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Why Force Regenerate Thumbnails Creates So Many Image Sizes (And How to Manage Them)

47 threads Sep 16, 2025 Plugin → Force regenerate thumbnails

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If you've used the Force Regenerate Thumbnails plugin, you might have been surprised to see it generating a long list of image sizes—far more than the few you set in your WordPress Media Settings. This is a common point of confusion, but it's not a bug in the plugin. Let's break down why this happens and what you can do about it.

Why Does This Happen?

The Force Regenerate Thumbnails plugin has a specific job: it deletes all existing thumbnails for an image and then regenerates every single image size that is currently registered with WordPress. The key term here is "registered."

Your registered image sizes come from three main sources:

  1. WordPress Core: The default 'thumbnail', 'medium', 'medium_large', and 'large' sizes you set in Settings > Media.
  2. Your Theme: Most themes add their own custom image sizes for sliders, galleries, featured images, and other layout elements.
  3. Your Plugins: Plugins like WooCommerce, bbPress, and many others register their own sizes to function properly (e.g., WooCommerce's product gallery images).

When you run the plugin, it processes all of these registered sizes. So, that long list of 100×100, 300×300, 768×432, and other dimensions isn't random; it's the complete set of sizes that your entire WordPress installation (core, theme, and plugins) has requested.

Common Solutions and Workarounds

1. Identify the Source of the Sizes

Before trying to remove sizes, you need to know what is creating them. While you can search through your theme and plugin code for add_image_size() function calls, an easier method is to use a plugin that lists them all. Some image optimization plugins include a feature that displays a complete list of all registered image sizes, often indicating which plugin or theme registered them.

2. Control Which Sizes Are Generated

By itself, Force Regenerate Thumbnails does not have a built-in interface to select specific sizes for regeneration; it processes all registered sizes. However, you can control the regeneration process by filtering the list of registered sizes before the plugin acts on them.

Some image optimization plugins offer a feature to disable the generation of specific, unused thumbnail sizes. If you use such a plugin to disable a size, Force Regenerate Thumbnails will respect that setting and will not regenerate that size, as it will have been removed from the list of registered sizes that WordPress provides.

3. Manually Clean Up Old, Unused Files

It's important to understand what Force Regenerate Thumbnails deletes. It only deletes the thumbnails for the sizes that are currently registered. When it runs, it removes the files for those specific sizes and then creates new ones. It does not automatically track down and delete old thumbnail files from sizes that are no longer registered by any theme or plugin.

To free up server space by deleting these legacy, unregistered files, you may need a different tool or a manual method, such as:

  • Using a dedicated plugin designed to find and remove unused image files.
  • Using a command-line script (if you have server access).

Key Takeaway

The behavior of Force Regenerate Thumbnails generating many sizes is by design. The plugin's core function is to ensure all currently active image sizes are correctly reformatted, which is crucial after changing a theme, modifying image quality settings, or altering image dimensions. The long list of sizes is a reflection of your site's entire ecosystem, not the plugin creating unnecessary files.

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