Runway is the most consequential single number for an early-stage startup. It's cash on hand divided by net burn. Most operators target at least 18 months of runway after a round closes, long enough to hit milestones, raise the next round, and absorb 3 to 6 months of fundraising slip without becoming desperate.
Runway shrinks faster than burn predicts when sales miss or when a vendor demands prepayment. Conservative founders model runway under several scenarios (revenue at plan, at 50% of plan, and at 0%).