What is Scroll depth?
Scroll depth can be read two ways, and only one of them leads to a decision. The first is depth per visit: this person got to 62 per cent of the page. The second is the share per mark: 100 per cent of visitors see the top, 48 per cent reach the halfway point, 19 per cent the end. Only the second reading answers the question you actually have, namely whether a given element reaches any meaningful share of your visitors at all.
That makes the metric above all a placement tool. Form, price, booking button, proof: everything the decision needs belongs above the mark a clear majority still reaches. Below it, the element does not exist for most visitors, however well it is written. The check takes a minute and settles long arguments about whether a page is too long.
The trap is that scroll depth does not measure reading. Someone who flicks to the bottom in two seconds produces the same value as someone reading every paragraph. On top of that, the percentage is relative to page height, so a long page scores worse for that reason alone. Scroll depth is therefore never read on its own, always against a goal: does the visitor reach the point where the action becomes possible?
Why does Scroll depth matter?
In an eyetracking analysis of more than 130,000 fixations from 120 participants, the Nielsen Norman Group measured how viewing time is distributed: 57 per cent on the first screenful, 74 per cent on the first two, 81 per cent on the first three. People do scroll - they simply read far less further down, and that distribution decides where your main button has to sit.
Scroll depth in practice
- 01A service page has its booking button at the very bottom. Scroll depth shows 22 per cent of visitors reach it, so a second button moves into the first screenful.
- 02After a blog post is shortened, 41 per cent instead of 26 per cent reach the closing offer - the same metric, measured against its own past.
- 03On a phone the mark half of all visitors reach sits at 38 per cent of the page, on a desktop at 55 per cent. The order of the sections is then set separately for the phone.
