@classmethod
def convert(cls, src_value: float, src_unit: str, des_unit: str) ->float:
    """Convert a quantity value from one unit to another unit.

        src_value : float
            the value to convert
        src_unit : str
            the current unit in which the quantity value is expressed
        des_unit : str
            the new unit in which the quantity value is to be expressed

        return value : float
            the value of the quantity expressed in the new unit
        """
    src_cf = cls.units.get(src_unit)
    base_value = src_value / src_cf
    des_cf = cls.units.get(des_unit)
    des_value = base_value * des_cf
    return des_value
