WordPress thumbnails are usually “wrong” for one of four reasons: the page is using an old derivative image, the intended dimensions are defined somewhere other than the setting you changed, cropping is configured differently than expected, or the browser is selecting another responsive image. Fix the source of the size first, then regenerate existing attachments and verify the rendered HTML.
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Why WordPress thumbnails have the wrong dimensions
WordPress normally keeps the original upload and creates several derivative files when the image is added to the Media Library. A derivative may be the built-in thumbnail, a medium or large image, or a custom size registered by the active theme or a plugin. The page usually requests one of those derivatives rather than the original file.
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Changing a setting or editing theme code changes what WordPress will create or request in the future. It does not automatically rebuild derivative files that already exist. That is why a newly uploaded image can look correct while an older featured image remains cropped, too small, or stretched.
The built-in Thumbnail size defaults to 150 × 150 pixels in current WordPress documentation. That number is only a default, not a guarantee that every thumbnail displayed on your site will be 150 × 150. A theme can register a separate post-thumbnail size, and a template can request another custom size entirely.
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First, identify which image WordPress is actually serving
Do not start by changing settings at random. First establish whether the problem is the file, the requested derivative, or the way the browser displays it.
- Open the page with the incorrect thumbnail in a desktop browser.
- Right-click the image and choose the browser’s developer-tool option to inspect it. In the Elements panel, select the actual
<img>element, not just its surrounding link or figure. - Record the displayed width and height, then read the image URL in the
srcattribute and in anysrcsetattribute. - Look for a filename suffix such as
-150x150,-300x200, or a custom suffix generated by the theme. If the URL has no size suffix, the page may be using the original full-size file. - Compare the URL’s intrinsic dimensions with the dimensions imposed by CSS. The image can be a perfectly correct file while CSS displays it in a box with a different aspect ratio.
This inspection gives you a concrete starting point. A -150x150 file confirms that WordPress is serving a square derivative; it does not, by itself, explain whether the square was intentionally hard-cropped or simply displayed in a square box. A custom suffix points you toward theme or plugin registration rather than the basic Media settings.
Check Settings > Media before editing code
In the WordPress administrator, go to Settings > Media. Review the values for Thumbnail, Medium, and Large. Pay particular attention to both the width and height fields and to the checkbox labeled Crop thumbnail to exact dimensions.
What the three built-in sizes control
- Thumbnail: the smallest built-in derivative. The documented default is 150 × 150 pixels.
- Medium: an intermediate derivative used by WordPress and themes that request the medium size.
- Large: a larger derivative for layouts that need more pixels than Medium but do not need the original upload.
The exact values on your site may differ because an administrator, an installation process, or a previous theme changed them. The important point is to compare the saved values with the dimensions your layout requires, rather than assuming the defaults apply.
Understand the crop checkbox
When Crop thumbnail to exact dimensions is enabled, WordPress creates a fixed box and removes portions of the source image to reach the requested width and height. A landscape photograph can therefore lose its sides when it is made square. When the option is disabled, WordPress scales the image proportionally, preserving the original aspect ratio; one dimension may be smaller than the value entered because the other dimension is the limiting side.
This setting affects the built-in thumbnail behavior. It does not override a theme’s independent crop instruction for a custom size. Save the setting only after deciding whether your design needs a fixed, uniform box or the complete image with its natural proportions.
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If the dimensions in Settings > Media do not match the file being requested, inspect the active theme and image-related plugins. WordPress themes commonly register image sizes in PHP.
set_post_thumbnail_size()
A theme can define the featured-image size with set_post_thumbnail_size(). Conceptually, a registration may look like this:
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set_post_thumbnail_size( 640, 360, true );The first two arguments are the requested width and height. The third argument controls geometry: true requests a hard crop to that box, while false requests proportional scaling. A theme’s post-thumbnail size can therefore differ from the administrator’s built-in Thumbnail setting.
Changing this registration affects images generated after the change. WordPress’s developer reference explicitly notes that set_post_thumbnail_size() does not resize existing featured images.
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add_image_size()
The theme or a plugin may instead use add_image_size() to register a named custom size:
add_image_size( 'card-landscape', 640, 360, true );The name is later passed to a template function when the image is requested. A custom size can have its own dimensions and crop behavior, independent of the built-in Thumbnail, Medium, and Large values. Search the active theme and plugins for both function names, and note every width, height, name, and crop argument that relates to the template showing the incorrect image.
Do not edit a parent theme if the site uses a child theme and you need the change to survive an update. More importantly, make sure you are inspecting the active theme, not an installed theme that is currently disabled.
