I’ve tried to make the concept less abstract by offering a specific metric for determining product/market fit. I ask existing users of a product how they would feel if they could no longer use the product. In my experience, achieving product/market fit requires at least 40% of users saying they would be “very disappointed” without your product. Admittedly this threshold is a bit arbitrary, but I defined it after comparing results across nearly 100 startups. Those that struggle for traction are always under 40%, while most that gain strong traction exceed 40%. Of course progressing beyond “early traction” requires that these users represent a large enough target market to build an interesting business.

You should measure your product/market fit as soon as possible because it will significantly impact how you operate your startup. If you haven’t reached product/market fit yet it is critical to keep your burn low and focus all resources on improving the percentage of users that say they would be very disappointed without your product. Avoid bringing in VPs of Marketing and Sales to try to solve the problem. They will only add to your burn and likely won’t be any better than you at solving the problem. Instead, you (the founders) should engage existing and target users to learn how to make your product a “must have.” Sometimes it is as simple as highlighting a more compelling attribute of your product – but often it requires significant product revisions or possibly even hitting the restart button on your vision. For more on getting to product/market fit, I recommend reading Marc Andreesen’s full post via archive.org (it has been removed from his blog).

This is a subject of much debate:

Achieving product/market fit requires at least 40% of users saying they would be “very disappointed” without your product.

There are many kinds of users and by the way users are not necessarily customers.