Marc Andreessen defined product-market fit as being in a good market with a product that can satisfy that market. Useful, but abstract. Sean Ellis quantified it: if forty percent or more of your users would be "very disappointed" if they could no longer use your product, you have achieved product-market fit. Below that threshold, you have not — and no amount of growth tactics will make a meaningful difference.
The honest evaluation of product-market fit requires resisting the optimism bias that afflicts every founding team. Customer testimonials, positive demo responses, and early sales are not evidence of PMF. Retention is evidence: do users who experience the product return? Engagement is evidence: do users who return use the product frequently and across its core value proposition? Organic growth is evidence: do satisfied users tell others without being incentivised to?
The metrics that most reliably indicate PMF in a B2B SaaS context: week-four retention above thirty percent for the acquired cohort, Net Revenue Retention above one hundred percent (existing customers expanding faster than they churn), and sales cycles shortening because customers are coming pre-educated through referrals and reputation rather than requiring full-cycle education.
Before PMF, the founder's job is to talk to users obsessively, ship rapidly, and iterate based on what you learn — not to scale. Hiring a large sales team, running broad marketing campaigns, and scaling engineering capacity before PMF generates burn without corresponding value creation.
The most common mistake is defining the successful segment of early users as representative of the full market. If your product has strong PMF with a specific user archetype but weak PMF with the broader audience you are targeting, you have partial fit that needs to be understood before scaling. The founders who scale premature PMF are the ones who raise Series B on the strength of one customer segment and discover at Series C that the rest of the market does not behave the same way.