Runway isn't a number you calculate once. It's a number that moves every time you hire, every time a customer churns, every time a vendor bill comes in higher than expected. Treat it like a dashboard, not a diary entry.
Gross burn vs. net burn
Gross burn is everything going out. Net burn is what's left after revenue comes in. Founders who only track net burn get surprised when a big customer leaves — suddenly the number that mattered was gross burn all along, and it was always higher than they let themselves notice.
The 18-month rule is a floor, not a target
Conventional advice says raise 18 months of runway. Treat that as the minimum you tolerate, not the number you aim for, because your actual burn will almost always be higher than your model — new hires ramp slower than expected, sales cycles stretch, and something always costs more than the line item you budgeted.
Build the sensitivity table, not just the base case
One spreadsheet row showing "18 months at current burn" tells you almost nothing. Build three columns: burn stays flat, burn grows 10% as you hire, and revenue comes in at half your forecast. Look at the worst column every month, not the best one. That's the number that should actually change your hiring decisions.
Set a trigger date, not just a trigger number
Decide today — not when you're already stressed — the exact calendar date by which you need a term sheet or a plan B, based on your worst-case runway. Write it down somewhere you'll actually see it. Founders who wait until the number itself feels scary have usually already burned the negotiating leverage they needed.