def API_to_SG(API):
    """
    Calculates API of a liquid given its specific gravity, as shown in
    [1]_.

    .. math::
        \\text{SG at}~60^\\circ\\text{F} = \\frac{141.5}{\\text{API gravity} +131.5}

    Parameters
    ----------
    API : float
        API of the fluid [-]

    Returns
    -------
    SG : float
        Specific gravity of the fluid at 60 degrees Farenheight [-]

    Notes
    -----
    Defined only at 60 degrees Fahrenheit.

    Examples
    --------
    >>> API_to_SG(60.62)
    0.7365188423901728

    References
    ----------
    .. [1] API Technical Data Book: General Properties & Characterization.
    American Petroleum Institute, 7E, 2005.
    
    """
    return 141.5 / (API + 131.5)
