How to get a list of all the possible thumbnail sizes set within a theme

What function can I use in a plugin to get the dimensions of every image size (in an array preferably) that is defined in a child theme?

Just for clarification I am not asking how to create a new image size.

Answers:

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Method 1

Found it here. The answer is:

global $_wp_additional_image_sizes; 
print '<pre>'; 
print_r( $_wp_additional_image_sizes ); 
print '</pre>';

Method 2

WordPress core doesn’t have a native method for getting intermediate image sizes (i.e. width and height), but the following helper function will get all registered image sizes along with their dimensions:

/**
 * Get all the registered image sizes along with their dimensions
 *
 * @global array $_wp_additional_image_sizes
 *
 * @link http://core.trac.wordpress.org/ticket/18947 Reference ticket
 *
 * @return array $image_sizes The image sizes
 */
function _get_all_image_sizes() {
    global $_wp_additional_image_sizes;

    $default_image_sizes = get_intermediate_image_sizes();

    foreach ( $default_image_sizes as $size ) {
        $image_sizes[ $size ][ 'width' ] = intval( get_option( "{$size}_size_w" ) );
        $image_sizes[ $size ][ 'height' ] = intval( get_option( "{$size}_size_h" ) );
        $image_sizes[ $size ][ 'crop' ] = get_option( "{$size}_crop" ) ? get_option( "{$size}_crop" ) : false;
    }

    if ( isset( $_wp_additional_image_sizes ) && count( $_wp_additional_image_sizes ) ) {
        $image_sizes = array_merge( $image_sizes, $_wp_additional_image_sizes );
    }

    return $image_sizes;
}

Which will output results similar to:

Array
(
    [thumbnail] => Array
        (
            [width] => 150
            [height] => 150
            [crop] => 1
        )

    [medium] => Array
        (
            [width] => 300
            [height] => 300
            [crop] => 
        )

    [medium_large] => Array
        (
            [width] => 768
            [height] => 0
            [crop] => 
        )

    [large] => Array
        (
            [width] => 1024
            [height] => 1024
            [crop] => 
        )

)

Method 3

If you only need the names of all image sizes, you can use get_intermediate_image_sizes:

<pre>
<?php print_r(get_intermediate_image_sizes()); ?>
</pre>

Method 4

Since WP 5.3 it is enough to use this function:

wp_get_registered_image_subsizes();

Method 5

If the other answer did not work, use this code, so it will run after WordPress is initialized.

add_action('init', 'get_all_image_sizes');

function get_all_image_sizes(){
    global $_wp_additional_image_sizes; 
    print '<pre>'; 
    print_r( $_wp_additional_image_sizes ); 
    print '</pre>';
}

Method 6

Since WP 4.7 you can use wp_get_additional_image_sizes() to get additional image sizes registered (excluding the default image sizes added by WP).

Since WP 5.3 you can use wp_get_registered_image_subsizes() to get all the image sizes registered.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0