What Size Cargo Box Do I Need?

Treat these as a starting point, then apply the constraints below — they will usually push you down a size rather than up. For skis specifically, length overrides everything, and we cover it in cargo boxes for skis.

The published dynamic roof load limit covers the crossbars, the box and the cargo together. Subtract the first two and what remains is your real payload — which on many cars is less than the box’s volume would suggest you could carry.

This is why a bigger box is not automatically better. A 600-liter shell on a roof with a modest limit simply gives you more space you are not allowed to fill, at the cost of extra weight and extra drag.

Find your figure in the owner’s manual, under the roof rack section, and read our weight limits guide for the dynamic-versus-static distinction before you interpret it.

Every box states a minimum and maximum crossbar spread, and its mounting points must land inside your bars. Short roofs can fall below a large box’s minimum, which no amount of goodwill fixes. Measure bar center to bar center; the method and the numbers are in crossbar spread and length.

Clearance is simpler and more often forgotten. Measure your garage opening, measure the car’s height, and buy inside the difference — remembering the bars add height before the box does. Then think about the parking structure you use and the drive-through on the way out of town.

A low-profile box at the same volume is the usual answer to a clearance problem, and it also costs less fuel. It is the one place where the clearance constraint and the efficiency argument point the same way.

A long box pushed back on the bars can be caught by a tailgate or trunk lid as it rises. On hatchbacks and SUVs with steeply raked rear glass this is common and it is not always obvious until the lid taps the box.

Check it before you buy by measuring the rear overhang you would need, and check it again after fitting by opening the tailgate slowly with a hand on it. If it does foul, moving the box forward is the fix — at the cost of more noise and more drag, because the box moves into cleaner, faster air.

Dual-side opening mitigates a lot of practical annoyance here, and it is worth paying for on any box you will load in a car park.

Soft bags, not hard suitcases. A roof box’s usable volume is its shape, and soft luggage fills a tapered molding in a way rigid cases never will. Two large soft bags will beat three small hard ones in the same box.

Light and bulky up, heavy and dense down. Sleeping bags, coats, towels and empty coolers belong in the box; tools, books, tinned food and anything with a motor belong in the trunk, where they do not count against the roof limit.

And load it so nothing can move. A box with a shifting load is noisier and changes the car’s balance mid-corner — the method is in how to load a cargo box.