Choose proportional scaling or a hard crop
The correct geometry depends on the component, not on a universal “best” setting.
| Decision point | Proportional scaling | Hard crop |
|---|---|---|
| Shape | Preserves the source aspect ratio; the rendered box may not have a uniform height. | Produces the registered width and height when enough source pixels are available. |
| What happens to the picture | No edges are deliberately removed. | Parts of the image are removed to fit the fixed box. |
| Good fit | Editorial photos, product images, and any content where the entire subject must remain visible. | Grid cards, avatars, and layouts that require every tile to align. |
| Main risk | Cards can have uneven heights or letterboxing supplied by CSS. | Faces, text, or other focal details can be cut off. |
| Where it is set | A disabled crop option or a false crop argument. |
The enabled crop option or a true crop argument. |
If a subject is repeatedly cut off, changing the dimensions alone may not solve the problem. You may need a less aggressive crop, a different aspect ratio, or a source image whose important content is positioned near the center of the crop area.
Correct the definition, then regenerate old attachments
Once you have identified the responsible setting or registration, correct it first. Regeneration before the definition is fixed simply recreates the wrong files.
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- Decide the target dimensions and whether the target is proportional or hard-cropped.
- Save the values in Settings > Media, or update the relevant
set_post_thumbnail_size()oradd_image_size()registration. - Confirm that the template actually requests that size. A corrected registration has no effect if the template still asks for another named size.
- Regenerate the existing attachments so WordPress rebuilds derivatives from the original uploads.
- Clear any page, object, or browser cache used by the site, then inspect an old image again.
Regeneration works from the original full-size attachment. It does not turn a previously tiny derivative into a higher-quality original, and it does not replace the original upload. If the original itself lacks sufficient pixels, the rebuilt derivative cannot contain detail that was never present.
Regenerate through an administrator tool
A regeneration plugin or other administrator tool can rebuild the Media Library without requiring command-line access. The exact labels vary by tool, but the safe workflow is consistent:
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- Choose the action that regenerates thumbnails or image sizes, not an action that deletes originals.
- Regenerate all sizes if several registrations changed, or select only the affected size when the tool permits that choice.
- Let the process finish; large libraries can require substantial time and server resources.
- Test both a previously uploaded image and a newly uploaded image after completion.
If a tool reports failures, note the attachment names and the size that failed. A failed regeneration can result from a missing original, insufficient server resources, or an image that the server cannot process. Do not assume a completion message means every attachment was rebuilt unless the tool reports the count and any errors.
Regenerate with WP-CLI
Sites with shell access can use WP-CLI. The general command is:
wp media regenerateWP-CLI also supports an --image_size filter when you need to target one registered size:
wp media regenerate --image_size=thumbnailUse the registered size name, not a filename suffix. For a custom size, the value must match the name supplied to add_image_size(). Targeting one size reduces unnecessary work when only that derivative changed; regenerating all sizes is appropriate when multiple built-in or custom definitions were corrected.
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Why a correct file can still display incorrectly
Finding the expected dimensions on disk does not prove that the page will display them. Since WordPress 4.4, native responsive images have been supported through srcset and sizes attributes in generated image markup. The browser uses those attributes, the viewport, and the layout’s declared size to choose a candidate file.
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Inspect srcset and sizes
The srcset attribute can list several width variants. The sizes attribute tells the browser how wide the image is expected to be at different layout conditions. If sizes describes a 300-pixel slot when the image is actually displayed at 640 pixels, the browser may select a file that is too small. Conversely, a wrong size declaration can cause a much larger file to be downloaded than necessary.
In developer tools, compare the displayed width with the candidate selected in the image properties or network panel. Then check whether the theme’s generated sizes value matches the CSS layout. Do not remove responsive attributes simply to force the original image; correct the requested size or layout description instead.
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Check CSS width, height, and aspect ratio
CSS can stretch a correctly proportioned file by assigning both width and height that do not share the image’s aspect ratio. It can also use object-fit: cover to fill a fixed box, which visually crops the image even when the underlying derivative was proportional. Inspect the image’s computed width, height, and object-fit behavior before regenerating files again.
Check the template’s requested size
A featured-image function may request post-thumbnail, while a card component requests a custom name. The URL suffix tells you which result was selected, but the PHP template tells you why. If the theme requests a custom size, changing the built-in Thumbnail setting will not change that component.
