Scroll depth measures how far down a web page a user scrolled, expressed as a percentage. Tracking arrival rates at the 25%, 50%, 75%, and 100% marks is standard practice.
Scroll depth matters because page views and time on page alone cannot tell you whether a user actually read the content. A user who opens a page and switches to another tab inflates time-on-page without consuming any content. Scroll depth more accurately reflects the degree to which users actually engaged with the material.
Google Analytics 4 (GA4) automatically tracks scroll depth via the scroll event. By default, the event fires when a user reaches 90% of the page. For finer granularity (25%, 50%, 75% thresholds), set up custom triggers in Google Tag Manager.
Scroll depth is indispensable for optimizing landing pages linked from shortened URLs. It reveals whether users scroll far enough to see the CTA button on a campaign destination page. If most users drop off at the 50% mark, you need to either move the CTA higher or improve the content around that point.
Combining scroll depth with heatmaps enables even deeper analysis. Scroll depth provides quantitative data on "how far users read," while heatmaps provide qualitative data on "where attention concentrated." Cross-referencing both pinpoints exactly where content improvements are needed. You can find related books on Amazon.