Conversion from free to paid is treated as a funnel metric. It is actually a single moment in the user's experience: the first time they hit a wall that the paid tier removes, and the wall is worth removing. Everything upstream is preparation; everything downstream is billing. The art of free-to-paid conversion is choosing that wall.
What makes a wall convert
The user has already felt the value of the product (the aha has happened). The wall is in the path of doing more of what they want to do, not in the path of starting. The paid tier removes the wall in a way the user understands and trusts. The wall is at the moment of momentum, not at the moment of arrival.
Common walls that do not work
Time-based trial expiration, when the user has not yet hit aha. Feature gates on capabilities the user has not yet wanted. Per-user limits, when the user is still solo. All of these block users before they have a reason to pay, which converts curiosity into churn.
How to choose your wall
Watch the highest-converting users. What were they doing when they upgraded? That action is the wall. Build the free tier so users reliably reach that action, and the paid tier so it removes the wall cleanly. Conversion stops being a funnel; it becomes a product feature.