What is product-market fit and how do you know you have it?

PMF is the most talked-about concept in startups — and one of the hardest to define. Here's what it actually means and how to measure it.

The Most Important Milestone

Marc Andreessen described product-market fit as "being in a good market with a product that can satisfy that market." In practice, PMF is the moment when your product starts pulling itself — when demand exceeds your ability to supply it, when customers tell their friends, and when you stop fighting for every sale.

Before PMF: growth is hard, retention is poor, and you're constantly pivoting. After PMF: growth is the challenge, not acquisition.

The Sean Ellis Test

The most commonly used measure of PMF is the Sean Ellis question: "How would you feel if you could no longer use this product?"

• Very disappointed — strong PMF signal
• Somewhat disappointed — weak signal, more work needed
• Not disappointed — no PMF

If 40% or more of active users say "very disappointed," you likely have PMF. If you're below 30%, you have significant work to do.

Retention as the True Signal

Surveys are useful but lagging. The most honest PMF signal is your retention curve:

• Does it flatten out at a meaningful level (>30% day-30 for consumer, >60% month-6 for B2B)?
• Are your best customers expanding their usage over time?
• Is churn declining as you fix things?

A retention curve that never flattens — that keeps declining toward zero — is the clearest signal you don't have PMF yet.

What PMF Is Not

PMF is not: - Getting your first 10 customers (many of whom are friends or early adopters) - Having positive NPS scores (NPS can be high even without retention) - Raising a round (investors bet on potential, not PMF) - Having a lot of signups (interest ≠ engagement)

Finding PMF: The Loop

PMF is found through a cycle of learning:

• Define your best customer (ICP)
• Get them using your product
• Measure retention and engagement
• Interview churned and retained users to understand the delta
• Iterate on the product (or the ICP) based on what you learn
• Repeat

Most startups that find PMF do so after 3–5 major pivots or product iterations.

Preserving PMF as You Scale

PMF isn't permanent. As you expand to new segments or geographies, your original PMF can degrade. Monitor cohort retention closely as you scale — it's the earliest signal that you're drifting away from your core value.