How can scrolling or reading time be measured?

The honest answer: not very accurately. Email clients don't expose the same interaction data that web browsers do. You can't attach a JavaScript event listener to a scroll action in an email. So any reading time or scroll depth measurement is an inference, not a direct count.

How reading time gets estimated

Most ESPs that offer reading time data are using a simple calculation: the time between when their tracking pixel loaded (the "open") and when the message was closed or the user navigated away. Some email clients like Gmail report approximate reading time categories: "glanced" (under 2 seconds), "skimmed" (2-8 seconds), "read" (over 8 seconds). These categories come from Litmus's Email Analytics panel, not from the ESPs directly.

Apple Mail is a particular problem here. Since Mail Privacy Protection launched in 2021, Apple pre-loads tracking pixels in the background before the recipient actually opens the email. That makes reading time data from Apple Mail unreliable. The pixel fires even if the person never looked at the message.

What scroll depth can tell you

Scroll depth is even harder to measure than reading time. Some ESPs use image-based hacks: a series of small tracking images placed at different heights in the email. If the image at 50% height loaded, the user probably scrolled at least halfway. It's a rough signal at best. Most email clients block remote images by default until the recipient enables them, which makes even this approach inconsistent.

The practical takeaway: use click rate and click-to-open rate as your real engagement signals. They're imperfect too, but they require an actual action. Reading time and scroll depth numbers are directional at best. If your click rate is healthy, people are reading. If clicks are low, it's the content or the offer, not whether they scrolled to the bottom.