WordPress’s documented default thumbnail size is 150 × 150 pixels, cropped square. That is a starting setting, not a universal recommendation for every site: the right dimensions depend on where the image appears, the theme’s layout, and whether the design needs a consistent crop. Check your own values at Dashboard > Settings > Media before changing them.
What thumbnail size should you use in WordPress?
Use 150 × 150 pixels as the reference point for WordPress’s documented default thumbnail, not as a rule that every image on every site should be square. WordPress lets site administrators change the thumbnail width and height, and themes or plugins can affect which image sizes are available. The dimensions of the uploaded original also constrain the result.
Choose based on the image’s job:
- Square list or grid: a square crop can make cards look consistent, but may cut off important parts of the subject.
- Wide card, archive, or featured-image slot: use a crop matching the shape of that component when its design requires consistent frames.
- Inline article image: preserving the original proportions is often more appropriate than forcing a square crop.
The key distinction is between an image file’s pixel dimensions and the size at which the page displays it. A 150-pixel image may be rendered smaller or larger by the layout; conversely, choosing a display width does not by itself change the underlying uploaded file. Avoid choosing a setting solely because a number is called “thumbnail,” “medium,” or “large.”
What the Media settings control
Open Dashboard > Settings > Media to inspect the built-in image-size settings. WordPress documents fields for thumbnail width and height, maximum width and height for Medium, and maximum width and height for Large. The thumbnail crop checkbox controls whether WordPress crops the thumbnail to the exact dimensions; otherwise, images are normally resized proportionally.
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| Setting | What it controls | How to choose |
|---|---|---|
| Thumbnail | Width and height, plus whether to crop to exact dimensions | Set a square only when the design calls for square frames. If the full image should remain visible, avoid forcing an exact crop. |
| Medium | Maximum width and height | Consider the content patterns where medium images are inserted and the space available in those layouts. |
| Large | Maximum width and height | Consider the theme’s content area and the context in which the image is used. |
| Full Size | The original uploaded image as an image-size choice | “Full Size” does not mean the browser must display the file at its full pixel width on every screen. |
Medium and Large are configurable maximums, not universal recommended numbers. The Classic Editor documentation explains that the theme’s content column can limit the displayed width of Large and Full Size images. A site’s available content width and component layout therefore matter more than a generic dimension copied from another site.
Generated image dimensions versus display dimensions
WordPress image-size settings concern image versions used when inserting or displaying content; the editor also provides controls that affect how an image appears in its block. These are related but distinct decisions:
- Resolution choice: available choices depend on theme support and the dimensions of the uploaded original.
- Aspect ratio and crop: these affect the visible frame. A crop can hide part of an image without editing the original upload.
- Width and height: these determine the displayed block dimensions. WordPress recommends leaving them on Auto in most cases so the image can adapt to the layout.
A fixed height can be harder to manage across different screen sizes. In the block editor, Cover fills the chosen area and crops when necessary; Contain keeps the entire image visible, which can leave empty space in the frame. Use Cover when a filled, consistent frame matters more than showing every edge. Use Contain when the complete image must remain visible.
For example, a landscape image placed in a square card can be set to Cover to fill the card, but some of its sides may not be visible. Contain preserves the whole landscape image, but the square frame may show unused space. Neither is universally better: pick the behavior that matches the content and design.
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- Identify the actual image slot. Is it an inline image, a square thumbnail, a wide card, or a featured image? Check the page on which that image appears rather than selecting dimensions in isolation.
- Measure the layout the image must serve. Consider the component’s shape and the theme’s content area. For a responsive layout, account for how that slot changes on narrow screens.
- Decide whether to crop. Use an exact crop for a design that requires uniform frames; preserve proportions when cropping would remove information or vary undesirably by image.
- Check the source image. WordPress cannot create detail that is absent from a small original. A requested size larger than the source may not produce the result you expect.
- Preview the result in context. Check representative images, including subjects near the edges, and review the layout at more than one screen width.