Use the URL suffix as a diagnostic clue
| What you see in the URL | Likely direction for investigation |
|---|---|
-150x150 |
The built-in thumbnail derivative or a size with matching dimensions is being served; check crop settings and whether regeneration occurred. |
A different numeric suffix such as -300x200 |
Check Medium or another registered size, then inspect the template and responsive candidates. |
| A custom name or suffix | Search theme and plugin code for add_image_size() and identify the function that requests that name. |
| No derivative suffix | The original full-size file may be used, or the URL may be rewritten by another image system; inspect the HTML and computed CSS. |
The suffix is evidence about the file being requested, not proof of the visual result. A CSS crop, a responsive selection, or a stale cache can still change what visitors see.
Verify the repair with old and new images
Test more than one attachment because regeneration and future uploads exercise different paths.
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- Choose an image uploaded before the settings or code change. Confirm that its URL now points to the intended derivative and that its intrinsic dimensions match the target.
- Choose an image uploaded after the change. Confirm that WordPress creates the same intended size without requiring another manual regeneration.
- Check a wide source, a tall source, and a source with an important subject near an edge. This reveals whether proportional scaling and cropping behave as designed.
- Inspect the image at the actual card or featured-image breakpoint, not only in the Media Library.
- Check both desktop and narrow viewport layouts so the browser’s responsive candidate selection is exercised.
- Confirm that the original full-size attachment remains available and unchanged.
Record the final dimensions and crop behavior for the component in your theme documentation. That makes a later theme change easier to diagnose because you can distinguish an intentional redesign from an accidental size regression.
Common failure modes and their fixes
The setting was changed, but old images are unchanged
Media settings do not rebuild existing derivatives. Regenerate the affected attachments after saving the new values.
Only featured images are wrong
Inspect set_post_thumbnail_size(), add_image_size(), and the featured-image template. The theme may use a post-thumbnail or custom size that bypasses the built-in Thumbnail setting.
The image is always square and loses the subject’s sides
Check whether the crop checkbox is enabled or the relevant registration passes true as its crop argument. Use proportional scaling or a less restrictive aspect ratio when the full subject must remain visible.
The file dimensions are right, but the visual box is wrong
Inspect CSS width, height, object-fit, and aspect-ratio rules. The problem is presentation rather than thumbnail generation when the correct file is being requested.
The browser downloads a small candidate on a wide layout
Inspect srcset and sizes. Correct the theme’s responsive size description or requested image size instead of deleting responsive markup.
Regeneration appears successful, but visitors see the old image
Check the image URL in a private browser window and clear the site’s page or object cache according to its normal operating procedure. Then verify the HTML again; a cache can preserve old markup even after new files exist.
A custom size never appears
Confirm that the registration runs on the active theme or plugin, that the name is spelled consistently, and that regeneration occurred after registration. A size definition alone does not alter existing attachments.
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| Question | If the answer is yes | Next action |
|---|---|---|
| Does the URL show an old or unexpected suffix? | The page is requesting a different derivative than expected. | Inspect the template and registered sizes. |
| Do Settings > Media values match the desired built-in size? | The built-in definition is probably correct. | Check theme registrations and regenerate old attachments. |
| Is the image’s original aspect ratio being preserved? | The result should not be forced into a fixed box by generation. | Check CSS for visual cropping or stretching. |
| Does the image have the right intrinsic dimensions but the wrong displayed dimensions? | The file is not the primary problem. | Inspect srcset, sizes, CSS, and template output. |
| Are only older uploads affected? | New generation rules are working. | Regenerate the existing library or selected attachments. |
| Are new and old uploads both wrong? | The active definition or display code is still incorrect. | Recheck the active theme, plugin registration, and requested name. |
What not to do
- Do not repeatedly change Media settings without regenerating old attachments; each change can leave another set of unused derivatives.
- Do not edit the original upload to compensate for a template requesting the wrong size.
- Do not assume a filename suffix identifies the template that requested it; verify the PHP registration and rendered markup.
- Do not force every image to a square crop when the component’s content needs a different aspect ratio.
- Do not remove
srcsetandsizesmerely because one candidate is wrong. Correct the responsive description or requested size. - Do not delete original attachments during cleanup unless you have independently verified that every required derivative and backup exists.
The shortest reliable repair sequence
For most sites, the repair can be reduced to this order: inspect one broken image’s URL and rendered dimensions; check Settings > Media; inspect set_post_thumbnail_size() and add_image_size(); choose proportional scaling or hard cropping deliberately; save the corrected definition; regenerate existing attachments with an administrator tool or wp media regenerate; then verify an old image, a new image, and the responsive HTML.
If those checks agree but the image still looks wrong, the remaining cause is usually outside thumbnail generation: CSS is changing the box, srcset/sizes is selecting an unexpected candidate, the template requests another registered size, or cached markup is being served. Following the URL from the rendered page back to its registration prevents all four problems from being treated as the same “thumbnail” setting.
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