def adjust_gravity_volume(vol, og, ng):
    """
    Returns the new volume needed to achieve the desired new gravity.
    This is unit independent and the return value can be used for liters
    and or gallons.

    New Volume = (Volume * original Gravity) / new Gravity

    :arg vol: Original volume of wort
    :arg og: The current gravity of the wort
    :arg ng: The desired gravity of the wort
    :returns: The amount to adjust the wort volume by
    """
    og = (og - 1) * 1000
    ng = (ng - 1) * 1000
    return vol * og / ng
