Almost everyone starts with the same question: how many litres do I need? It is a reasonable place to begin, but capacity on its own has sent plenty of people home with a box that does not fit the car, does not fit the gear, or blocks the tailgate every time they open it.
Here is the order we actually work through with people on the floor.
Start with the longest thing you carry
Volume is easy to market and easy to misread. A 400-litre box and a 450-litre box can hold roughly the same amount of duffel bags, but only one of them may swallow a pair of 175 cm skis without a fight.
So before anything else, measure your longest item — skis, snowboards, fishing rods, a stroller frame — and treat that as the floor for the box's internal length. As a rough guide:
- Up to 165 cm skis or most snowboards: a short or mid-length box will do.
- 175 cm and up: you want a long box, typically 200 cm or more externally.
- Bags, camping gear, luggage only: length matters less — buy for volume and shape.
Measure your crossbar spread
Every box has a minimum and maximum spread it can clamp to — the distance between the centres of your two crossbars. Too narrow and the box will not sit securely; too wide and the mounting points will not line up at all.
Measure yours before you shop. It takes thirty seconds with a tape measure and it eliminates a whole category of boxes that were never going to work on your roof.
The single most common reason a box goes back on the rack after a test fit is not weight or length — it is that the crossbars are too close together for the box's mounting range.
Mind the rear hatch
On hatchbacks and most SUVs, a long box mounted too far back will collide with the tailgate as it swings up. You usually will not notice in the parking lot; you notice the first time you open the back in a garage.
The fix is usually to slide the box forward on the bars, but that only works if you have enough bar in front of you. If your vehicle has a short roof and a steeply raked hatch, a shorter box mounted centrally is the better answer than a long one shoved forward.
Check the roof's weight limit, not just the box's
Two numbers matter here and they are not the same:
- Your vehicle's dynamic roof load — how much the roof can carry while moving. This is in the owner's manual and it includes the rack and the box itself, not just what you put inside.
- The box's own capacity — usually generous enough that the roof limit is what actually constrains you.
On a typical crossover with a 75 kg dynamic limit, a 20 kg rack plus an 18 kg box leaves you under 40 kg of gear. That is a lot of ski clothing and not much firewood. Plan around the smaller number.
Think about where it lives in the off-season
A long box is wonderful on the road and awkward in a garage. If storage is tight, that is a legitimate reason to size down — or to ask us about a wall hoist when you pick the box up.
Where we come in
We keep new and used boxes in stock in Hooksett precisely so this does not have to be a guessing game. Bring the vehicle, tell us what you are hauling, and we will test fit before you buy — crossbar spread, hatch clearance and all. If it does not fit, you have not spent anything.
If you are trading up from a box you already own, read how our trade-in credit works before you come in, and bring the old box with you.