How to Measure Product-Market Fit: Beyond the Sean Ellis Score

The Sean Ellis 40 percent rule ("how would you feel if you could no longer use this product?") gives you a directional signal about whether your current users would miss the product. But it tells you nothing about whether the market is large enough, whether the founder can execute on the opportunity, or whether the team can ship the next twelve months of work without falling apart.

A founder with 50 percent must-have users in a market of 800 buyers has a great lifestyle business and a hard ceiling. A founder with 28 percent must-have users in a $40B market with the right team and capital has a venture-scale company in the making. The number is the same kind of input — the conclusion is opposite.

Market readiness asks: is there a real, large enough, reachable market that has the budget and urgency to buy? It includes total addressable market, ideal customer profile clarity, competitive landscape, pricing power, and channel feasibility.

Founder readiness asks: is the founder making decisions based on evidence or on protective patterns? It includes the founder's ability to talk to customers without selling, to update beliefs when data contradicts assumptions, to name the metrics they actually run on, and to detect their own cognitive blind spots.

Execution readiness asks: can the team turn decisions into shipped reality? It includes velocity, prioritisation discipline, the capacity to run experiments and read results, and the operational hygiene to compound learning week over week.

Retention curves over time. Cohort retention is the most honest signal in the business. Flatten above zero and you have a product. Decay to zero and you do not.

Time to value. How many minutes, days, or weeks until a new user gets the outcome they came for. Shorter time to value is the strongest correlate of organic growth in consumer products.

Sales cycle length and discount depth. If your enterprise sales cycle is shrinking and your discounts are shrinking, the market is pulling. If both are growing, you are pushing.

Net revenue retention. For B2B, NRR above 110 percent is the strongest single predictor of durable PMF — because it shows existing customers expanding their use and spend voluntarily.

It is normal for one signal to be strong while another is weak. Strong retention with weak organic growth often means the product works but the message does not. Strong organic signal with weak retention often means the marketing is overpromising what the product delivers.

Treat these conflicts as diagnostic information, not noise. The pattern of where you score well and where you score badly is often more useful than any single number. A balanced 60/100 across all three dimensions is a different business from an 80 in market and a 40 in execution — and the 90-day plan looks completely different in each case.