def cappa2npsh(cappa, head):
    """Calculate the npsh required for a given pump's typical number.

    :param cappa (float): typical number
    :param head (float): head [m]
    :return npsh_req (float): neat positive suction head required [m]
    """
    npsh_req = .25 * cappa**(4/3) * head

    return npsh_req