Unit economics
Retention rate
The share of a cohort still active after a given interval — the same fact as churn, read from the surviving side.
Formula
Retention(n) = Customers from the cohort active in period n ÷ Cohort size
- Cohort
- everyone who first bought in the same period
- n
- periods elapsed since acquisition, counted from the cohort's own start
Retention read by cohort shows the shape of the curve, and the shape is the finding: a curve that flattens means the business has a real base, a curve that keeps falling means it is renting customers and will have to keep buying them.
The flattening point matters more than any single month's number. A business whose curve settles at 22 % has a durable fifth of every cohort; one whose curve is still falling at month twelve has none.
Mixing cohorts hides the curve entirely. A base that is growing fast reports improving retention while every individual cohort is decaying, because new arrivals keep replacing the departed in the denominator.
A definition is free. Being answerable for the figure it produces is the part that is bought, and this term is a working part of the engagements below.
- Client-base reactivation
- The cheapest customer already gave you their contact details.
The definitions are the easy part. Whether the figure on your dashboard was computed this way is a different question, and usually the more expensive one.