Runway Math for Non-Finance Founders: The Spreadsheet I Wish I'd Had at Seed

Most non-finance founders get their first real lesson in runway math from an investor who says "your burn looks high." By that point, you are usually three months from the problem, not three months from the solution.

I have acted as the de facto CFO (through the CTO and operator role) for five seed-stage companies, and I have walked through runway conversations with more than thirty founders. The confusion is almost always the same: the difference between gross burn and net burn is unclear, the relationship between headcount and cash is not modeled explicitly, and there is no scenario plan.

This is the spreadsheet model I hand founders at pre-seed and seed. It is not complex. Complexity is not the goal; clarity is.

The Three Numbers That Actually Matter

Before building the model, understand the three numbers that everything else derives from.

Gross burn is the total amount of cash you spend each month, before any revenue.

Net burn is gross burn minus monthly revenue. This is the number that determines how fast you are consuming your bank balance.

Runway is current cash divided by net burn, expressed in months. This is the only number your investors will ask about in every meeting until you are profitable.

The mistake I see most often: founders report gross burn when investors ask about burn. A company spending $80,000/month with $30,000 in monthly recurring revenue has a gross burn of $80,000 and a net burn of $50,000. These are not interchangeable. Quoting the wrong one by accident in a board meeting creates a credibility problem.

Start with Headcount, Not Categories

The most reliable way to build a burn model is to start with headcount, not line-item categories. At seed stage, 70% to 85% of your burn is compensation. If you get that right, the rest follows.

# Monthly Burn Model (example seed-stage company)
headcount:
  - role: Founder/CEO
    monthly_cash: 8000
    equity_vesting: true
  - role: Founder/CTO
    monthly_cash: 8000
    equity_vesting: true
  - role: Senior Engineer
    monthly_cash: 12000
    equity_vesting: true
  - role: Designer (part-time contractor)
    monthly_cash: 4000
    equity_vesting: false
total_headcount_cost: 32000

non_headcount:
  infrastructure: 1200
  saas_tools: 800
  legal_and_accounting: 1500
  growth_and_marketing: 3000
  miscellaneous: 500
total_non_headcount: 7000

gross_burn: 39000
monthly_recurring_revenue: 11000
net_burn: 28000

bank_balance: 560000
runway_months: 20.0  # 560000 / 28000

That is a clean, honest model. Build it in a spreadsheet. Update it every month. Share it with your board.

The Gross-to-Net Bridge

Walk your investors through this explicitly every time you discuss burn:

The parentheses on revenue are intentional: this is the standard format. Revenue reduces burn; it does not add to a separate column.

Scenario Planning: The Three Cases

Never present runway as a single number. Present three scenarios.

Base case: Headcount stays flat, revenue grows at the rate it grew last month. This is the business-as-usual projection.

Bear case: One key hire attrition (add 2-month replacement cost), revenue growth stalls at 50% of the current rate, two unplanned infrastructure events add $5K. This is the stress test.

Bull case: You close the two pilots in your pipeline this month, revenue doubles over the next quarter. This shows what upside looks like without requiring it.

The bear case is the one you manage to. If your bear case runway is under 12 months, you are already behind on fundraising.

The Capital Efficiency Ratio

This is the metric I wish someone had explained to me at seed stage. Capital efficiency is defined as: cumulative ARR generated divided by cumulative net cash burned.

A capital efficiency ratio of 1.0 means you generated $1 of ARR for every $1 you burned. Best-in-class seed-stage SaaS companies run between 0.8 and 1.5. Below 0.5 is a problem you need to be able to explain. Above 2.0 is extraordinary and should be in your fundraising deck.

Capital Efficiency = Cumulative ARR / Cumulative Net Burn

Example:
  Cumulative net burn at month 18: $504,000
  Current ARR:                     $396,000
  Capital Efficiency:              0.79 (396,000 / 504,000)

0.79 is in the acceptable range for a company still finding product-market fit. If you raised $750K and are at 18 months with $396K ARR, you need to show an improving trend in the ratio to have a credible Series A story.

Track this monthly from the first dollar of revenue. The trend matters more than the absolute number.

The Raise vs. Extend Decision Tree

The question founders get wrong most often: when do you raise versus when do you extend runway through cost cuts?

Is your net burn-to-revenue ratio improving month over month?
  Yes: Stay the course. Raise when you have 9 to 12 months of runway left.
  No:  Ask next question.

Is gross burn growing faster than revenue?
  Yes: You have a unit economics problem. Fix it before raising.
  No:  Ask next question.

Are you within 6 months of runway with no growth momentum?
  Yes: Emergency. Cut costs immediately, seek a bridge, or accelerate revenue.
  No:  Ask next question.

Do you have strong growth momentum and tight runway?
  Yes: Raise aggressively now. Do not let momentum decay during a slow process.
  No:  Revisit base-case assumptions. Something is not modeled correctly.

The most common mistake: waiting too long because founders are uncomfortable with the fundraising process. A company with 14 months of runway that waits until 7 months is fundraising from pressure rather than strength. Start the process at 12 months.

What I Got Wrong

At my second startup, I managed burn intuitively instead of from a model. I knew roughly what payroll was and roughly what was in the bank. When I finally built the explicit model at month 14, I discovered we had three months less runway than I thought, because I had been accounting for revenue on an invoiced basis rather than a collected basis.

Three months of error in runway estimation, at a seed-stage company, is the difference between starting your Series A process from strength and starting it in a panic. Build the model in month one. Update it in month two. Never manage burn intuitively.

The second lesson: model contractor costs explicitly. It is easy to mentally categorize contractors as variable costs and undercount them in burn planning. If a contractor is working 20 hours a week on your core product, they are functionally a part-time employee. Model them that way.

The third lesson: do not average your burn across the last three months without adjusting for one-time events. A month with a $15,000 legal invoice skews your rolling average and makes your burn look worse than the underlying run rate. Flag one-time items explicitly in the model and calculate adjusted burn alongside raw burn.

The One Table to Share with Your Board

Every board meeting, bring this and nothing else for the financial slide:

Six rows. That is the entire financial conversation at seed stage for most boards. If you are spending more time than this on financial reporting and you have fewer than ten employees, your reporting is more complex than your stage requires.