$480,000 in the bank. $60,000 average monthly burn. Eight months of runway.
Eight months sounds like a plan. It's actually a rounding error wearing a plan's clothes. Here's what that formula cannot see.
(If you run a services or trades business and “runway” sounds like startup talk — the operators' version is the same math with a different name: which week do I stop making payroll without touching the line of credit? Everything below applies.)
Cash doesn't leave monthly. It leaves on specific days.
“Average monthly burn” is a smoothie. Your actual outflows are chunks: payroll on the 3rd and the 18th, rent on the 1st, the annual insurance premium that lands in one week and zero others, the quarterly tax payment, the vendor who invoices Net 30 but calls on day 35.
Average the chunks and you get a burn number that is true over a quarter and false in any given week. The formula says you spend $13,846 a week. No week actually looks like that. Some weeks spend $4,000. Payroll weeks spend $38,000.
Runway math built on averages is a forecast of a business that doesn't exist.
Money in is a promise, not a schedule
The formula's other input — implicitly, your revenue — is worse. Cash ÷ net burn assumes your receivables arrive the way they're supposed to. They don't. You know they don't. The invoice that's due on the 15th pays on the 40th. The client who's “processing it this week” for three weeks.
A single $45,000 invoice slipping four weeks doesn't change your average burn at all. It can absolutely change whether you make payroll on August 3rd.
The failure mode: right on average, dead on Tuesday
Walk through a real shape of a real business. $480,000 in cash. It bills about $187,000 a month and spends about $247,000 — an average net burn of $60,000 a month. “Eight months of runway,” says every calculator.
Now lay the same business out week by week, the way cash actually moves. You invoice on time. But this quarter, most of your receivables are running late — the kind of late that never shows up in an average. You hold a $150,000 minimum buffer, because you're not reckless.
| Week | Came in | Went out | What happened | Ending cash |
|---|---|---|---|---|
| 1 | $12,000 | $62,000 | Payroll + rent land together; two expected invoices don't | $430,000 |
| 2 | $8,000 | $41,000 | Quiet week | $397,000 |
| 3 | $51,000 | $74,000 | One big receivable finally arrives — into payroll + the annual insurance premium ($28,000) | $374,000 |
| 4 | $6,000 | $39,000 | The other receivables are “processing” | $341,000 |
| 5 | $9,000 | $89,000 | Payroll + the quarterly tax payment ($41,000) | $261,000 |
| 6 | $4,000 | $43,000 | Still “processing” | $222,000 |
| 7 | $10,000 | $84,000 | Payroll + rent + the vendor who calls on day 35 | $148,000 — buffer breached |
And week 7 is rarely where it ends. In real businesses that week is when the line of credit quietly gets drawn — on the bank's terms, at the bank's pace, because nobody saw the date coming. A breach you see five weeks out is a negotiation; a breach you discover on Tuesday is a scramble.
Seven weeks. The calculator said eight months. Nothing dramatic happened — no customer churned, no crisis hit. Your billed revenue was fine; roughly $225,000 of it just hadn't arrived yet, while payroll, insurance, and taxes arrived exactly on schedule. Timing did this — timing the formula is blind to by construction, because dividing two numbers deletes the calendar.
Businesses rarely die of average burn. They die of a specific week — usually one with payroll in it.
The question “months of runway” can't answer
Ask the formula: “Can I afford two senior hires?” It says: burn goes from $60k to $84k, runway goes from 8 months to 5.7. Sure — if you're comfortable.
But the actual question is: on what date does my cash cross my buffer, and does hiring move that date into a payroll week? That question needs a forecast with a calendar in it: every expected invoice with a realistic pay date, every bill with a due date, every recurring obligation on its actual schedule, payroll on the weeks payroll happens — projected forward and re-checked against reality every week.
Finance people have a name for this: the 13-week cash flow forecast. It's what a fractional CFO builds when a company gets serious about cash — usually once a quarter, in a spreadsheet, for $5,000–$15,000 a month. Thirteen weeks because it's long enough to see a quarter ahead and short enough that line-by-line honesty is still possible.
The 13-week forecast doesn't output “8 months.” It outputs things you can act on: cash crosses the $150k buffer the week of September 14. The driver is the insurance premium colliding with two at-risk receivables. Collecting either one moves the breach out five weeks.
One number is a mood. The other is a to-do list.
“But my AI finance tool shows runway”
Check what's under it. A wave of AI CFO tools now put “Runway: 19 months” in a dashboard tile with a health badge next to it. Click through the marketing and the math is the same cash ÷ average burn — the calculator, restyled, with a chat interface reading it back to you in a confident voice.
An AI narrating the naive formula is not more accurate than the naive formula. It's the same blindness, faster.
The test to run on any tool (or any spreadsheet, or any advisor): “What date do I breach my buffer, and which specific invoices and bills produce that date?” If it can't answer with a date and a list of documents, it's doing division, not forecasting.
What we do instead
XPlor builds the 13-week forecast automatically from your actual books — every open invoice, every bill, every recurring payment it detects, cash basis, always — and maintains it weekly instead of quarterly. Runway shows up as a breach date and weeks-to-zero, not months-of-average. Click any week and see exactly which documents produced the number. And every Monday, last week's forecast is scored against what actually happened, so you know how much to trust it — we publish how that works.
No black box. No division dressed up as foresight.