Roof Box Sizes Explained: How to Pick One That Fits Your Car and Your Luggage

A roof box offers a practical way to carry extra luggage, but choosing the right roof box size isn’t always straightforward. Capacity, dimensions, vehicle compatibility and roof load limits all play a role in finding the best option.

Many buyers assume that a larger roof box is always the better choice. In reality, the ideal size depends on your vehicle, the type of items you carry, how often you travel and your manufacturer’s roof load limits. Selecting the wrong size can affect boot access, driving comfort and even fuel efficiency.

This guide explains everything you need to know about roof box sizes, including standard capacities, how to match a roof box to your vehicle, and the key measurements to check before you buy. You’ll also find practical examples, expert tips and common mistakes to avoid so you can make a confident decision.

What Does Roof Box Size Mean?

A roof box size refers to the combination of a roof box’s storage capacity, physical dimensions and weight limit. While capacity is usually measured in litres, it’s equally important to consider the box’s length, width and height, as these determine what you can carry and whether the box will fit your vehicle safely.

When comparing roof boxes, you’ll commonly see specifications such as:

• Capacity (litres)
• External dimensions
• Internal dimensions
• Maximum payload
• Empty weight
• Maximum ski length (where applicable)

Many people focus solely on litre capacity, but two roof boxes with the same advertised volume can offer noticeably different usable storage because of their shape. A long, low-profile design may be ideal for skis and snowboards, while a shorter, taller model may accommodate more suitcases or camping equipment.

Typical Roof Box Size Categories

Although manufacturers vary slightly, roof boxes generally fall into these categories:

Roof Box Size	Capacity	Best For
Small	250–350 litres	Weekend breaks, couples, everyday storage
Medium	350–450 litres	Small families, longer holidays
Large	450–550 litres	Family holidays, camping equipment
Extra Large	550+ litres	Large families, bulky luggage, extended trips

This provides a useful starting point, but capacity alone shouldn’t determine your decision. Vehicle size, roof dimensions and roof load limits are equally important considerations.

Standard Roof Box Sizes Explained

Understanding the available size ranges makes it much easier to choose a roof box that suits both your car and the way you travel.

Small Roof Boxes (250–350 Litres)

Small roof boxes are ideal if you occasionally need additional storage without significantly increasing your vehicle’s overall size.

They’re well suited to:

• Couples travelling for weekends away
• Small hatchbacks
• Carrying sports equipment
• Extra luggage during holidays
• Everyday flexibility

Because they’re generally shorter and lighter, small roof boxes are often easier to fit, remove and store when not in use. They also tend to create less aerodynamic drag than larger models.

Medium Roof Boxes (350–450 Litres)

For many motorists, this is the most versatile category.

A medium-sized roof box typically provides enough space for several suitcases, pushchairs, or holiday essentials without overwhelming the roof of the vehicle.

They’re particularly suitable for:

• Family holidays
• Estate cars
• Crossovers
• Medium-sized SUVs
• Road trips around the UK

Many families find that a medium roof box strikes the best balance between storage capacity, practicality and ease of driving.

Large and Extra-Large Roof Boxes (450 Litres and Above)

If you regularly travel with bulky equipment or have a growing family, a larger roof box can provide valuable additional storage.

Large models are commonly chosen for:

• Camping holidays
• Ski trips
• Family holidays with four or more passengers
• Long-distance touring
• Carrying multiple large suitcases

However, bigger isn’t always better. A longer roof box may interfere with tailgate clearance on some hatchbacks and SUVs, while a heavier box reduces the amount of luggage you can safely carry within your vehicle’s maximum roof load.

Before selecting a large roof box, always check your vehicle handbook for the permitted dynamic roof load and ensure the combined weight of the roof bars, roof box and luggage remains within the manufacturer’s specified limit.

How to Measure Your Vehicle Before Buying a Roof Box

Choosing the right roof box size starts with understanding your vehicle’s dimensions and carrying capacity. Even the best roof box won’t perform well if it doesn’t fit your car or if it exceeds the manufacturer’s recommended roof load.

Before you buy, take a few simple measurements and checks.

Check Your Vehicle’s Maximum Roof Load

Every vehicle has a maximum dynamic roof load, which is the total weight the roof can safely support while driving. This figure includes:

• The roof bars
• The roof box
• Everything packed inside the roof box

For example, if your vehicle has a maximum roof load of 75 kg, your roof bars weigh 5 kg and your roof box weighs 18 kg, you have 52 kg remaining for luggage.

