WordPress | File size from URL

size_from_url.php

<?php 
// Return a neat file size
function nice_file_size($file_url, $precision = 2) {
    $nice_file_size = null;
    $attachment_id = attachment_url_to_postid($file_url); // Gets ID of file from URL
    $file_path = get_attached_file( $attachment_id ); // Gets path using ID
    if ( is_writable( $file_path ) ) { // Locally an error is thrown due to file not being writable
        $size = filesize( $file_path ); // NB filesize only works with the path to the file NOT a URL
        $base = log($size, 1024);
        $suffixes = array('', 'KB', 'MB', 'GG', 'TB');
        $nice_file_size = round(pow(1024, $base - floor($base)), $precision) .' '. $suffixes[floor($base)];
    }
    return $nice_file_size;
}
?>

<?php
// Example
echo nice_file_size($download_url, 0);
?>

Updated May 2022

I discovered that, when working locally, files are typically unable to be accessed due to permissions. This causes filesize to print an error. I've therefore added a condition if ( is_writable( $file_path ) ) { which ensures the file is accessible before running filesize. Probably not a bad condition to have in place regardless of whether you're working locally or not.