Most small SaaS teams can recite their MRR from memory. Ask the same team how many months of cash they have left, and the answer usually takes longer: someone has to log into the bank account, guess at a few upcoming charges, and do the math on the spot.
That gap is more dangerous than it looks. CB Insights, which has tracked startup failure causes for over a decade, found that running out of cash is the single most common reason startups fail, cited by roughly 70% of failed founders as a top cause. Not because the founders were careless, but because runway is a number you have to calculate, while revenue is a number your dashboard hands you for free.
The formula, and why the burn number is the hard part
Runway is simple math: cash in the bank divided by monthly burn rate. A team with $150,000 in the bank and $12,500 in average monthly burn has 12 months of runway.
The part people get wrong isn't the division. It's the burn number. A single unusual month, an annual software renewal, a one-time contractor payment, can make burn look far worse or better than it really is. The fix is averaging burn over three to six months rather than reading it off the most recent bank statement, so one noisy month doesn't distort the number that decisions get made on.
A second number worth knowing: burn multiple
Runway tells you how much time you have. It doesn't tell you whether the money you're spending is actually working. That's what burn multiple measures.
Burn multiple = net burn ÷ net new annual recurring revenue (ARR), a framework popularized by investor David Sacks at Craft Ventures. If you burn $50,000 to add $50,000 in net new ARR that month, your burn multiple is 1.0x: every dollar spent produced a dollar of new recurring revenue. If you burn $150,000 to add that same $50,000, your burn multiple is 3.0x, and something is off in how the money is being spent, not just how much of it there is.
Sacks illustrates the point with his own example: a company burning $2M in a quarter to add $1M in ARR has a 2x multiple, which he calls reasonable for an early-stage startup. A company burning $5M to add that same $1M has a 5x multiple, which he calls terrible and a signal to cut costs immediately.
Two companies can post identical ARR growth and look the same on a revenue chart, while one of them is burning cash at more than double the rate of the other to get there. Burn multiple is what catches that difference; runway alone won't.
How much runway is actually enough
There's real disagreement on the exact number, but the guidance clusters more than it conflicts.
| Guidance | Recommended runway |
|---|---|
| Standard baseline | 12 to 18 months (First Round Review) |
| Tighter fundraising markets | 24 to 36 months (First Round Review) |
| Right after a seed round | 24-plus months (SaaStr) |
SaaStr puts it bluntly in its own coverage of the current fundraising climate: "Plan for 24+ months of runway at seed. The days of quick flips to Series A are largely behind us." Both sources treat runway as a number with actions attached to it, not a passive one: start planning the next raise well before you're down to your last 6 months, not after.
A monthly rhythm that keeps both numbers honest
Here's the version of this we walk founders through at Finsight, scaled for a small team without a finance function:
- Once a month, pull the actual cash balance and compare it to what you expected. Note anything that surprised you, and figure out why before you move on.
- Recalculate burn as a trailing 3-month average, not last month alone.
- Recalculate runway using that updated burn number, and calculate burn multiple against that month's net new ARR. Write both down somewhere you'll actually see again next month.
- Watch payroll separately. For most early-stage SaaS teams, payroll is the largest single line item in burn, which makes it the biggest lever if runway needs to move.
None of this requires a finance background or new software. It requires the same 20 minutes, on the same day, every month, treated as a non-negotiable meeting with yourself rather than something that happens right before a board update or a fundraise, which is exactly the moment you'd rather already know the answer.
Want this handled instead of DIY'd?
Finsight's Founder FP&A Framework builds the monthly cash rhythm above directly into your existing tools, reviewed by someone who's done this for other founders, not just your own spreadsheet.