def to_plato(sg):
    """
    Convert specific gravity to plato (extract)

    (-1 * 616.868) + (1111.14 * sg) – (630.272 * sg^2) + (135.997 * sg^3)
    """
    plato = (-1 * 616.868) + (1111.14 * sg) - (630.272 * sg ** 2) + (135.997 * sg ** 3)
    return plato