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WordPress’s documented default thumbnail size is 150 × 150 pixels, cropped square. It is a starting default, not a universal recommendation: the right size and crop depend on your theme, the image’s use, and the space where it appears. Check your own values at Dashboard → Settings → Media before changing them.
What thumbnail size should you use in WordPress?
For the built-in Thumbnail size, WordPress documents a default of 150 × 150 pixels with a square crop. The setting can be changed, and a theme or plugin may affect the image sizes or choices available on a particular site. Treat 150 × 150 as the documented default—not proof that your site currently generates or displays every thumbnail at that size. WordPress’s Edit Media documentation describes the default and where it can be adjusted.
Choose dimensions for the actual layout. A small square image used in a compact list has different needs from a landscape card image or a wide featured image. Also distinguish the dimensions WordPress generates from the dimensions a browser displays: a file can be served at one pixel size and rendered smaller or responsively in the page.
There is no single recommended Medium or Large value that suits every site. Inspect the theme’s content width and the contexts where each size is used, then preview representative pages on desktop and mobile.
What the built-in WordPress image sizes mean
Go to Dashboard → Settings → Media. WordPress provides fields for Thumbnail width and height, Medium maximum width and height, and Large maximum width and height. The thumbnail option can crop images to exact dimensions; without an exact crop, images are normally resized proportionally. The settings documentation explains these controls at Settings Media screen.
| Size | What the setting controls | How to choose it |
|---|---|---|
| Thumbnail | Target width and height, with an option to crop to exact dimensions. | Use a square crop when the design calls for a consistent square tile. Otherwise preserve the image’s proportions. |
| Medium | Maximum width and height for this generated size. | Base the limits on the content patterns that use Medium images, not a supposed universal WordPress standard. |
| Large | Maximum width and height for this generated size. | Consider the theme’s content area and the largest typical display context. |
| Full Size | The original uploaded image, subject to how it is displayed in the page. | “Full Size” does not mean the browser must render the image at its full pixel width on every screen. |
These built-in choices describe image sizes for inserting into post content; they do not guarantee that every theme component uses them. In the Classic Editor, the selected image size and the theme’s content column affect the result. The Classic Editor image guide describes the legacy display choices and content-column constraint. Check your theme’s cards, archives, and featured-image areas separately.
How to change thumbnail dimensions in Settings
- In the WordPress dashboard, open Settings → Media.
- Under Thumbnail size, set the desired width and height. Enable the exact-dimension crop option only if the design needs every thumbnail cropped to that shape.
- Set the maximum width and height for Medium and Large according to where those sizes are used on your site.
- Save the settings, then check images in the contexts that matter, including a post, an archive or card grid, and a narrow screen.
Changing these settings does not change the proportions of the original upload. It changes the size rules WordPress uses when generating image variants. If older uploads do not show the expected result after a change, check whether the needed generated size exists; a setting change alone should not be treated as proof that all existing files have been regenerated.
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Block editor sizing: resolution, aspect ratio, and display
In the block editor, the resolution choices available for an image depend on the theme and the original upload. Resolution selection is separate from the block’s display controls: aspect ratio and width or height settings affect how the image appears in the layout, and a crop in the block does not change the original image file.
Start with Auto for responsive layouts
WordPress recommends leaving width and height on Auto in most cases. A fixed height can be harder to manage across screen sizes, especially when images have different source proportions. Use explicit dimensions only when your layout requires them and you have checked how they behave at smaller viewport widths. See the current Image block documentation for the available editor controls.
Choose Cover or Contain for the visual goal
- Cover fills the selected area and crops parts of the image when necessary. It suits a uniform card or tile when consistent framing matters more than showing every edge.
- Contain keeps the full image visible. It may leave unused space in the selected area, but avoids cropping important content.
When a subject’s position matters, use the block’s focal-point control to guide the crop. Preview the result at the page’s real display sizes: a crop that works on a wide card may cut off the subject in a narrower layout.
When to register a custom image size
Use a named custom size when a theme component—such as a card, archive tile, or featured-image slot—needs a predictable generated crop that differs from the built-in sizes. This requires theme or plugin code; it is not a value to paste into the Media settings fields.
add_image_size( 'card-thumbnail', 640, 360, true );This illustrative example registers a 640 × 360 crop named card-thumbnail; those dimensions are not a universal recommendation. The true argument requests a crop to the target dimensions using center positioning. If crop is omitted or false, WordPress uses proportional resizing instead of forcing that exact crop. The API reference explains the arguments and behavior: add_image_size().
Register a custom size only when your theme or code will request and use it. Registration alone does not make every image context use that size. Also consider whether existing uploads have a generated file for the new size; after introducing a size, older images may need their variants regenerated with an appropriate site workflow. Confirm the image output and crop rather than assuming the code changed existing files.
How to decide between proportional resizing and cropping
- Choose proportional resizing when the whole image should remain visible and the layout can accommodate varying aspect ratios.
- Choose an exact crop when a grid needs uniform dimensions and losing some edges is acceptable.
- Use a block-level display crop when you want to control how an image fills a particular block without changing the original file.
- Use a custom registered size when a theme component repeatedly needs a specific generated variant rather than an editor-only display adjustment.
Before committing to a crop, test images with subjects near each edge, portrait and landscape orientations, and text or logos that should not be cut off. If the same crop must be dependable across a theme component, use a defined size and verify the theme is requesting it.
Image dimensions, display size, and file weight
Pixel dimensions describe the image file, while CSS and the editor’s layout determine its rendered size. Uploading an image that is smaller than the space in which it must appear can make it look soft; generating or serving an unnecessarily large image can waste bandwidth. Match source and generated sizes to the actual layout, then check that the browser is using an appropriate image variant.
Dimensions are not the only factor in loading time: file format and compression also affect file weight. WordPress discusses these general considerations in Image size and quality. Its older numeric file-size guidance should not be treated as a current universal performance target.
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The thumbnail is square, but the subject is cut off
The exact-dimension thumbnail crop or a Cover display can remove image edges to fill a square or other fixed area. Disable exact cropping if proportional output is desired, or adjust the image block’s aspect ratio and focal point. For a repeated theme crop, revise the custom size and test the subject framing.
The image looks stretched or has empty space
A fixed width and height can distort an image if the display behavior does not preserve its proportions. Use Auto dimensions as a starting point. If the whole frame must remain visible in a fixed area, Contain may leave empty space; Cover fills the area by cropping instead.
The editor does not offer the size you expect
Available resolution options depend on the theme and the original upload. Check whether the theme registers a custom size and whether the original file is large enough to supply the expected variant. A named size must also be used by the relevant theme or block code.
A size change did not alter older uploads
The Media screen configures image-size rules; existing uploads may not already have files generated to match newly changed dimensions or added custom sizes. Verify which image URL and dimensions the page is using, then use an appropriate image-regeneration workflow if needed.
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The file’s intrinsic pixel width and its rendered width are different. A theme can constrain the display to the content column even when the selected source image is larger. Inspect the page layout and the image’s rendered dimensions rather than assuming Full Size must occupy the whole screen.
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