The weight of the blanket should be no more than 8-10% of your body weight. So using 10% as an average, if you weigh 80kg, you will want a weighted blanket of around 8kg / 17lbs. It’s safe to go lighter at first if you're unsure what weight would give you the most benefit.
Weighted blankets for kids are much lighter and smaller, and some of the best weighted blanket brands make sizes for children too. In all cases, weighted blankets should never be used for children under the age of two, or any child under the weight of 20lbs, as it can be extremely dangerous. Children should never use a weighted blanket that is intended for adults, so keep yours out of reach from little hands when not in use.
Remember, weighted blankets can be heavy to move, so if you don’t think a child or adult is physically able to remove it easily if they wish, then the blanket should not be used.