starting cash | 925 | 915 | 555 | 550 |
| Online store | 150 | 150 | 150 | 150 |
| Marketplace payouts | – | 100 | – | 100 |
| Wholesale payments | – | – | – | 90 |
| Total inflows | 150 | 250 | 150 | 340 |
| Inventory | – | 550 | – | – |
| Freight and duties | – | – | 40 | – |
| Payroll and payroll taxes | 60 | – | 60 | – |
| Rent | 30 | – | – | – |
| Marketing | 30 | 30 | 30 | 30 |
| Software and overhead | – | 30 | – | – |
| Loan payment | – | – | – | 20 |
| Sales tax | – | – | 25 | – |
| Insurance (quarterly) | 40 | – | – | – |
| Estimated income tax (quarterly) | – | – | – | – |
| Total outflows | 160 | 610 | 155 | 50 |
| Net cash flow | (10) | (360) | (5) | 290 |
| Ending cash | 915 | 555 | 550 | 840 |
| Above / (below) the $600K floor | 315 | (45) | (50) | 240 |
| Month 3: weeks 9–13 (illustration, $K) | Wk 9 | Wk 10 | Wk 11 | Wk 12 | Wk 13 |
|---|---|---|---|---|---|
| Starting cash | 840 | 870 | 760 | 765 | 765 |
| Online store | 150 | 180 | 180 | 180 | 180 |
| Marketplace payouts | – | 100 | – | 100 | – |
| Wholesale payments | – | – | – | 70 | – |
| Total inflows | 150 | 280 | 180 | 350 | 180 |
| Inventory | – | 300 | – | 300 | – |
| Freight and duties | – | 30 | – | – | 20 |
| Payroll and payroll taxes | 60 | – | 60 | – | 60 |
| Rent | 30 | – | – | – | – |
| Marketing | 30 | 30 | 30 | 30 | 30 |
| Software and overhead | – | 30 | – | – | – |
| Loan payment | – | – | – | 20 | – |
| Sales tax | – | – | 25 | – | – |
| Insurance (quarterly) | – | – | – | – | – |
| Estimated income tax (quarterly) | – | – | 60 | – | – |
| Total outflows | 120 | 390 | 175 | 350 | 110 |
| Net cash flow | 30 | (110) | 5 | 0 | 70 |
| Ending cash | 870 | 760 | 765 | 765 | 835 |
| Above / (below) the $600K floor | 270 | 160 | 165 | 165 | 235 |
| The quarter by month (illustration, $K) | Month 1 (wks 1–4) | Month 2 (wks 5–8) | Month 3 (wks 9–13) | Quarter |
|---|---|---|---|---|
| Starting cash | 700 | 925 | 840 | 700 |
| Total inflows | 870 | 890 | 1,140 | 2,900 |
| Total outflows | 645 | 975 | 1,145 | 2,765 |
| Net cash flow | 225 | (85) | (5) | 135 |
| Ending cash | 925 | 840 | 835 | 835 |
| Lowest weekly ending cash | 620 (wk 2) | 550 (wk 7) | 760 (wk 10) | 550 (wk 7) |
The quarter in one line: $700K starting cash + $2,900K in − $2,765K out = $835K ending cash.
Step 5: the floor. A typical month of operating expenses in this example is $300K: two payrolls $120K, rent $30K, four weeks of marketing $120K, software and overhead $30K. Two months is $600K, so that is the floor.
What the forecast shows. Over the quarter cash rises from $700K to $835K, and every month ends above the floor ($925K, $840K, $835K). A monthly forecast would show no problem. The weekly view does: the $550K supplier payment in week 6 is followed in week 7 by payroll, sales tax and freight, in a week with no marketplace payout. Cash falls to $555K in week 6 and $550K in week 7, $50K under the floor. It recovers in week 8, when the marketplace payout and a wholesale payment arrive.
What to do with five weeks of notice. Two options change timing only, and either one keeps cash above the floor every week:
- Collect earlier. Bring the $90K wholesale payment forward from week 8 to week 6. Cash is $645K in week 6 and $640K in week 7.
- Pay later. Ask the supplier to take $450K in week 6 and the remaining $100K in week 8. Cash is $655K in week 6 and $650K in week 7.
In both cases the totals for the quarter are the same and the lowest balance of the quarter becomes $620K (week 2). Doing both leaves $140K of room above the floor in week 7.
| Ending cash (illustration, $K) | Wk 5 | Wk 6 | Wk 7 | Wk 8 |
|---|---|---|---|---|
| As forecast | 915 | 555 | 550 | 840 |
| With the wholesale payment collected in week 6 | 915 | 645 | 640 | 840 |
| With the supplier payment split | 915 | 655 | 650 | 840 |
| With both | 915 | 745 | 740 | 840 |
Collecting earlier is the “accelerate collections” option from Step 5. The others named there are cutting costs and arranging financing before it is needed.
The Monday Morning Ritual
Every Monday: update actual results for last week, roll the forecast forward one week, and review the next 4 weeks for any concerns. This takes 15-20 minutes once the model is built. It’s the highest-ROI 20 minutes of your week.
For a real example, see how a $12M e-commerce brand used a rolling 13-week cash flow forecast, reviewed by the founder every Monday morning.
Do This Monday
- Open a spreadsheet. 15 columns: labels, 13 weeks and a total. Rows: starting cash, each inflow source, each outflow category, ending cash.
- Fill in everything you know with certainty: payroll dates, rent, committed payments. Then layer in expected collections from your AR aging.
- Set a recurring Monday morning calendar block: ‘Cash Flow Forecast Update.’ 20 minutes. Non-negotiable.
For ongoing cash management beyond the forecast, see our Cash Flow Management services, we build and maintain these systems for growing companies every day.
If you want help building a cash flow forecasting system, book a free consultation →
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