There is no single numeric recommendation for Medium or Large that fits every WordPress theme. If your theme’s card is a fixed shape, the component’s intended crop is a better guide than a general-purpose “best size” list. If it is not a fixed-shape component, retain flexibility rather than cropping every upload to a square.
Change the built-in thumbnail size
- In the WordPress dashboard, go to Settings > Media.
- Review the Thumbnail size width and height values alongside the exact-dimension crop option.
- Enter dimensions suited to the site’s thumbnail use. Enable exact cropping only if the intended output should match those dimensions as a crop.
- Review the Medium and Large maximum width and height values in the same screen against the layouts where those sizes are used.
- Save the settings, then inspect affected pages and images to confirm the result.
These settings let you configure the built-in sizes; do not assume that changing a value will alter every theme-specific image treatment or automatically make every existing image look as intended. Check the site’s actual pages after the change. Where a particular theme component needs its own predictable crop, a named custom size may be a better fit than changing a built-in size used in other contexts.
Register a custom size for a theme component
Developers can register a named image size using WordPress’s add_image_size() function. For example:
add_image_size( 'card-thumbnail', 640, 360, true );This illustrative example registers a 640 × 360-pixel size called card-thumbnail and asks WordPress to crop to the target dimensions. The example dimensions are not a universal recommendation; choose values to match the component you are building.
The function’s crop argument matters. Its default behavior is proportional scaling. Setting it to true crops to the specified dimensions using center positions. A centered crop is simple, but it may not keep an off-center subject in frame. Preview real site content before relying on a crop for faces, product details, text, or other edge-sensitive subjects.
A custom size is developer work, typically added as part of a theme or plugin. It is useful when one component needs a named, repeatable image treatment while other parts of the site need different sizes or proportions. Registering a size does not establish that every existing image already has a generated version for it; verify the output on the site rather than assuming that adding the code alone updates all media.
Check the image in the block editor
When editing a page or post, select the image block and use the block’s available resolution and display controls. The precise choices depend on the theme and uploaded image. In most cases, begin with width and height set to Auto; then use aspect ratio and scaling options to get the intended appearance.
- Choose an available resolution that suits the image’s role and the source file.
- Use a custom aspect ratio only when the design calls for a particular frame.
- Choose Cover to fill that frame with a crop, or Contain to keep the full image visible.
- Inspect the focal point when a crop is used, especially if the important subject is not centered.
These display controls do not mean the original file has been permanently cropped. They control its presentation in the block. If a different generated image size is required for a theme component, use the appropriate site-level or developer-level approach rather than treating a visual block adjustment as a replacement for it.
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Common problems and what to check
The thumbnail is not square
Check the Thumbnail width and height and whether exact-dimension cropping is enabled at Settings > Media. Also check whether the image is being displayed through a theme component or block with its own aspect-ratio behavior.
The subject is cut off
An exact crop or Cover presentation may remove edges, particularly when the subject is off-center. Use a less aggressive crop, adjust the focal point if that control is available, or preserve the full image with Contain or proportional sizing.
The image looks blurry or too small
Check the original upload’s pixel dimensions and the resolution selected for the block. A small original has limited detail; increasing its displayed dimensions does not restore detail that was not in the source.
The image does not fill the expected width
Check the block’s display dimensions and the theme’s content area. In the Classic Editor, Large and Full Size display can be constrained by the theme’s content column; the original file’s width is not necessarily the width rendered on the page.
A changed setting does not appear in an existing page
First confirm the page’s selected resolution and any block-level aspect-ratio or sizing controls. Then inspect whether the image is presented by a theme-specific component rather than the built-in size you changed. Confirm the result on the actual page; do not assume the Media setting overrides every display choice.
Performance and image weight
Dimensions and file weight are connected but not interchangeable. Larger files can take longer to load, but pixel dimensions alone do not state how large a compressed image file will be. Choose dimensions appropriate to the layout, and treat compression or other image optimization as a separate concern rather than a reason to force every image into one size. Avoid relying on old generic file-size thresholds as though they were a current guarantee for every site or connection.
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