Every founder and finance lead at a startup needs one number: how many months until cash runs out? Getting that number wrong — or not having a model that updates it reliably — is one of the most common and preventable finance failures in early-stage companies.
This guide covers how to calculate runway correctly, how to build the Excel model, and how to think about scenarios when the number is tighter than you’d like.
The Basic Runway Formula
Runway (months) = Current Cash Balance ÷ Net Monthly Burn Rate
Net burn rate = total monthly cash outflows − total monthly cash inflows
If you have $1.8M in the bank and you’re spending $100k/month while bringing in $40k/month in revenue:
- Net burn = $100k − $40k = $60k/month
- Runway = $1,800,000 ÷ $60,000 = 30 months
This is the static calculation. It’s useful for a quick gut check, but it assumes your burn rate never changes — and it never doesn’t change.
Gross Burn vs. Net Burn
Gross burn rate — total cash going out the door each month (payroll, rent, software, services). Doesn’t net against revenue.
Net burn rate — gross burn minus cash inflows (revenue, interest, other). This is the number that determines when you run out of money.
Early-stage investors typically ask about both. Gross burn shows cost structure; net burn shows the actual cash consumption rate.
A startup with $200k gross burn and $120k in revenue has $80k net burn. That’s a very different position from a company with $200k gross burn and $20k revenue — even though the gross burn is identical.
Why Static Runway Calculations Lie
The simple formula breaks down because burn rates change:
- Revenue grows (or doesn’t) — your net burn narrows or widens over time
- Headcount changes — a new hire adds $15k–$30k/month in fully-loaded cost overnight
- Seasonality — quarterly software renewals, annual insurance, marketing campaigns
- Capital events — a new equity round or debt draw changes your cash balance on a specific date
- Variable costs — customer success, hosting, and COGS scale with revenue
A static calculation gives you one number. A month-by-month model gives you a curve — and the curve tells you things the single number doesn’t.
Building the Month-by-Month Model in Excel
A proper runway model has five categories:
1. Starting Cash
Your current bank balance as of the model start date. Include all accounts you can draw on.
2. Financing Inflows
Equity raises, debt draws, grants, tax credits. Enter these as one-time inflows in the month you expect to receive them. For a planned raise, model it in the month you expect close — not when you start talking to investors.
3. Revenue
Break this out by stream if you have multiple. For each line: starting amount, annual growth rate, and frequency. A simple ARR model: enter current MRR and a 40% annual growth rate, and the model calculates each month’s revenue automatically.
4. Fixed Costs
Payroll (your largest cost), rent, SaaS subscriptions, professional fees. Enter each as a monthly, quarterly, or annual item.
5. Variable Costs
COGS, hosting/infrastructure costs that scale with usage, sales commissions, customer success headcount. These should be tied to revenue assumptions, not entered as fixed numbers.
The Cash Flow Waterfall
Each month’s calculation:
Opening Cash
+ Financing Inflows
+ Revenue
− Fixed Costs
− Variable Costs
− CapEx
= Closing Cash
The closing cash in Month N becomes the opening cash in Month N+1. When closing cash first hits zero, that’s your cash-out date.
Scenario Planning: The Most Important Part
A single runway number is fragile. Investors, boards, and good finance people want to know: what if?
Build three scenarios:
Base case — your honest forecast. Not conservative, not aggressive. What you actually expect given what you know today.
Pessimistic — growth is slower, a key customer churns, hiring takes longer. How long do you have if things go moderately worse? This isn’t your worst case — it’s a realistic downside.
Optimistic — a big deal closes, growth accelerates, a raise comes in. When does cash flow positive in this scenario?
The three scenarios should differ primarily in revenue growth assumptions and major deal timing. Costs are usually more predictable.
If your pessimistic case shows less than 12 months of runway, you should be fundraising or cutting costs now — not in 6 months when the runway is down to 6 months.
Interpreting the Output
Months of Runway
18+ months = comfortable position. 12–18 months = start fundraising. Under 12 months = urgency.
Cash Zero Date
More useful than “months of runway” for board presentations. “We have 14 months” lands differently than “we run out in May 2027.”
Breakeven Month
The first month net cash flow is positive. Some companies reach this before they need to raise again; others raise first.
Net Burn Rate
Every $10k reduction in monthly burn adds roughly 2 weeks of runway at typical burn levels. Run the numbers before making headcount or spending decisions.
What to Do When Runway Is Shorter Than Expected
If the model shows under 18 months in your base case:
Option 1: Fundraise now. Start conversations before you’re at 12 months. Closing a round from 18 months is easier than closing at 8 months.
Option 2: Reduce burn. A 20% cost reduction often extends runway by 40–60% because operating leverage kicks in. Run the scenario in the model before making decisions.
Option 3: Accelerate revenue. Model what happens if you close the two big deals in your pipeline. If closing them puts you past breakeven, that changes the strategy.
Option 4: Add non-dilutive capital. Venture debt, revenue-based financing, grants, tax credits. Enter these in the Financing tab and see how they shift the cash-out date.
Usually the answer is some combination. The model is the tool for testing them without committing.
The Excel Approach
Most founders start with a simple spreadsheet: one row per cost, one column per month. It works until it doesn’t — which happens when you have 20+ cost lines, multiple revenue streams with different growth rates, seasonal items, and you want to run three scenarios without copy-pasting the whole model three times.
At that point, you need a structured workbook:
- Separate input tabs per category (financing, revenue, fixed costs, variable costs, CapEx)
- Scenario selector — one dropdown changes the entire model
- Dashboard — KPI cards you can drop into a board deck
- Cash flow waterfall — month-by-month detail you can filter and analyze
The Startup Runway Calculator is exactly this. Enter your numbers once, flip between scenarios with a dropdown, and get a board-ready dashboard automatically.
$49, one time. Get it here →
Or try the free 12-month version — same structure, limited to 5 input rows per tab.
KDesk Accounting builds audit-ready Excel tools for finance teams. Browse all templates →