Researchers at the US National Centre for Atmospheric Research calculated the density of a cumulus cloud to be around 0.5 grams (0.018 ounces) of water per cubic meter, so a 1 cubic kilometer (0.24 cubic mile) wide cloud would contain 1 billion cubic meters (35 million cubic feet). If you calculate the number of cubic meters times by the density, 1,000,000,000 x 0.5, you’ll be left with the total weight of a cumulus cloud of that size – 500,000 kilograms (1.1 million pounds).
I think the term “weight” - while useful at sea level - can easily confuse people when it comes to clouds, as the density of the water in a cloud doesn’t have much to do with the weight of the cloud - see my elaborated edited response (unless I mis-guesstimated) - the mass of a cloud changes so drastically with altitude that the relative or even the absolute weight of water (vapor) therein is going to give people the wrong idea.