What happens to posts and URLs when you delete a WordPress category?

What core does, step by step

When you delete a category on Posts → Categories, WordPress runs through this sequence:

1. Child categories are re-parented. Each direct child gets the deleted category's own parent, or becomes top-level if there was none.
2. Posts are detached. The category is removed from every post that had it.
3. Orphans get the default. A post left with no category at all is assigned the default category set under Settings → Writing. Posts that had other categories keep those and gain nothing new.
4. Menu items are removed. Navigation menu entries that pointed to the category are deleted with it.
5. The term is gone. Its name, slug and description are deleted from the database. There is no Trash and no undo.

The default category itself cannot be deleted; WordPress refuses until you choose a different default.

Tags follow the same steps except the third: tags have no default, so a post simply has one tag fewer.

What happens to URLs

Three of those rows deserve attention.

The archive returns 404 with no redirect from core. If it ranked or had links, that is lost until you redirect it.

Child archives move because their path contained the deleted parent. WordPress finds a category by the last segment of its path, so the old child address is redirected to the new one. That keeps links working, though the child may now sit at a level you did not intend.

Posts move on sites with the category in the permalink. A post that was at /grinders/burr-review/ may now be at /uncategorized/burr-review/, which is rarely what you want. Core does redirect a post requested under a category path that no longer applies, so old post links arrive, but the new address is the real problem. Reassign those posts to a proper category before deleting.

Delete, or merge?

Deletion suits a category that is empty or was a mistake. When a category has posts and you want to retire it, what you usually mean is a merge:

1. Filter posts by the old category on the Posts screen.
2. Add the destination category to all of them with bulk edit.
3. If categories are in your URLs, set the destination as primary so addresses are predictable. Hydrogen SEO's primary category picker does this per post.
4. Redirect the old archive to the destination archive with a 301. In Hydrogen SEO, add the rule in the Redirection Manager.
5. Delete the old category. Posts keep the destination category; none fall into the default.
6. Test the old archive address, one child archive if there were any, and two or three posts.

Doing it in this order means no post is ever uncategorised and no address is ever a 404.

Before you press Delete

A short check saves a clean-up later:

• Is the archive indexed or linked? Search site:example.com/category/grinders/ and look at its row in Search Console's Pages performance report. If it gets clicks, it needs a redirect, not just deletion.
• Does it have children? Decide where they should live. Moving them up a level may or may not be what you want.
• Is it used anywhere by ID or slug? Theme options, query blocks, widgets and shortcodes that list "posts from category X" will show nothing or fall back to all posts after deletion.
• Is it in your permalink structure? If so, work out the new post URLs first.
• Do you have a backup? Since terms cannot be restored, recreating a category by hand means re-assigning every post.

After deleting, watch the 404 log for the old paths for a few weeks. Hydrogen SEO's 404 monitor lists them with hit counts, and anything that keeps getting requests deserves a redirect to the nearest live archive.