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
