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Site Accelerator CDN

Site Accelerator from Jetpack is our Content Delivery Network (CDN), optimized to speed up your site by delivering faster images and static files.

As content delivery is one of several factors that impact site speed, a content delivery network (or CDN) can help make your site faster by:

  • Delivering your content from high-speed and dedicated data centers.
  • Allowing more files to be downloaded simultaneously by the browser.
  • Distributing data centers in different geographic locations to improve download speeds and provide redundancy.

By making use of a WordPress CDN like Site Accelerator, you can more efficiently distribute server load and save bandwidth, reducing your overall hosting costs.

Site Accelerator is automatically enabled on sites with the WordPress.com Creator and Entrepreneur plans.

About Site Accelerator

The following section explains the different actions Site Accelerator uses to improve your site’s performance.

Image Load Times

The Image CDN is an image acceleration and editing service. By duplicating your photos and storing them on our servers, you can diminish the strain on your website’s main server and provide faster image loading times for your audience.

Specifically, Site Accelerator:

  • Filters content but doesn’t change your database.
  • Acts on images in posts and pages, as well as featured images/post thumbnails via the image_downsize filter. Examples of WordPress functions using the image_downsize filter include:
  • All images are compressed on the fly using either a combination of pngquant and OptiPNG or jpegoptim. Lossless compression is available by specifying a quality setting of 100 in the query arguments.
  • Applies to old and new posts and can be turned on or off easily, applying the changes in just a few minutes.
  • Serves WebP image data to browsers that support the WebP image format (note that the file extension won’t change).

Static File Load Times (Asset CDN)

We host static assets from our servers as a content delivery network (CDN)—like the JavaScript and CSS shipped with the WordPress core, Jetpack, and WooCommerce—which further alleviates the load on your server.

Specifically, Site Accelerator:

  • Filters the URLs of assets that are loaded with every WordPress page.
  • Only acts on assets shipped with WordPress core, Jetpack, and WooCommerce.

Image Resizing

Site Accelerator resizes photos by checking the img element’s width and height attributes and serving an image resized to those dimensions or to the width of the containing element (whichever is smaller).

If there is no size set on the element, Jetpack will constrain images to the size indicated when adding the image to a post or to your theme’s “content width” setting.

Finally, if a content width isn’t set in your theme, the site accelerator will default to 1000px. This is to ensure that sites don’t attempt to serve images larger than the theme is intended to be able to display.

We remove the width and height arguments to prevent your images from skewing when the resized image doesn’t have the same dimensions as the original. This is particularly important when you switch from one theme to another, and the new theme might be narrower than the previous one. One of the benefits of this is that we will automatically resize your images so they don’t exceed the width supported by your theme.

Other Facts

The Site Accelerator is:

  • Open source: available via SVN at http://code.svn.wordpress.org/photon/ and on Trac at http://code.trac.wordpress.org/browser/photon
  • Uses multiple domains: In order to take advantage of parallel downloads we support multiple sub-domains for Site Accelerator. If you tend to have many images per page you can split them across i0.wp.com, i1.wp.com, and i2.wp.com. To take use of caching, it can be useful to use the same sub-domain for files in subsequent calls.
  • SSL supported: You can use Site Accelerator on SSL pages without mixed content warnings. Just use https://i{0,1,2}.wp.com instead of http://i{0,1,2}.wp.com

Enable Site Accelerator

You can go to Settings → Performance and activate the following settings:

  • Speed up image load times: We host your images from our servers, providing faster image loading for your readers.
  • Speed up static file load times: We host static assets like JavaScript and CSS from our servers, alleviating the load on your site.

The Site Accelerator can only retrieve publicly available images. If your site is set to private, your images (including in the media library) will return a 403 Forbidden message if “Speed up image load times” is activated.

Check Site Accelerator is Working

To check if images are loading from the CDN, you can follow these steps:

  1. Wait a few moments and then load your site in a different web browser to avoid any caching issues.
  2. Check an image’s URL in your browser’s inspector or open the image in its own tab. The images served from our CDN have URLs that begin https://i0.wp.com, https://i1.wp.com, https://i2.wp.com or similar.
  3. If you don’t see i0.wp.com or similar, see our troubleshooting steps below.

Troubleshoot Site Accelerator

If you have enabled Site Accelerator and the images are broken on your website, follow these steps:

  1. Wait a few moments and then load your site in a completely different web browser than you normally use to make sure it’s not your browser cache interfering with the URL change. It can take a few minutes for these changes to be reflected on your entire site.
  2. Check that your Jetpack connection is working properly.
  3. Try to disable all of your plugins except Jetpack before checking your images again. If they are displaying properly, you’ll know that another plugin is interfering with site acceleration. You can find out which plugin is responsible by enabling each plugin one by one and checking the broken images after each one.
  4. If Site Accelerator still isn’t working after testing your plugins, there may be a conflict with the current theme. To test this, switch to one of the default themes such as Twenty-Twenty Four. If this resolves the issue, the problem can be identified as related to the theme, and we’d recommend contacting the theme’s author for further assistance.

