Activation rate is a cohort metric. Take all users (or accounts) who signed up in a given period, count how many completed your defined activation event within the window, and divide: activation rate = activated users ÷ new users in the cohort. A team with 1,000 signups in June where 280 reached the activation moment by day 14 has a 28% activation rate for that cohort.
Two design choices make or break the number. First, the activation event: it must be the action that correlates with users sticking around, not a vanity step like 'completed onboarding tour.' Second, the window: activation is usually time-boxed (day 1, day 7, day 30) because a user who reaches value on day 90 is behaviorally different from one who gets there in the first session. Always report the event definition and the window alongside the percentage — a bare '28%' is uninterpretable.