How to calculate engagement decay curves?

An engagement decay curve shows you how subscriber interest decreases with time. It answers the question: if someone signed up six months ago and you've been sending them weekly emails, how does their engagement today compare to their engagement in week one? The answer, for most lists, is that it drops. The curve tells you by how much and when.

The basic calculation

To build a decay curve, you need to group subscribers by how long they've been on your list and compare engagement rates across those cohorts. The simplest version:

1. Pull your subscriber list with signup dates.
2. Calculate each subscriber's open rate (or click rate) over a fixed period, say the last 90 days of sending.
3. Group subscribers into buckets by list age: 0-30 days, 31-90 days, 91-180 days, 181-365 days, over 365 days.
4. Calculate average engagement for each bucket.
5. Plot the averages against the time bucket on the x-axis.

The resulting line is your decay curve. If engagement is 45% in the 0-30 day bucket and 12% in the over-365-day bucket, you have a steep decay. If it's 45% dropping to 35%, your list retains interest unusually well.

What to do with it

The decay curve tells you where your re-engagement triggers should fire. If engagement falls off sharply after 90 days, that's when to send a win-back campaign. If the drop is gradual, you have more time. It also tells you the realistic lifespan of a subscriber for planning purposes: if most subscribers are disengaged within six months, your list growth rate needs to outpace that churn.

A few practical caveats. Apple Mail Privacy Protection inflates open rates for iOS users, which can flatten the early part of the curve artificially. Use click rate as your primary signal if Apple Mail is a significant share of your audience. And make sure you're comparing like-to-like: subscribers from different acquisition sources often have very different decay profiles.