How to Calculate Startup Runway: The Complete Guide for Founders

By Vishesh Khurana — Mar 15, 2026

Every startup founder eventually confronts the same question: how long can we keep going before we run out of money? The answer is your runway, and calculating it correctly can mean the difference between a well-timed fundraise and a fire sale.

The Basic Runway Formula

At its simplest, startup runway equals your current cash balance divided by your monthly net burn rate. Net burn is total monthly expenses minus total monthly revenue. If you have $600,000 in the bank and you spend $80,000 per month while earning $30,000, your net burn is $50,000 and your runway is 12 months.

This formula is useful as a starting point, but it carries a dangerous assumption: that your burn rate stays constant. In practice, burn fluctuates. Revenue is lumpy. Contracts slip. Hires start at different times.

Why Static Calculations Fail

Consider a real scenario. You calculate 14 months of runway based on current numbers. But next quarter, a key customer churns, reducing revenue by $8,000 per month. You also onboard two engineers, adding $25,000 in monthly payroll. Suddenly your net burn jumps from $50,000 to $83,000, and your runway drops to 7 months.

The Monte Carlo Approach

Instead of calculating a single runway number, sophisticated founders use Monte Carlo simulation. This technique runs your financial model thousands of times, each with slightly different inputs drawn from realistic probability distributions. The result is a probability distribution showing your P10 (pessimistic), P50 (median), and P90 (optimistic) runway.

Key Variables That Affect Runway

Revenue growth rate is the single biggest lever. Customer churn works in the opposite direction. Hiring pace is usually the largest controllable expense. Payment terms and accounts receivable also matter.

Practical Steps for Founders

Start by connecting your actual financial data rather than working from estimates. Calculate your gross and net burn rates over the past six months. Then project forward with multiple scenarios: baseline, optimistic, and pessimistic.