Formula
Active users from cohort at interval N / Original cohort size x 100
How to measure
Group users by signup period, then measure what percentage of each cohort remains active at each subsequent interval, typically plotted as a retention curve.
Why track it
Cohort retention reveals whether product or onboarding changes are actually improving retention over time, which a single blended number would hide.
Example
The March signup cohort retains at 45% by month three, up from 30% for the January cohort, showing improvement after an onboarding redesign.