Runway 101: the Formula, the Fundraising-Trap Correction, the One Number That Matters
The runway formula is short:
runway = cash ÷ monthly net burn
net burn = monthly expenses − monthly revenueIf you have $300,000 in the bank, burn $25,000/month, and earn $5,000/month:
- Net burn: $20,000/month
- Runway: 15 months
Simple. Most founders get this right. They get the implications wrong.
What founders systematically underestimate
Burn grows. The number you have today is not the number you’ll have in 6 months. Common reasons:
- The senior engineer hire you’ll need in month 4: +$12K-$16K/month
- Server costs scaling with users: +$1K-$5K/month
- Legal and compliance bills: $5K-$15K, often as one-time hits
- Sales and marketing acceleration before fundraise: +$3K-$10K/month
- Tools and infrastructure that scale with team: +$1K-$3K/month
A realistic projection adds 30-50% to current burn over 12 months. The “$25K/month” startup is usually a $40K/month startup by month 12.
Revenue ramps slower than projected. Almost universally. Enterprise sales cycles run 3-6 months. Pilot-to-paid conversion is typically 30-50% of forecast. Churn in early cohorts runs 20-40%. Seasonal effects shift Q4 decisions to Q1.
A safe rule: assume revenue achieves 60% of projection with a 2-month delay. Run the model. If it still works, the plan has resilience.
The fundraising trap
Fundraising takes time. Specifically:
| Phase | Typical duration |
|---|---|
| First investor meetings to first term sheet | 2-3 months |
| Term sheet to wire | 1-2 months |
| Buffer for falling-through deals | 1-2 months |
| Total | 4-7 months |
The trap: a startup with 12 months of runway that starts fundraising at month 8 actually has 4 months to close a deal. Investors can smell desperation, and they price it in.
The rule: start fundraising when you have 9-12 months of runway remaining. That gives you leverage to negotiate, walk away from bad terms, and signal that you don’t need the money — which paradoxically is when investors want most to give it to you.
What 9 months of runway costs
If you have $400K in the bank and burn $30K net per month, you have 13.3 months of runway. To get to “raise from strength,” you need to be raising at month 1-4 — which feels too early to most founders.
The alternative — waiting until month 8 with 5 months left — produces:
- Visible time pressure that investors price into terms
- Lower valuation (10-30%)
- More restrictive terms (liquidation preferences, anti-dilution, board control)
- Higher risk that the deal doesn’t close in time
The “I’ll start fundraising next quarter when our metrics are better” reasoning often costs more in valuation than waiting buys in metric improvement.
Three scenarios to model
Best case. Revenue ramps as projected, burn stays roughly stable. Result: 18-22 months of runway. You can be patient.
Base case. Revenue at 60% of projection with 2-month delay, burn grows 30-40% over the year. Result: 12-15 months. Standard fundraising window.
Worst case. Revenue at 30% of projection, burn grows 50%, plus one black-swan cost (regulatory issue, key cofounder departure, major churn event). Result: 6-9 months. Defensive measures needed: cut burn now, accelerate fundraise, possibly take bridge round at unfavorable terms.
If your worst-case is under 6 months, you have a problem you should solve now — not when it materializes.
Revenue changes everything (when it actually arrives)
Even modest revenue dramatically extends runway:
| Monthly net burn before revenue | After $5K revenue | After $15K revenue |
|---|---|---|
| $30,000 | $25,000 (+20% runway) | $15,000 (+100%) |
| $50,000 | $45,000 (+11% runway) | $35,000 (+43%) |
Revenue’s leverage on runway is non-linear. Small revenue at the start of the burn-rate growth curve helps disproportionately.
What changes the math
Three big modifiers:
- Founder compensation. Three founders at $0 salary for 18 months saves ~$300-500K of cash. Most early-stage founders subsidize the company this way, then transition to market comp at series A. Plan for both phases.
- Geography. Remote-first lowers facilities cost but engineers still command top-market rates. Fully-distributed teams can be 30-50% cheaper than fully-SF teams; partial-distributed varies.
- Bootstrapping vs venture. Bootstrapped startups care about the path-to-profit timeline more than runway-to-fundraise. Different optimization, same cash-flow math underneath.
Where this framework breaks
- Series B+ companies. Different math: gross margins, CAC payback, LTV/CAC. Burn matters but in the context of unit economics, not survivability.
- Capital-intensive hardware/biotech. Multi-year cycles between funding rounds, clinical trial costs, production scale-up. Adapted runway frameworks needed.
- Profitable bootstrap. Once monthly profit is positive, “runway” stops being the relevant metric. Replaced by: “how long can the company sustain itself if revenue drops 50%.”
What to actually do
- Compute current runway honestly, with realistic burn growth.
- Identify your fundraising trigger: total runway minus 7 months.
- If trigger is past, start fundraising today.
- Build three scenarios (best/base/worst) and know the worst-case floor.
- Update monthly. The model is only as useful as its currency.
Open the Startup Cost Calculator → and run your real numbers. The output is the fundraising-trigger month and the worst-case-survival month — the two numbers that should drive operational decisions.