get_upload_space_available()

function get_upload_space_available() {
	$allowed = get_space_allowed();
	if ( $allowed < 0 ) {
		$allowed = 0;
	}
	$space_allowed = $allowed * MB_IN_BYTES;
	if ( get_site_option( 'upload_space_check_disabled' ) )
		return $space_allowed;

	$space_used = get_space_used() * MB_IN_BYTES;

	if ( ( $space_allowed - $space_used ) <= 0 )
		return 0;

	return $space_allowed - $space_used;
}