Home Forums ACF PRO Use a current ACF image as the featured image Reply To: Use a current ACF image as the featured image

  • I have managed to work it out here is the answer if anyone needs it. This would go in your function.php file

    // retrieves the attachment ID from the file URL
    function get_image_id($image_url) {
        global $wpdb;
        $attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url ));
        return $attachment[0];
    //Set the featured image on save of the post
    add_action('save_post', 'set_featured_image_from_image');
    //set featured image as main image.
    function set_featured_image_from_image()
            $has_thumbnail = get_the_post_thumbnail($post->ID);
            //If there is not featured image get the other image.
            if (!$has_thumbnail) {
            //get the url of the ACF image and get the ID of that image
                $image_url = get_field('main_image');
                $image_id = get_image_id($image_url);
                if ($image_id) {
                    set_post_thumbnail($post->ID, $image_id);

    I am outputting just the url from the image so I had to reverse get the ID. Those who are using the full object should be able to get the ID more easily.