How to Calculate Startup Runway: A Complete Guide
Master the art of runway calculation. Learn gross vs net burn, factor in revenue growth, and know exactly when to start fundraising.
What Is Startup Runway?
Runway is the number of months your startup can survive before running out of cash, assuming your current burn rate and revenue trajectory continue. It's your "time to zero" — the clock ticking between your last fundraising round and your next.
Simple definition: Runway = (Cash in Bank) ÷ (Monthly Burn Rate)
But the real world is more complex. Revenue grows, expenses change, one-time costs hit. This guide will show you how to calculate runway accurately and plan your fundraising timeline.
Gross Burn vs Net Burn
Before calculating runway, you need to understand the difference between gross burn and net burn. This is a common source of confusion.
| Metric | Definition | When to Use |
|---|---|---|
| Gross Burn | Total monthly expenses (salaries, rent, software, etc.) | Pre-revenue or early-stage |
| Net Burn | Expenses minus revenue (actual cash outflow) | Revenue-generating startups |
Important: Always use net burn for runway calculations if you have revenue. Using gross burn will give you a falsely optimistic runway number.
The Basic Runway Formula
For a simple scenario without growth or changes:
Example: You have $500K in the bank. Your monthly expenses are $40K, and you're making $10K in revenue.
| Item | Value |
|---|---|
| Cash in Bank | $500,000 |
| Monthly Expenses (Gross Burn) | $40,000 |
| Monthly Revenue | $10,000 |
| Monthly Net Burn | $30,000 |
| Runway | 16.7 months |
Factoring in Revenue Growth
Most startups don't have flat revenue — it grows (hopefully!). Ignoring growth can lead to underestimating your runway.
If your revenue grows 10% per month, your net burn decreases over time. The simple formula doesn't capture this.
With growth: A startup with $50K/month expenses, $10K/month revenue growing at 15%/month, and $400K in cash has ~14 months of runway — significantly longer than the 10 months the simple formula would predict.
When to Start Fundraising
Here's the golden rule of startup fundraising:
Start fundraising when you have 6 months of runway left.
Why 6 months? The fundraising timeline:
- 1-2 months: Prep (pitch deck, financial model, investor list)
- 2-3 months: First meetings and follow-ups 1-2 months: Due diligence, term sheets, and closing
If you wait until 3 months of runway, you're negotiating from weakness. Investors can smell desperation, and you'll likely accept worse terms.
Don't Forget Expense Growth
Just as revenue grows, so do expenses. Hiring more people, upgrading tools, moving to a bigger office — all of these increase your burn rate.
Calculating unit economics (CAC, LTV, and LTV:CAC ratio) is critical for understanding your startup's efficiency. Use our Unit Economics Calculator to track these key metrics.
A realistic runway calculation should account for:
- Planned hires and their impact on burn
- Increasing infrastructure costs as you scale
- Marketing and sales expansion
What's a Good Runway?
Runway expectations vary by stage:
| Stage | Target Runway | Why |
|---|---|---|
| Pre-Seed / Idea | 12-18 months | Time to build MVP and validate |
| Seed | 18-24 months | Time to reach meaningful traction metrics |
| Series A | 18-24 months | Time to scale and prepare for Series B |
| Series B+ | 24+ months | Longer cycles, more stability sought |
Rule of thumb: Always aim for 18+ months of runway post-raise. Less than 12 months is dangerous territory.
One-Time Expenses Matter
Some expenses hit once and then they're gone. But they still reduce your runway. Common one-time costs:
- Legal fees for incorporation or fundraising
- Equipment purchases (laptops, servers)
- Office setup or relocation
- Key hires' signing bonuses
Subtract these from your cash balance before calculating runway, or treat them as an additional monthly burn in the relevant month.
Use a Runway Calculator
Doing runway calculations by hand is tedious and error-prone. A good runway calculator should:
- Factor in both revenue and expense growth rates
- Show month-by-month projections
- Highlight your "fundraise deadline"
- Calculate when you'll break even (revenue covers expenses)
- Handle one-time expenses
Calculate Your Runway
Use our free Runway Calculator to see exactly how many months of runway you have. Factor in revenue growth, expense growth, and one-time costs.
Try Runway Calculator →Key Takeaways
- Use net burn, not gross burn — revenue reduces your actual cash outflow.
- Account for growth — both revenue growth (extends runway) and expense growth (shortens it).
- Start fundraising at 6 months — never wait until you're desperate.
- Aim for 18+ months after each raise to give yourself time to execute.
- Recalculate monthly — runway changes as your business evolves.
Final tip: Track your runway religiously. It's the single most important metric for your survival. Create a habit of updating your runway projection at the end of every month.
Use our free Runway Calculator to see exactly how many months of cash you have left. No signup required.
Try Runway Calculator Free →