A roofbox’s capacity in litres tells you its volume. It is a useful starting point, but it does not tell you exactly how many bags will fit — or whether your particular pushchair, tent or suitcase will go inside.
What does the litre figure mean?
One litre is a measure of volume. A 400-litre roofbox therefore has more stated capacity than a 300-litre model, but the extra space may be spread across its length, width or height.
For a familiar comparison, picture an 80-litre black bin bag. A 400-litre roofbox has the same stated volume as five of these bags — a useful way to visualise the extra space.
This is a guide to volume rather than a guaranteed bag count: what you pack, the shape of the bags and the roofbox’s curved interior will affect how much fits.
Why not compare it with a car boot?
A roofbox may have a similar stated capacity to a car boot, but the spaces are very different shapes. A roofbox is generally longer and shallower, with a curved lid.
Something that fits comfortably in your boot may be too tall or too wide for the roofbox, even when the litre figures are similar.
Will my bags fit?
Soft bags, clothing and bedding usually make better use of a roofbox’s shape than rigid suitcases. They can fit around the curves and into smaller spaces.
A bag labelled “80 litres” will not necessarily occupy exactly 80 litres once packed. Its shape, how full it is and the gaps between bags all affect the space needed. Dividing the roofbox capacity by the bag capacity will therefore not give a reliable bag count.
If you have an item that must fit, measure its length, width and height as it will be packed. Send us those dimensions so we can help assess a suitable box.
Check usable dimensions, not just overall size
The external dimensions include the roofbox’s shell. The usable space inside is smaller, and the curved lid means that the full internal height is not available everywhere.
Allow room for the lid to close and lock properly, and for the contents to be secured with the supplied straps. Never force the lid down over luggage.
More litres do not mean more permitted weight
Capacity in litres measures space. Capacity in kilograms measures weight. A larger roofbox can hold more bulky items without necessarily allowing a heavier load.
The permitted luggage weight depends on your vehicle, roofbar system and roofbox. Your vehicle’s roof-load allowance includes the weight of the bars, fitting equipment, box and contents.
For more detail, see the weight guidance in our searchable FAQs or read Thule’s guide to packing a roofbox safely, covering load limits, weight distribution and securing your luggage.
Choose the space you need for your car and trip
The largest box is not automatically the right choice. Its dimensions, fitting position and clearance for your tailgate also need to suit your vehicle.
Tell us how many people are travelling and what needs to go in the roofbox. Mention any particularly large items and provide their dimensions. We’ll help you choose from the suitable equipment available for your dates.
Our guide to choosing the right roofbox explains the other practical considerations.
Not sure how much space you need?
You don’t need to arrive with a model already chosen. Tell us about your car, your dates and your clobber — we’ll help with the rest.
Request your personalised quotation — we’ll reply in less than two working hours.
View our guide prices or call 01438 289030.
Advice from Nigel, Owner of Welwyn Roofbox Hire.
Based on our experience helping customers choose suitable Thule equipment and professionally fitting it for their cars and journeys.
Updated October 2026.