Source

File: wp-includes/functions.php

function get_dirsize( $directory, $max_execution_time = null ) {
	$dirsize = get_transient( 'dirsize_cache' );

	if ( is_array( $dirsize ) && isset( $dirsize[ $directory ]['size'] ) ) {
		return $dirsize[ $directory ]['size'];
	}

	if ( ! is_array( $dirsize ) ) {
		$dirsize = array();
	}

	// Exclude individual site directories from the total when checking the main site of a network,
	// as they are subdirectories and should not be counted.
	if ( is_multisite() && is_main_site() ) {
		$dirsize[ $directory ]['size'] = recurse_dirsize( $directory, $directory . '/sites', $max_execution_time );
	} else {
		$dirsize[ $directory ]['size'] = recurse_dirsize( $directory, null, $max_execution_time );
	}

	set_transient( 'dirsize_cache', $dirsize, HOUR_IN_SECONDS );
	return $dirsize[ $directory ]['size'];
}