Imagine you open a brand new coffee shop in your neighborhood.
On Day 0 (The Grand Opening), the hype is real. You put up balloons, give out free samples, and 100 people walk through the door to try you out.
The real test is the very next morning, Day 1. The balloons are gone. How many of those exact same 100 people came back to buy a coffee on their way to work? If only 5 returned, your coffee might not be meeting expectations.
Fast forward to Day 30. The hype has faded. Maybe 3 people from that first day are still visiting every morning. These are no longer just "users"—they are your "Regulars." They have formed a habit.
Key Takeaway: Retention measures how well you turn passing visitors into loyal regulars. If nobody comes back on Day 1, spending more money on marketing (inviting more people to the shop) will just drain your budget.
Retention is the percentage of users who return and actively use your product after their first visit.
$$ Retention(Day N) = \frac{\text{Users Active on Day N}}{\text{Users who Started on Day 0}} \times 100\% $$
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❕ Short Notation Example:
$$
Ret30d = 10\%
$$
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To calculate this, you must define exactly what "Started" and "Active" mean for your product.
The user's First Interaction with your product. This definition changes based on your business model:
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❕Note on Day 0
Day 0 is simply the day the first interaction happened. Because of this, Day 0 Retention is almost always 100% (everyone who started, started). The real measurement begins on Day 1.
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The user performed a Key Action on a specific day after their start date.