Ranked by how often people hit the star Favourite on a tool. Your own favourites stay on this device; the count is everyone together.
This is a count of how many people pressed Favourite on a tool — not page views, not time on page, and not a judgement about which tool is best. It is deliberately the weakest useful signal available: one bit per browser, no account, no calculator inputs, nothing that could identify anybody. What it buys in honesty it gives up in precision, and it is worth being clear about how.
A calculator published eighteen months ago has been in front of far more people than one added last month, so the top of this list is partly a list of what has been here longest. A new tool starting at zero is not unpopular; it is new. The changelog shows when each one arrived, which is the correction to apply when reading this page.
A mortgage calculator has an audience of millions; an O-ring gland design calculator has an audience of people designing seals. The second is not worse, and for the person who needs it, it is the more valuable page by a wide margin. Popularity measures audience size, which is mostly a fact about the subject rather than about the tool.
The tally accepts one vote per browser per tool. That keeps a single enthusiastic user from moving the order, but it also means clearing your storage and voting again would count twice. There is no login to prevent that, and adding one to protect a popularity list would be a poor trade. Read the order as a rough grouping rather than a ranked table.
If you want the catalogue rather than the ranking, every tool is listed A–Z, grouped by subject on the home page, and the underlying reference material is in the Rulebook with its sources.