Guide: Startup Burn Rate
Calculating a startup's Runway—the amount of time before cash reserves run out—is the most critical survival metric for any founder. However, most basic calculators rely on "Static Runway," which takes your current bank balance and divides it by your current monthly loss. This is a dangerous oversimplification. In reality, startups are dynamic entities. If your product is succeeding, your Monthly Recurring Revenue (MRR) is compounding every month, meaning your net burn shrinks over time. Conversely, as you scale, your expenses (new hires, larger server costs, expanded marketing budgets) also compound. To truly understand when your company will hit the wall, you must use a "Dynamic Runway" model that projects compounding revenue growth against compounding expense growth month-over-month. This advanced model determines whether your startup is "Default Alive" (will reach profitability before running out of money) or "Default Dead".
How to Use This Tool
Enter your current Cash in Bank—the total liquid capital you have available to deploy. Next, input your current Monthly Revenue and your current total Monthly Expenses (including payroll, rent, and software). The calculator will subtract revenue from expenses to find your current Net Burn. Crucially, input your expected Revenue Growth per month (a healthy early-stage SaaS might target 5% to 10% MoM growth). Finally, enter your expected Expense Growth per month, as costs inevitably rise as you scale operations.
The Math Behind It
The engine iterates month-by-month in a financial loop. For month one, it subtracts the Net Burn (Expenses - Revenue) from the Cash Reserve. For month two, it increases the Revenue by the Revenue Growth percentage, and increases the Expenses by the Expense Growth percentage. It subtracts this new Net Burn from the remaining cash. The algorithm repeats this compounding loop until either the cash balance drops below zero, or the Revenue surpasses the Expenses (achieving profitability).
Understanding Your Results
Dynamic Runway tells you exactly how many months you have left to operate under your specific growth assumptions. The Zero Cash Date translates that runway into a literal doomsday calendar date, providing an urgent deadline for when you must secure your next round of venture capital funding or achieve profitability.
Real-World Example
A startup has $750,000 in the bank. They currently generate $15,000 in monthly revenue but spend $65,000, creating a current Net Burn of $50,000. Under a flawed static model, they have exactly 15 months of runway ($750k / $50k). However, they are growing revenue at 5% month-over-month, while keeping expense growth strictly constrained to 2%. When running the dynamic calculation, the compounding revenue slowly closes the gap on the expenses. Instead of dying in 15 months, the dynamic model reveals they actually have 18 months of runway, giving the founders a critical extra 90 days to close a Series A funding round.
Frequently Asked Questions
What is the difference between Gross Burn and Net Burn?
Gross Burn is the total amount of money your company spends each month (e.g., $65,000). Net Burn is your total expenses minus your total revenue (e.g., $65,000 - $15,000 = $50,000). Investors care deeply about Net Burn because it dictates your actual runway.
What does it mean to be 'Default Alive'?
Coined by Y Combinator founder Paul Graham, 'Default Alive' means that if your expenses remain constant and your revenue continues growing at its current historical rate, your company will reach profitability before your cash reserves run out. You do not need to raise more money to survive.
Why is Month-over-Month (MoM) growth so important?
Early-stage startups are valued almost entirely on their growth velocity. A 5% MoM growth rate equates to roughly 80% annualized growth. Compounding monthly growth quickly turns small revenue numbers into massive valuations over a 3-to-5 year horizon.
Should I include venture debt in my cash reserves?
Yes, if the debt is liquid and immediately accessible to pay payroll and operating expenses. However, you must also model the monthly debt servicing (interest payments) into your monthly expenses, which will accelerate your burn rate.