Limitations of Site Accelerator

  • Site Accelerator doesn’t do cache invalidations. Static assets are tied to the public version of WordPress, Jetpack, or WooCommerce that you’re using. For images, if you want to “refresh” an image, you will need to change its file name. Adding random query arguments, commonly known as “cache busters,” will not work.
  • Theme and plugin assets are not supported by Site Accelerator at this time.
  • It’s not possible to automatically purge all site images from Jetpack. If there’s an image or photo no longer on your server that you’d like to remove, please contact support with a direct link to the file as it appears on your site. These will begin with i0.wp.com, i1.wp.com, i2.wp.com, or i3.wp.com. Since images can only be purged manually by individual URLs, there’s a limit to the number we can remove.
  • Site Accelerator only handles .gif, .png, .jpg, and .webp images from servers that listen on port 80 for HTTP and port 443 for HTTPS. This applies to about 99.99% of the web servers in the world. If you are having issues, try using the jetpack_photon_reject_https filter.
  • We won’t “upscale” an image in most circumstances. If your original image is 1000px wide and you ask for Jetpack to make it 5000px, we’ll serve the original 1000px image. Upscaled images are usually of poor quality, which we avoid.
  • Site Accelerator will not to apply computationally expensive filters to large images (colorize, brightness, ulb, contrast, smooth, etc.).
  • If your server takes longer than 10 seconds when an image is being retrieved for our CDN, the process will time out, and your image will appear to be broken. Try to upload a differently-named image with a smaller dimension or file size if this happens.
  • It’s not possible to choose or limit where in the world your images will be served from. We have servers located all over the world, and the server that ultimately loads your image is dependent on a variety of factors, including the visitor’s location.
  • Site Accelerator doesn’t support animated PNGs.
  • Site Accelerator doesn’t serve audio (.mp3, .wav, .flac, etc.) or video (.mp4, .wmv, .flv, etc.) files. If you’d like to host videos on our CDN, check out Jetpack’s Video Hosting feature.

Site Accelerator API

Images can be cropped, resized, and filtered by using an API controlled by the following GET query arguments:

ResourceDescription
GET?w=Set the width of an image in pixels:
http://i0.wp.com/s.ma.tt/files/2012/06/DSC01205.jpg?w=300


GET?h=Set the height of an image in pixels:
http://i0.wp.com/s.ma.tt/files/2012/06/MCM_0629-1600x1064.jpg?h=200


GET?crop=x,y,w,hCrop an image by percentages x-offset,y-offset,width,height (x,y,w,h). Percentages are used so that you don’t need to recalculate the cropping when transforming the image in other ways such as resizing it. For example:

Original image: 4-MCM_0830-1600×1064.jpgcrop=12,25,60,60 takes a 60% by 60% rectangle from the source image starting at 12% offset from the left and 25% offset from the top.
http://i0.wp.com/s.ma.tt/files/2012/06/4-MCM_0830-1600x1064.jpg?crop=12,25,60,60


crop=160px,160px,788px,788px takes a 788 by 788 square starting at 160 by 160.


http://i0.wp.com/s.ma.tt/files/2012/06/4-MCM_0830-1600x1064.jpg?crop=160px,160px,788px,788px


crop=160px,25,1400px,60 shows you can also mix the parameters types, for example a 1400 pixels by 60% rectangle from the image starting at 160 pixels by 25%.


http://i0.wp.com/s.ma.tt/files/2012/06/4-MCM_0830-1600x1064.jpg?crop=160px,25,1400px,60


GET?resize=Resize and crop an image to the exact width,height pixel dimensions. Set the first number as close to the target size as possible and then crop the rest. Which direction it’s resized and cropped depends on the aspect ratios of the original image and the target size.
http://i0.wp.com/s.ma.tt/files/2012/06/9-DSC01406-1600x1066.jpg?resize=400,220


http://i0.wp.com/s.ma.tt/files/2010/11/MCM_4443.jpg?resize=200,400


This is useful for taking an image of any size and making it fit into a certain location while losing as little of the image as possible.


GET?fit=Fit an image to a containing box of width,height dimensions. Image aspect ratio is maintained.
http://i0.wp.com/s.ma.tt/files/2010/10/MCM_4049.jpg?fit=300,300


http://i0.wp.com/s.ma.tt/files/2010/10/MCM_4214.jpg?fit=300,300


For example, fit=100,100 on a landscape image with dimensions 400×300 will result in an image that is 100×75, while fit=100,100 on a portrait image with dimensions 300×400 will result in an image that is 75×100.


