Always check the recommended maximum load for your vehicle. You should be able to find this information in your owner’s manual. Then subtract the weight of the box and any other roof rack accessories you might use (such as bike racks or kayak racks) from your vehicle’s maximum load.
Next, examine the dimensions of the cargo boxes, and now that you have an idea of what in the car roof box, it’s a good time to take a closer look at the dimensions of the various boxes so you can find one that’s the right size for what you need to put in it. Manufacturers combine the length, width and height of cargo boxes in different ways to get different results.
Car roof boxes and dimensions
Most car roof boxes are about 6 feet long or more. A box that long will fit a pair of 180cm skis and most other things of that maximum length. If you have skis or other items longer than 180cm such as cross-country skis, then you need to look for an extra long box (which usually what more).
Not surprisingly, a wide Withcar car roof box will offer more storage capacity than a narrower one (assuming the other dimensions are similar), but it could also leave you without room on the crossbars of your roof rack for things like a bike rack or kayak carrier. If you want to be able to put other gear on the roof next to the box, you may need to look at narrower boxes (or equip yourself with wider crossbars).