How to Change Thumbnail Default Size in WordPress

When you upload an image to WordPress, it creates different sizes of that image. One of those is the thumbnail size. This is the most miniature version often used in blog post previews, galleries, and widgets. But what if the thumbnail looks too small or doesn’t fit your layout? You can fix that by changing the default thumbnail size.

In this article, I will provide step-by-step instructions for changing the thumbnail size in this guide.

What Is a Thumbnail in WordPress?

A thumbnail is a miniature version of your original image. WordPress uses it to keep your pages light and fast. It also helps show previews of your posts without loading the full-size image. By default, WordPress sets the thumbnail size to 150 x 150 pixels. However, this size may not work for all themes or designs, so you can change it to fit your needs.

Step 1: Go to WordPress Media Settings

1. Log in to your WordPress dashboard.
2. Go to Settings > Media.

Here you will see three image size options:

• Thumbnail size
• Medium size
• Large size

You can set your width and height for the thumbnail.

Step 2: Enter New Thumbnail Dimensions

Under Thumbnail size, you will see two boxes:

• Width (in pixels)
• Height (in pixels)

Enter your desired size. For example:

• 300 x 200 if you want rectangular thumbnails
• 200 x 200 for square thumbnails

There is also a checkbox: Crop thumbnail to exact dimensions (normally, thumbnails are proportional).

• If you check this, WordPress will crop images to match your size.
• If you leave it unchecked, WordPress will scale the image without cropping.

Choose what works best for your design.

Step 3: Save Your Changes

After setting the new size, scroll down and click Save Changes.

This will apply the new size to any images you upload.

But what about the old images? Let’s fix that next.

Step 4: Regenerate Thumbnails for Old Images

Changing the size only affects new uploads. Your old images still use the old size.

To fix this, you need to regenerate the thumbnails.

Use a Plugin:

Install the plugin Regenerate Thumbnails.

1. Go to Plugins > Add New
2. Search for Regenerate Thumbnails
3. Click Install Now, then Activate

Now go to Tools > Regenerate Thumbnails

Click the button that says: Regenerate Thumbnails For All x Attachments

This may take a few minutes, depending on how many images you have. After it finishes, all your old images will match the new thumbnail size.

Optional: Use a Custom Size in Code

You can set thumbnail sizes in code if you build a custom theme or want more control.

Add this to your functions.php file:

set_post_thumbnail_size( 300, 200, true );

• The first number is the width
• The second is height
• True means crop to the exact size

You can also create more sizes:

add_image_size( ‘custom-thumb’, 400, 250, true );

Then use this size in your theme:

the_post_thumbnail( ‘custom-thumb’ );

Only use this method if you know how to edit theme files.

Best Practices for Thumbnail Sizes

• Choose a size that fits your theme layout
• Don’t make thumbnails too big—keep your site fast
• Constantly regenerate old thumbnails when you change the size
• Test how they look on mobile and desktop