GET?lb=Add black letterboxing effect to images, by scaling them to width, height while maintaining the aspect ratio and filling the rest with black. 
http://i0.wp.com/developer.files.wordpress.com/2013/03/letterboxing-example.jpg?lb=310,250

Original image

?lb=310,250

GET?ulb=Remove black letterboxing effect from images with ulb. This function takes only one argument, true. 
http://i0.wp.com/developer.files.wordpress.com/2012/11/black-letterboxing-example.jpg?ulb=true

Original image

?ulb=true

GET?filter=The filter GET parameter is optional and is used to apply one of multiple filters. Valid values are: negate, grayscale, sepia, edgedetect, emboss, blurgaussian, blurselective, meanremoval.

Original image

filter=negate

filter=grayscale

filter=sepia

filter=edgedetect

filter=emboss

filter=blurgaussian

filter=blurselective

filter=meanremoval

GET?brightness=Adjust the brightness of an image. Valid values are -255 through 255 where -255 is black and 255 is white. Higher is brighter. The default is zero.brightness=-40 will darken an image by 40 and brightness=80will brighten an image by 80. 
http://i0.wp.com/s.ma.tt/files/2011/06/MCM_9517-1600x1065.jpg?brightness=-40
http://i0.wp.com/s.ma.tt/files/2011/06/MCM_9517-1600x1065.jpg?brightness=0
http://i0.wp.com/s.ma.tt/files/2011/06/MCM_9517-1600x1065.jpg?brightness=80

brightness=-40

brightness=0

brightness=80

GET?contrast=Adjust the contrast contrast of an image. Valid values are -100 through 100. The default is zero.contrast=-50 will decrease contrast by 50 and contrast=50 will increase contrast by 50. 
http://i0.wp.com/s.ma.tt/files/2011/06/MCM_9517-1600x1065.jpg?contrast=-50
http://i0.wp.com/s.ma.tt/files/2011/06/MCM_9517-1600x1065.jpg?contrast=0
http://i0.wp.com/s.ma.tt/files/2011/06/MCM_9517-1600x1065.jpg?contrast=50

contrast=-50

contrast=0

contrast=50

GET?colorize=Add color hues to an image with colorize by passing a comma separated list of red,green,blue (RGB) values such as 255,0,0 (red), 0,255,0 (green), 0,0,255 (blue). 
http://i0.wp.com/s.ma.tt/files/2024/02/IMG_3775.jpeg?colorize=100,0,0
http://i0.wp.com/s.ma.tt/files/2024/02/IMG_3775.jpeg?colorize=0,100,0
http://i0.wp.com/s.ma.tt/files/2024/02/IMG_3775.jpeg?colorize=0,0,100

colorize=100,0,0




colorize=0,100,0




colorize=0,0,100



GET?smooth=The smooth parameter can be used to smooth out the image. 
http://i0.wp.com/s.ma.tt/files/2011/06/MCM_9230-1600x1064.jpg?smooth=1

Original image

smooth=1

From the comments area of the PHP manual on the function: “Applies a 9-cell convolution matrix where center pixel has the weight arg1 and others weight of 1.0. The result is normalized by dividing the sum with arg1 + 8.0 (sum of the matrix). Any float is accepted, large value (in practice: 2048 or more) = no change”. Simply put, 0 appears to be maximum smoothing with higher numbers being less smoothing.

GET?zoom=Use zoom to size images for high pixel ratio devices and browsers when zoomed. Not available to use with crop. Zoom is intended for use by scripts such as devicepx.js which automatically set the zoom level. Valid zoom levels are 1, 1.5, 2-10. 
http://i0.wp.com/s.ma.tt/files/2012/02/MCM_4246-1600x1064.jpg?w=310&zoom=2

Original image

zoom=2

GET?quality=Use the quality parameter to manage the quality output of JPEG and PNG images. Valid settings are between the values between 20 and 100. For JPEGs a setting of 100 will output the image at the original unaltered quality setting. However when specifying a quality of 100 for PNGs, the image is compressed using lossless compression and produces an image with identical quality as the original. Note that if the requesting web browser supports the WebP image format, then PNG and JPEG images will automatically be converted to the WebP image format by the server. When specifying 100 as the quality in this instance, a lossless compressed image will be produced, which in some instances may result in a file larger than the original.Note that the default quality setting for JPEGs is 89%, PNGs 80%, and WebP images is 80%. 
http://i0.wp.com/ma.tt/files/2014/09/8084136238_169f1ca1f0_o.jpg?w=310&quality=50

Original image

quality=50

GET?strip=Use the strip functionality to remove JPEG image Exif, IPTC, comment, and color data from the output image to reduce the image’s file size without having any visual data removed from the image. If orientation data is included in the Exif metadata, the image will be rotated accordingly.There are 3 valid settings for this parameter:
  • all: strips all extraneous data.
  • info: removes Exif, IPTC and comment data from the output image.
  • color: removes any ICC color profiles.
http://i0.wp.com/ma.tt/files/2012/06/13-MCM_0885.jpg?w=310&strip=all

Original image

strip=all

Last updated: April 30, 2024

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