Guide

Roof box sizes explained: litres to luggage

Roof boxes are sold by the litre — 320, 420, 500 — which is precisely no help when what you're packing is two children, a buggy, and a week of clothes for Kerry. Here's how to translate litres into actual luggage, and how to pick a size that suits your car as well as your boot.

What a litre figure actually measures

The litres on the label are the gross volume of the shell — every corner of a curved, tapered wedge, measured as if you could fill it with water. You can't fill it with suitcases. Hard cases waste the tapering ends and the low front, so in practice you'll fit meaningfully less rigid luggage than the number suggests. Soft duffel bags and hold-alls squash into the corners a hard case can't reach, which is why seasoned packers bring duffels, not wheelie cases, to a roof box.

For a rough mental anchor: a standard cabin bag (55 × 40 × 20cm) is about 44 litres. That's the unit the rest of this guide counts in.

The sizes, in luggage terms

• 300–330 litres — roughly three cabin bags' worth, more if it's soft bags. The overflow box: sleeping bags, wellies, the beach tent, the awkward light stuff that fills a boot. A couple's full luggage, or a family's spillover while the cases stay in the boot.
• 400–450 litres — the family sweet spot. A 420-litre box takes four cabin bags, with room around them for the soft odds and ends. A family of four's clothes for a week travel up top while the boot keeps the buggy, the food box, and the dog.
• 460–500+ litres — the big wedges, usually long rather than tall. Five-plus cabin bags, or a big family's full holiday load. Many boxes this size are cut long for skis (look for an internal length around 180cm or more if skis are the point), which is also what makes them a poor match for small cars — more on that below.

One honest caveat: shapes vary between makers, so two 420-litre boxes can swallow different amounts of the same luggage. Treat the bands above as a guide, not gospel.

The family-of-four question

The question behind most size searches: does 320 litres do a family of four? It does if the box is carrying the overflow — bulky soft stuff up top, cases in the boot. It doesn't if you want all the luggage on the roof and the boot left free. For that, 400 litres and up is the comfortable answer, and it's why the 420-ish class is the one you see on half the cars heading west on the M4 in August.

Bigger isn't automatically better

Before falling for the biggest box on the page, three checks against your actual car:

• Length and the tailgate. A long ski box on a short hatchback can hang over the rear enough that the boot won't open fully — you find this out with the box loaded, in the rain. Check the box's length against your roof before committing.
• Roof load limit. Your car's handbook states a maximum roof load, commonly 50–100kg, and that figure includes the bars and the box itself. A big box can weigh 20kg empty, bars another 5kg or so, which may leave less packing allowance than you'd think.
• Height. Bars and box together add somewhere around half a metre to the car. Worth remembering before the height barrier at the multi-storey, not after.

Not sure? Borrow the size first

Sizes are genuinely hard to judge from a listing photo, and a wrong guess is expensive when you've bought the box. It's not when you've borrowed it. Renting a 320 for one trip and a 420 for the next tells you more than any chart — this one included — and if the size you need isn't listed near you, you can post what you're after and the nearest owners will see it. If you're weighing up owning one at all, the sums are in renting vs buying a roof box.