The question comes up at every catalogue overhaul: two, three or four levels of categories? The answer depends less on a general rule than on the size of your catalogue and on how your customers search.
The three-click rule, and what it is worth
The idea that everything must be reachable in three clicks has been circulating for twenty years. Usability research has heavily qualified it: visitors do not count their clicks, they give up when they feel they are not making progress.
A five-click journey where each step visibly narrows the field is experienced better than a two-click journey whose second step lands on a list of six hundred references.
What does remain true, from the search engines’ point of view: click depth from the home page influences crawl frequency. A page six clicks deep is crawled less often and treated as less important.
The useful formulation is therefore not “three clicks maximum” but: your commercially important pages must be close to the home page, the others can sit further away.
The right number of levels
Three benchmarks by catalogue size.
Fewer than 200 references: two levels are enough. A three-level tree on a small catalogue produces categories with three products in them, which is worse than no subcategory at all.
From 200 to 3,000 references: three levels, the most common and most balanced setup. Universe, family, subfamily.
Beyond 3,000 references: three levels as well, complemented by faceted filters. Moving to four levels is almost always a mistake: it signals that you are using the tree for what facets should be doing.
Category or facet: the criterion
This is the structural decision, and it has a fairly clear answer.
A criterion deserves a category when it matches the way customers name what they are looking for, and when it is searched for on search engines. “Cordless drills” is a category: people type it.
A criterion belongs to a facet when it serves to narrow a list without constituting a search intent in itself. Colour, price, availability, rating.
The practical test: if you can picture writing a specific, useful introduction for that page, it is a category. If you would have nothing particular to say about it, it is a facet.
Some criteria sit on the border and deserve both treatments: brand, for instance, often exists both as a facet and as a dedicated page.
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How many products per category
Two thresholds frame a healthy tree.
The lower threshold, around eight to ten products. Below that, the subcategory helps nobody: it adds a click to reach a list the visitor would have scanned effortlessly at the level above. Merge it.
The upper threshold, around two to three hundred products. Beyond that, category navigation is no longer enough and the visitor needs filters. That is not necessarily a problem, provided the facets are effective.
A useful check, once a year: list your categories with their number of active products, sort, and look at both ends. The obvious corrections show up in a few minutes.
The symptoms of a badly calibrated tree
Too deep: categories with fewer than five products, a dropdown menu that requires three hovers, level-4 pages that get no traffic, and customers who use internal search rather than the menu.
That last signal is the most telling. A high internal search usage rate on a mid-sized catalogue indicates that the navigation is not understood.
Too flat: categories with several hundred products, intensive filter use right from arrival, and a high bounce rate on category pages.
The role of the menu
The tree and the menu are two different things, and confusing them produces unreadable menus.
Your tree may have three levels, your menu does not have to display all of them. A mega menu showing two levels with a few highlighted entries from the third is more effective than an exhaustive listing.
Conversely, the menu can contain entries that are not categories: seasonal selections, guide pages, new arrivals. These cross-cutting entries answer intents the tree does not cover.
Changing an existing tree
Three precautions, because reorganising categories touches URLs.
Every deleted or merged category must be redirected to its replacement, permanently. A category that had been receiving traffic for years loses all of it without a redirect.
The default categories of products must be rechecked after the reorganisation. They determine product URLs and the breadcrumb, and they are easily lost during a bulk move.
Product positions in the new categories start from scratch. Plan the time to reorder them on your main categories.
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