How do I know if I've found product market fit?

Short Answer I would not call it product market fit because you hit a certain number of users, revenue, or signups. I'd look for pull. People keep using the product. They pay for it. They come back without you chasing them. They tell other people about it. And if you took it away, some of them would genuinely be upset. That is a much better signal than a big signup number.

Why I Think This I think founders make product market fit sound much more complicated than it needs to be. They start looking for a magic number. “Do I need 100 customers?” “Is $10,000 MRR enough?” “Is 40% retention good?” “Should I run the 40% test?” These are useful questions. But I don't think they are the first question I'd ask. I'd ask: “If I stopped pushing this product tomorrow, would people still want it?” That's the part I care about. There is a big difference between people liking your product and people needing it. Someone saying, “This is really cool,” doesn't tell me much. Someone using it every week, paying for it, asking when the next feature is coming, and getting annoyed when it stops working tells me a lot more. I've seen this difference come up again and again when working with founders. People will tell you they like an idea. They'll tell you they would use it. They may even sign up. But then they don't come back. That's why I would look at behaviour before opinions. Retention is probably the signal I'd trust most. If every new group of users slowly disappears, you don't have product market fit yet. If the curve starts settling and a group of users keeps using the product, that's much more interesting. I'd also look for people pulling the product out of you. They ask for it.  They tell other people about it.  They find their own ways to use it.  They complain when something breaks.  They don't need you to keep convincing them. That's what real demand starts to look like. There is a useful test for this too. Sean Ellis asks active users: “How would you feel if you could no longer use this product?” If 40% or more say they would be “very disappointed”, that's a useful signal that you may have found strong product market fit. But even Sean Ellis describes the 40% number as a benchmark based on his experience, not some law of nature. I'd use the test.  I just wouldn't worship the number. Superhuman is a good example. When they first ran the test, only 22% of their users said they would be very disappointed without the product. Instead of deciding the product was bad, they looked closely at the users who loved it most, figured out who they were, and focused the product around them. Their score eventually reached 58%. That's the part I like. The test didn't tell them, “You have failed.” It told them, “These are the people who really care. Now figure out why.” So if you have 20 customers and 15 of them would be genuinely annoyed if you disappeared tomorrow, I'd take that seriously. If you have 5,000 signups and most of them used your product once, I'd be much less excited. I'd rather have 20 people who really need the product than 5,000 people who think it's interesting. If I Were Starting From Scratch... I'd forget about trying to prove product market fit with one number. I'd make a list of my active customers and look at what they actually do. Who keeps using the product? Who pays without me chasing them? Who comes back after the first few weeks? Who has referred someone else? Who gets annoyed when something breaks? Then I'd talk to the customers who seem to love it most. I'd ask what they would do if the product disappeared tomorrow. I'd also run the Sean Ellis question if I had enough active users for the result to mean something. Then I'd look at all of it together. If the same pattern keeps showing up, I'd start thinking about growth. If I'm still spending most of my time convincing people to care, I'd stay focused on the product.