The 40% Rule
Sean Ellis says if 40% of users would be very disappointed if your product disappeared, you have PMF.
Retention Curves
Flat retention curves mean users stick around. If it curves down to zero, you have a problem.
Organic Growth Signals
Word-of-mouth growth, declining CAC, and organic traffic are the best PMF indicators.
Net Promoter Score
NPS > 50 is excellent. NPS > 70 is world-class. Measure it regularly and track trends.
What to Do When You Have It
Double down. Raise money. Scale the team. But do not get complacent - PMF can slip away.