Formula
Unsubscribes / Emails delivered x 100
How to measure
Divide the count of unsubscribes triggered by a specific send by the number of emails delivered in that send.
Why track it
A rising unsubscribe rate is an early warning that email frequency or relevance has drifted away from what the audience signed up for.
Example
A send to 20,000 subscribers producing 60 unsubscribes reports a 0.3% unsubscribe rate.