On average, most boxes can hold up to 100kg depending on the type of car and what is being carried. However, always refer to your car’s handbook or manufacturer for specific weight limits. Investing in a cargo box can be a great way to increase storage space and help you make the most out of every journey. The amount a rooftop cargo carrier can hold depends on the size of the box you have purchased. Boxes come in a range of sizes and shapes, so it is important to choose one that will fit your car’s roof bars and accommodate the items you wish to carry.