Cash Flow, Burn Rate & Runway — Don’t Run Out of Money
Most startups don’t die because their idea was bad. They die because they run out of cash. You can have happy customers, a growing product, and a great team — and still go to zero on a Friday when payroll hits and the bank account can’t cover it. This section teaches you the small handful of numbers that tell you exactly how many days you have left, and the levers you can pull to buy more time.
First, what is "cash flow"?
Cash flow is simply the money actually moving in and out of your bank account over time. Cash in = money customers pay you, plus money investors give you. Cash out = every dollar that leaves — salaries, rent, software, ads, taxes, paying suppliers.
Note the word actually. Cash flow is not the same as "profit." You can make a sale today (so it counts as revenue) but not get paid for 60 days. The sale helps your profit now, but no cash has arrived. Your bank balance is what keeps the lights on, so we track the cash.
Burn rate: how fast the tub is draining
Burn rate is how much cash your company uses up each month. There are two flavours, and mixing them up is a classic mistake.
| Term | Plain meaning | Formula |
|---|---|---|
| Gross burn | Total cash you spend in a month, ignoring any money coming in. | = all monthly cash spending |
| Net burn | How fast your bank balance is actually shrinking after counting revenue. | = cash spent − cash received |
Gross burn answers the worst-case question: "If every customer vanished tomorrow, how fast would I burn cash?" Net burn answers the real-life question: "How fast is my balance really dropping right now?"
• Gross burn = $150,000 (the total you spent).
• Net burn = $150,000 − $50,000 = $100,000 per month.
Your bank balance dropped by $100,000 that month.
One more case: if your revenue is bigger than your spending, your net burn is negative — your balance is growing. People call that "default alive" or "cash-flow positive." Good place to be.
Runway: how many months until empty
Runway is the number of months you can keep going before the cash runs out, if nothing changes. It is the single most important survival number for a founder.
| Formula |
|---|
| Runway (months) = Cash in the bank ÷ Net monthly burn |
Runway = $600,000 ÷ $100,000 = 6 months.
That means: in 6 months, the bathtub is empty. Mark the date on your calendar — that is your "zero-cash date." Today is the day to start acting on it, not the month before.
Reading your cash position and building a simple forecast
Your cash position is just one number: how much real, spendable cash sits in your accounts today. Don’t count money you’re "owed but haven’t received" — that’s not cash yet.
A forecast is your best guess of that balance going forward. The most trusted tool founders use is the 13-week cash flow forecast — a week-by-week view of the next quarter (13 weeks ≈ 3 months). Why weekly instead of monthly? Because startups don’t run out of money on a tidy monthly schedule — they run out in the specific week payroll lands and the account is short. Weekly granularity catches that.
How to build a simple one, in three columns per week:
- Starting cash — the balance at the start of the week.
- Cash in — customer payments you truly expect to arrive that week (by the date money hits the account, not the invoice date).
- Cash out — payroll on its real pay dates, rent, supplier payments, taxes, subscriptions.
Then: Ending cash = Starting + In − Out. That ending number becomes next week’s starting number. Roll it forward 13 weeks.
13-WEEK CASH FORECAST (simplified) Week Start + In - Out = End ------------------------------------------- W1 600,000 40,000 150,000 490,000 W2 490,000 60,000 90,000 460,000 W3 460,000 30,000 170,000 320,000 <- payroll W4 320,000 50,000 90,000 280,000 ... W13 ~10,000 ... ... ~ ZERO <- danger
Default alive vs default dead (Paul Graham)
Startup investor Paul Graham gave founders a simple, brutal question. Assume your spending stays the same and your revenue keeps growing at the rate it has lately. Do you reach profitability (the point where revenue covers all costs, so net burn hits zero) before the money runs out?
- Default alive: Yes — on your current path, you’ll become self-sustaining before the cash is gone. You don’t need anyone’s permission to survive.
- Default dead: No — you’ll hit zero unless you raise more money or change something big.
The key insight: this is about trajectory, not just runway. Runway tells you how long you last. Default-alive tells you whether your growing revenue will catch up to your spending in time. Graham says ask this honestly around month 8–9 of building, because the answer changes every decision.
The five levers to extend runway
If your zero-cash date is too close, you have five things you can pull on. Most founders should pull several at once.
| Lever | What it means | Effect on cash |
|---|---|---|
| 1. Cut burn | Reduce spending — pause hiring, trim tools, lower ad spend, renegotiate rent. | Less cash out, fast. |
| 2. Raise prices | Charge more per customer (often the fastest profit lever — see the pricing section). | More cash in, no extra cost. |
| 3. Collect receivables faster | Get customers to pay sooner (deposits up front, shorter payment terms, chase late invoices). | Pulls future cash into now. |
| 4. Delay payables | Pay your own suppliers later (negotiate longer terms — fairly, not by stiffing them). | Keeps cash in your account longer. |
| 5. Grow revenue | Sell more — more customers, more usage, upsells. | More cash in over time. |
Working capital: the timing that swings your cash
Working capital is the cash tied up in the day-to-day gap between paying out and getting paid in. Three things drive it:
- Accounts receivable (AR) — money customers owe you but haven’t paid yet. The longer they take, the more of your cash is "stuck" outside your bank.
- Accounts payable (AP) — money you owe suppliers but haven’t paid yet. Paying later keeps cash with you longer.
- Inventory — cash frozen in stock sitting on a shelf, not yet sold.
Together these form the cash conversion cycle: how many days from spending a dollar (on stock or work) until that dollar comes back as customer cash. Shorter is better — it means you self-fund growth instead of needing to raise money just to operate. A rough version: days to sell inventory + days customers take to pay − days you take to pay suppliers.
The danger zone: act at ~6 months, not 2
Raising money and cutting costs both take longer than founders expect. A funding round commonly takes 3 to 6 months from "start talking to investors" to "money in the bank." So if you wait until you have 2 months of runway to start, you’re already too late — you’ll be negotiating from desperation, on bad terms, or not at all.
RUNWAY DEPLETION — act early, not at the cliff
Cash
$600k |*
| *
| *
| * <-- 6 mo left: START raising / cut now
$300k | *
| *
| * <-- 3 mo: hard to fix, weak position
| *
$0 |________________*___________
0 1 2 3 4 5 6 (months)
What good founders do every week
- Know two numbers cold: cash in the bank, and net monthly burn. You should be able to say them from memory at any moment.
- Know your zero-cash date. A single calendar date is more motivating than "a few months."
- Update the 13-week forecast weekly, ideally every Monday, and compare it to what actually happened.
- Re-check "default alive or dead?" monthly. If you’re dead, decide the fix this month, not next quarter.
- Run a worst-case scenario: what if revenue is 30% lower and a big payment slips? If that breaks you, fix the plan now.
- Start raising or cutting at ~6 months of runway, while you still have leverage and time.