Responsive Images in WordPress with ACF

Posted in Web Development on the 4th of January 2016 by @aaronrutley

WordPress 4.4 shipped with an awesome new feature, Responsive Images. This feature works great for any images within the standard WYSIWYG editor. The srcset and sizes attributes are added to each img tag to inform the browser of the available sizes for each image.

If you’re using a plugin like Advanced Custom Fields to store and display images within a custom theme you’ll need to modify your themes to use this new feature. I’ve coded up a simple helper function to make this a little easier.

You can add this function to your themes functions.php file or a functionality plugin:

 * Responsive Image Helper Function
 * @param string $image_id the id of the image (from ACF or similar)
 * @param string $image_size the size of the thumbnail image or custom image size
 * @param string $max_width the max width this image will be shown to build the sizes attribute 

function ar_responsive_image($image_id,$image_size,$max_width){

	// check the image ID is not blank
	if($image_id != '') {

		// set the default src image size
		$image_src = wp_get_attachment_image_url( $image_id, $image_size );

		// set the srcset with various image sizes
		$image_srcset = wp_get_attachment_image_srcset( $image_id, $image_size );

		// generate the markup for the responsive image
		echo 'src="'.$image_src.'" srcset="'.$image_srcset.'" sizes="(max-width: '.$max_width.') 100vw, '.$max_width.'"';


To use this within your custom theme, open an image tag, call the function and pass 3 parameters:

  • The image ID (I’m getting this from an ACF image field which returns an ID)
  • The image size (I’ve used a custom image size, added via the add_image_size )
  • The max width (In this case the max width this image is 640px)

It will return the srcsrcset and sizes attributes, you can assign your class and alt text as needed, see this example:

<img class="my_class" <?php ar_responsive_image(get_field( 'image_1' ),'thumb-640','640px'); ?>  alt="text" /> 

This will generate the responsive images markup, for all of the image sizes you have set (that have the same aspect ratio), similar to this:

<img class="my_class" 
srcset="http://aaronrutley.dev/wp-content/uploads/2015/10/wp-tips-320x75.jpg 320w, 
http://aaronrutley.dev/wp-content/uploads/2015/10/wp-tips-640x150.jpg 640w, 
http://aaronrutley.dev/wp-content/uploads/2015/10/wp-tips-1280x300.jpg 1280w, 
http://aaronrutley.dev/wp-content/uploads/2015/10/wp-tips-1920x450.jpg 1920w"
sizes="(max-width: 640px) 100vw, 640px" 
alt="text" >

Bonus Tips:

In WordPress 4.4 max srcset size is set to 1600px wide by default, if you have images larger than that size that you’d like included in the srcset you can use this filter:

add_filter( 'max_srcset_image_width', 'ar_max_srcset_image_width', 10 , 2 );

function ar_max_srcset_image_width() {
	return 2200;

If your custom image sizes are similar to the default WordPress image sizes you might want to remove some of those sizes from the srcset array with this snippet:

add_filter('intermediate_image_sizes_advanced', 'ar_remove_default_image_sizes');

function ar_remove_default_image_sizes( $sizes) {
    unset( $sizes['medium_large']);
    unset( $sizes['large']);
    return $sizes;

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