Marc Andreessen defined product-market fit as 'being in a good market with a product that can satisfy that market.' It sounds simple, but achieving it is the single hardest thing a startup will do. The uncomfortable truth is that most startups never find it.
The Sean Ellis Test
The most widely-used quantitative measure of product-market fit is the Sean Ellis test: ask your users 'How would you feel if you could no longer use this product?' If 40% or more answer 'very disappointed,' you have achieved product-market fit. Below 25%, you have significant work to do. Between 25-40%, you are on the right track but need iteration.
Qualitative Signals
Beyond surveys, watch for these organic signals: Users start recommending your product without being asked. Retention curves flatten (users who stay past week 2 continue using the product). Usage frequency increases over time rather than decreasing. Users start requesting features that extend the core product rather than changing it.
The Iteration Framework
If you have not reached product-market fit, do not scale. Instead, iterate systematically. Segment your users by engagement level. Interview your most engaged users to understand what they love. Interview churned users to understand what drove them away. Look for the pattern.
Common PMF Mistakes
The most common mistake is declaring product-market fit prematurely. A spike in signups from a press mention is not PMF. A single large customer is not PMF. PMF is sustained, organic demand from a repeatable customer segment. Another mistake: pivoting too quickly. Give each iteration enough time and traffic to generate statistically meaningful results.
After Product-Market Fit
Once you have genuine PMF, the entire company changes. Your focus shifts from iteration to scaling. Hire aggressively, invest in infrastructure, and pour fuel on the acquisition channels that are working. But never stop measuring -- PMF can be lost if the market shifts or competitors catch up.
