PMF is a curve, not a feeling
Product-market fit (PMF) is the point at which a product satisfies real demand well enough that users keep coming back without heavy, constant prompting. Founders describing PMF as "a feeling in the room" aren't wrong that it's felt, but a VC needs the number underneath the feeling, and that number is almost always a retention curve.
Reading a retention curve
Plot the percentage of a signup cohort still active in each week after joining. A product with no PMF shows a curve that keeps sliding toward zero. A product with real PMF shows the curve flatten, settling at some non-zero level because a core group has genuinely adopted the habit.
Signals beyond the curve
| Signal | What it suggests |
|---|---|
| Retention curve flattens above ~35-40% (consumer) or ~70-80% (B2B) | A real, durable core has adopted the product |
| Users organically invite others without being asked | The product is solving a real, sharable problem |
| Customers get upset about a short outage | The product has become load-bearing, not just nice-to-have |
| Growth requires constant paid acquisition to sustain | Weak signal, often means retention, not top-of-funnel, is the real gap |
None of these alone proves PMF, and thresholds vary hugely by category, a weekly-use consumer app and an annual-contract enterprise tool have very different "good" retention shapes. What matters is triangulating several signals rather than trusting a founder's enthusiasm on its own.
Checkpoint
- PMF: users keep coming back without heavy, constant prompting.
- Retention curve: the clearest quantitative read on whether PMF is real; look for it to flatten, not slide to zero.
- Organic referral and reaction to outages are supporting signals, not substitutes for the curve.
If anything here still feels unclear, ask before moving to Lesson 10.