Optimize images with Thumbor

Thumbor can be used for free to resize, compress, and transform images on-demand.

Katie Hempenius
Katie Hempenius

Thumbor is a free, open source image CDN that makes it easy to compress, resize, and transform images. This post lets you try out Thumbor firsthand without needing to install anything. We've set up a sandbox Thumbor server for you to try out at The image that you're going to experiment with is available at


This post assumes that you understand how image CDNs can improve your load performance. If not, check out Use image CDNs to optimize images. It also assumes that you've built basic websites before.

Thumbor URL Format

As mentioned in Use Image CDNs to Optimize Images, each image CDN uses a slightly different URL format for images. Figure 1 represents Thumbor's format.

A Thumbor URL has the following components: origin, security key, size, filters and image.
Thumbor's URL format


Like all origins, the origin of a Thumbor URL is composed of three parts: a scheme (which is almost always http or https), a host, and a port. In this example, the host is identified using an IP address, but if you're using a DNS server it might look like thumbor-server.my-site.com. By default, Thumbor uses port 8888 to serve images.

Security Key

The unsafe part of the URL indicates that you're using Thumbor without a security key. A security key prevents a user from making unauthorized changes to your image URLs. By changing the image URL, a user could use your server (and your hosting bill) to resize their images, or, more maliciously, to overload your server. This guide won't cover setting up Thumbor's security key feature.


This part of the URL specifies the desired size of the output image. This can be omitted if you don't want to change the size of the image. Thumbor will use different approaches like cropping or scaling to achieve the desired size depending on the other URL parameters. The next section of this post explains how to resize images in more detail.

Try it now:

  1. Click the following URL to view the image served at its original size in a new tab:

    Image at original size
    Original image
  2. Resize the image to 100x100 pixels:

Image at 100x100 pixels
Image resized to 100x100 pixels


Filters transform an image. The filters part of the URL segment starts with filters: followed by a colon-separated list of filters; this can be omitted if you are not using any filters. The syntax for individual filters resembles a function call (for example grayscale()) containing zero or more arguments.

Try it now:

  1. Apply a single filter: a Gaussian blur effect with a radius of 25 pixels:

    Blurred image
    Blurred image
  2. Apply multiple filter. Convert to grayscale and rotate the image 90 degrees:

Grayscale image that has been rotated 90 degrees
Grayscale, rotated image

Transforming Images

This section focuses on the Thumbor functionalities most relevant to performance: compression, resizing, and conversion between file formats.


The quality filter compresses JPEG images to the desired image quality level (1-100). If no quality level is provided, Thumbor compresses the image to a quality level of 80. This is a good default: quality levels 80-85 typically have little noticeable effect on image quality, but usually decrease image size by 30-40%.

Try it now:

  1. Compress the image to a quality of 1 (very bad):

    Low-quality image
    Low-quality image
  2. Compress the image using Thumbor's default compression settings:

Compressed image with no noticible quality issues
Compressed image


To resize an image while maintaining its original proportions use the format $WIDTHx0 or 0x$HEIGHT within the size portion of the URL string.

Try it now:

  1. Resize the image to a width of 200 pixels while maintaining original proportions:

    Image that is 200 pixels wide
    Image resized to a width of 200 pixels
  2. Resize the image to a height of 500 pixels while maintaining original proportion:

Image that is 500 pixels tall
Image resized to a height of 500 pixels

You can also resize images to a percentage of the original by using the proportion filter. If size is specified in conjunction with the proportion filter, the image will be resized, and then the proportion filter will be applied.

Try it now:

  1. Resize the image to 50% of the original:

    Image that is 50% the size of the original
    Image resized to 50% the size of the original
  2. Resize the image to a width of 1000 pixels, then resize the image to 10% of its current size:

Image that is 100 pixels wide
Image resized to a width of 100 pixels

These methods are just a few of Thumbor's many cropping and resizing options. To read about other options, check out Usage.

File Formats

The format filter converts images to jpeg, webp, gif, or png. Keep in mind that if you're optimizing for performance you should use either JPEG or WebP as PNG and GIF files tend to be significantly larger and do not compress as well.

Try it now:

  1. Convert the image to WebP. If you open the Network panel of DevTools the document's Content-Type response header shows that the server returned a WebP image:
DevTools screenshot showing the content-type (WebP) of an image
The content-type response header shown in DevTools

Next Steps

Try out other filters and transformations on the hero.jpg image.

If you're following along using your own Thumbor installation, check out the appendix below that explains how and why to use the thumbor.conf file.

Appendix: thumbor.conf

Many of the configuration options discussed in this post, plus many others, can be established as defaults by setting up and using a thumbor.conf configuration file. Settings in the thumbor.conf file will be applied to all images unless overridden by the URL string parameters.

  1. Run the thumbor-config command to create a new thumbor.conf file.

    thumbor-config > ./thumbor.conf
  2. Open your new thumbor.conf file. The thumbor-config command generated a file that lists and explains all Thumbor configuration options.

  3. Configure settings by uncommenting lines and changing the default values. You may find it useful to set the following settings:

  4. Run Thumbor with the --conf flag to use your thumbor.conf settings.

    thumbor --conf /path/to/thumbor.conf