When you update your theme, WordPress deletes the theme folder and reloads the new theme.
Hence any changes or additions to the theme will get removed.
You have 2 options.
Put the calculator in a plugin and install it.
Use a child theme ( http://codex.wordpress.org/Child_Themes ) and put the calculator in it so that when you update the current theme (which becomes the parent theme ), the child theme is unaffected.