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What size is a WordPress thumbnail?
WordPress documentation defines the built-in thumbnail default as 150 × 150 pixels, cropped as a perfect square. This is a documented default, not a guarantee that every theme or plugin will display every image at that size. Themes can request their own image variants, and a featured-image card, gallery or archive template may use one of those instead.
The original upload is retained separately. WordPress creates thumbnail, medium, large and other variants when an image is uploaded, subject to the site’s settings and available server processing. The generated thumbnail is therefore a derivative file, not a forced resize of the original.
How to change the thumbnail size in WordPress
- Sign in to the WordPress administration area.
- Open Settings > Media.
- Under Thumbnail size, enter the required Width and Height in pixels.
- Choose whether to enable Crop thumbnail to exact dimensions.
- Save the changes.
Exact cropping enabled
With exact cropping enabled, WordPress makes the thumbnail fit both numbers precisely. A setting of 300 × 200 produces a fixed landscape shape, but portions of a differently proportioned source image may be removed. This is useful for grids in which every card must align.
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Exact cropping disabled
Without the checkbox, the values act as maximum dimensions and WordPress preserves the source image’s proportions. One side can therefore be smaller than the value entered. A portrait image will not be stretched simply to fill a landscape box.
Changing these fields affects thumbnails generated after the change. Existing uploads may still have the old dimensions; changing a setting does not automatically recreate every derivative already stored in the media library. A regeneration tool or a command-line workflow may be needed for older files, and the appropriate method depends on your hosting and site policy.
Generated thumbnail versus what visitors see
Several WordPress controls are easy to confuse because they all use words such as “size,” “crop” or “thumbnail.” Identify which layer you are changing before troubleshooting.
| Control | What it changes | Typical effect |
|---|---|---|
| Settings > Media > Thumbnail size | Generated thumbnail dimensions and, optionally, its crop | Creates a derivative file such as a square 150 × 150 image |
| Theme or plugin image size | A named derivative registered in code | Cards, featured images or galleries may use dimensions different from the built-in thumbnail |
| Image block resolution | Which available image variant is inserted | Choices can include Thumbnail, Medium, Large and Full Size |
| Image block width, height, aspect ratio or Scale | How the selected image is displayed | Changes presentation without editing the original upload |
| Cover block | How an image fills a display area | May crop parts of the image to fill the chosen area |
The block editor’s available resolution choices depend on theme support and the dimensions of the uploaded image. The classic editor commonly lists Thumbnail, Medium, Large and Full Size, but the exact choices on a particular site can differ.
Why a thumbnail may look cropped differently
The thumbnail file is square
If Crop thumbnail to exact dimensions is enabled, WordPress removes portions of the source to create the requested shape. Important faces, logos or text near an edge can disappear when the focal point is not centered.
The block is applying a display crop
An Image block can use an aspect ratio or a Cover-style presentation. The editor may show a square or landscape frame even though the selected file is not square. This is a display decision; it does not rewrite the original image file.
The theme is using another size
A theme can register a named image size for a particular template. The developer reference for add_image_size() documents proportional resizing, hard cropping and crop-position controls. The reserved names thumb and thumbnail are aliases, but a theme may also define other names. If changing Settings > Media has no effect on a card or featured image, inspect the template’s registered sizes rather than assuming it uses the built-in thumbnail.
Choosing dimensions for common layouts
There is no universal best thumbnail size. Start with the largest pixel dimensions the layout actually needs, then match the shape required by the design.
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- Square avatar or grid: use equal width and height with exact cropping when every tile must align.
- Landscape article cards: choose the card’s ratio and use an exact crop only if uniform rows matter.
- Portrait covers: preserve proportions unless the design deliberately uses a fixed portrait frame.
- Variable editorial images: leave exact cropping off when losing the edges would be worse than uneven shapes.
Pixel dimensions also affect layout and file size. WordPress’s image size and quality documentation explains that larger dimensions generally provide more detail but can require more storage and transfer. Do not create a huge thumbnail merely to compensate for a CSS box that is actually small.
How to diagnose a thumbnail that will not change
Check whether the file is new
Upload a test image after saving the new Media settings. If the new image has the expected dimensions but older images do not, the setting is working and the existing derivatives need regeneration.
Inspect the inserted resolution
Select the image in the editor and review its resolution setting. Selecting Medium, Large or Full Size means the page is not using the thumbnail variant, even if the image appears small on screen.
Check the theme and plugins
Featured-image behavior varies by theme. The Page/Post Settings sidebar documentation notes that featured-image use depends on the active theme. A plugin or custom template may also request a registered size with its own crop rules.
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Browser, page, CDN and image-optimization caches can continue serving an older derivative. Purge only the relevant cache after confirming the file itself has the new dimensions, and avoid deleting originals without a restorable backup.
Consider the source image
If the upload is smaller than the requested dimensions, WordPress cannot invent detail. The editor may offer fewer resolution choices, and enlarging a small source can look soft. The Image block documentation explains that available choices vary with the uploaded image’s dimensions and theme support.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Custom image sizes for developers
When a design needs a size that the Media screen does not provide, register a named size in theme or plugin code. The documented function supports proportional resizing, hard cropping and a crop position. A conceptual example is:
add_image_size( 'card-landscape', 640, 360, true );Here, true requests an exact crop. A crop-position array can be used when the focal point should favor a particular area, as described in the developer reference. Registration alone does not guarantee that an old upload has that derivative; the file must be generated for the attachment, and the template must request the registered name.
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WordPress thumbnail checklist
- Confirm whether you need a generated file size or only a display crop.
- Check the current Thumbnail width and height under Settings > Media.
- Decide whether exact dimensions matter more than preserving the full frame.
- Verify which resolution the editor or template inserts.
- Check for theme or plugin sizes registered with
add_image_size(). - Test a newly uploaded image before regenerating older derivatives.
- Inspect caches and source-image dimensions if the result still looks unchanged.
Frequently asked questions
Does WordPress always create a 150 × 150 thumbnail?
150 × 150 pixels is the documented default. Administrators can change it, and themes or plugins can use additional registered sizes.
Will changing the thumbnail setting resize my original photo?
No. WordPress keeps the original upload and creates derivative image variants. A block’s display crop also does not edit the original file.
Why is the thumbnail option missing in my editor?
Resolution choices depend on the active theme, the editor and the dimensions of the uploaded image. A theme can also present a custom image-size name instead of the built-in label.
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