Short answer: The number of people and the type of trip determine the roof box size; a rough guideline is about 100 liters per person. For a four-person family holiday, 350–450 liters is usually enough; for camping, skiing, or longer trips, 450–480 liters offers more room. But capacity alone is not enough: the box's length must fit the longest item you plan to carry, and its total weight must comply with your vehicle's roof load limit.
How many liters do you need?
- 350 liters — weekend and intercity trips for 2–3 people; suitcases, bags, and soft items.
- 370 liters — the same use, with a little more room.
- 450 liters — a family holiday for 4; large suitcases, beach and camping gear.
- 480 liters — long holidays, camping, and winter sports; bulky but lightweight loads.
The cost of choosing a larger size is not capacity, but weight and wind resistance: a larger box weighs more, creates more wind resistance, and increases fuel consumption even when empty. Choosing one size larger than you need may seem sensible “just in case,” but in practice you pay the price on every trip.
Three Measurements to Check Before Capacity
- Length: If you plan to carry skis, a fishing rod, or a long tent pole, the box's internal length—not its capacity—is what matters. Measure the longest item you plan to carry.
- Weight: The roof box's empty weight counts against your vehicle's roof load limit. That means you cannot put 450 liters of weight in a 450-liter box—your actual limit is the one in your vehicle manual. The crossbars on Thunder Carrier roof bars are rated for 85 kg while driving; if your vehicle's limit is lower, your vehicle's limit applies.
- Will the Tailgate Open? When long boxes extend toward the rear, the tailgate may not open fully on some vehicles. Test it by opening the tailgate after installation.
What Do You Mount a Roof Box On?
A roof box does not go directly on the roof; it is mounted on roof bars . If your vehicle does not have roof bars, you need to get them first; the type of roof rail on your vehicle determines which series to choose.
Why Does the Material Matter?
A roof box is exposed to sun, rain, and temperature changes throughout the year. The aerodynamic shell of Thunder Carrier roof boxes is made from UV- and impact-resistant ABS using vacuum-forming technology; its surface is reinforced for scratch resistance with an acrylic (PMMA) layer. Critical parts such as the locking mechanism and mounting feet are made from 30% glass-fiber-reinforced polyamide (PA6), while all metal fasteners are Geomet coated. The box closes with a central security lock.
Note: roof boxes are bulky items and are shipped by freight carriers to prevent damage during transit; delivery takes 3–5 days.
Frequently Asked Questions
How do you choose the right roof box capacity for your vehicle?
A rough guideline is about 100 liters per person: 350–370 liters for 2–3 people, 450 liters for a family holiday for 4, and 480 liters for longer holidays and camping. Before capacity, however, check three things: the box length needed for your longest item, your vehicle's roof load limit, and whether the tailgate opens fully after installation.
Are there downsides to buying a large roof box?
Yes. A larger box weighs more, and that weight counts against your vehicle's roof load limit; it also creates more wind resistance, increasing fuel consumption even when empty. You pay for choosing one size larger “just in case” on every trip.
Can I put a 450 kg load in a 450-liter roof box?
No—liters measure volume, not weight. The amount you can carry is determined by the lowest of three limits: your vehicle's roof load limit in its owner's manual, the roof bars' capacity (85 kg while driving for Thunder Carrier), and the roof box's own limit. The empty weight of the box also counts toward that limit.
Is a roof box mounted directly on the vehicle roof?
No, it is mounted on roof bars. If your vehicle does not have roof bars, you need to get them first; the type of roof rail on your vehicle determines which series to choose.
What material is a roof box made from?
The aerodynamic shell of Thunder Carrier roof boxes is made from UV- and impact-resistant ABS using vacuum-forming technology, and its surface is reinforced with a scratch-resistant acrylic (PMMA) layer. The locking mechanism and mounting feet are made from 30% glass-fiber-reinforced polyamide (PA6); metal fasteners are Geomet-coated.
Are roof boxes shipped by freight carrier?
Yes. Roof boxes are bulky items and are shipped by freight carriers to prevent damage during transit; delivery takes 3 to 5 days.
Related Guides
Related Articles
Related product: Thunder Carrier Roof Box - 350 L