Runway is the number that actually keeps founders up at night, and it falls straight out of burn. A company with $1.2M in the bank burning $100K a month has 12 months of runway — and since raising a round takes months, it really needs to start fundraising around month 6. As an investor, when you read an update saying burn went from $80K to $150K, the real question is whether revenue and progress grew to match, or whether the company just got more expensive without getting more valuable.
It helps to separate gross burn (total monthly spend) from net burn (spend minus revenue), because a company with real revenue can have high gross burn but modest net burn. Burn isn't bad on its own — spending to grow is the entire point of venture funding — but burn without corresponding traction is the classic path to a down round or a shutdown. The healthiest signal is burn that's deliberate: rising in step with hiring and growth, with a clear plan for the next raise well before the runway runs out.