Simple calculation:

Maximum roof load − roof bars − roof box = maximum luggage weight

Always refer to your vehicle handbook for the correct roof load, as it varies between manufacturers and models.

Measure Available Roof Space

The overall length of your roof affects which roof box sizes will fit comfortably.

Measure:

• The distance between the front and rear roof bars
• Available roof length
• Roof width
• Clearance above the windscreen
• Space between the roof box and the rear tailgate

A roof box that extends too far towards the back of the vehicle may prevent the boot from opening fully—particularly on hatchbacks and SUVs.

Check Roof Bar Compatibility

Not every roof box fits every set of roof bars.

Before purchasing, make sure:

• The roof bars are compatible with your vehicle.
• The roof box supports your roof bar style (square, aero or wing bars).
• The crossbar spacing falls within the manufacturer’s recommended range.
• The mounting system matches your roof bars.

If you’re unsure which roof bars or roof box will fit your vehicle, choosing compatible products together helps avoid installation issues. Explore the suitable roof bars and roof box options available from A&S Towbars to ensure a secure, vehicle-specific fit.

Match the Roof Box to Your Vehicle Type

A roof box should complement your vehicle’s size and proportions. Selecting a model that’s too large can affect handling, reduce accessibility and create unnecessary wind resistance, while one that’s too small may not provide enough storage.

Hatchbacks

Hatchbacks generally benefit from compact or medium roof boxes between 250 and 400 litres.

A shorter roof box helps maintain access to the rear tailgate while providing enough space for holiday luggage, shopping or outdoor equipment.

Ideal for:

• Weekend trips
• Couples
• Small families
• Everyday storage

Estate Cars and SUVs

Estate cars and SUVs usually have longer roofs, making them suitable for medium to large roof boxes ranging from 400 to 550 litres.

These vehicles are popular choices for:

• Family holidays
• Camping equipment
• Pushchairs
• Multiple suitcases
• Longer journeys

Their larger roof area often provides greater flexibility when positioning the roof box without affecting boot access.

MPVs and Larger Vehicles

If you regularly travel with several passengers or bulky equipment, an extra-large roof box may be appropriate.

These are particularly useful for:

• Family camping holidays
• Ski trips
• Extended road trips
• Large sports equipment
• Outdoor adventure gear

Even with larger vehicles, avoid choosing the biggest roof box simply because it fits. Consider what you actually carry and whether a lower-profile model could offer better aerodynamics and easier handling.

Choose Your Roof Box Size Based on What You Carry

Instead of asking, “What’s the biggest roof box I can fit?”, a better question is, “What do I need to carry most often?”

Choosing a roof box based on your luggage and travel habits helps you avoid paying for unnecessary capacity while ensuring you have enough space when it matters.

Weekend Breaks and Everyday Use

If your extra storage needs are occasional, a smaller roof box is usually the most practical option.

A 250–350 litre model is ideal for:

• Two to three cabin-sized suitcases
• Sports equipment
• Pushchairs
• Shopping overflow
• Weekend luggage

Smaller boxes are also lighter, easier to remove and simpler to store when not in use.

Family Holidays

Families travelling with children often benefit from a 400–500 litre roof box.

This size comfortably accommodates:

• Four medium suitcases
• Soft travel bags
• Children’s travel essentials
• Beach equipment
• Folding chairs
• Lightweight outdoor gear

A medium-to-large roof box often provides the best balance between storage capacity and everyday practicality.

Camping and Outdoor Adventures

Camping equipment tends to be bulky rather than heavy, making larger roof boxes particularly useful.

A 500 litre or larger roof box is well suited to carrying:

• Sleeping bags
• Air mattresses
• Folding camping chairs
• Tents
• Outdoor clothing
• Lightweight cooking equipment

Packing lighter, bulkier items overhead also leaves more room inside the vehicle for passengers and heavier luggage.

Ski Holidays and Long Equipment

If you’re transporting skis or snowboards, focus on internal length rather than litre capacity.

A long, aerodynamic roof box may hold fewer litres than a taller model, yet still accommodate skis that wouldn’t fit inside a shorter box. Checking the manufacturer’s maximum ski length specification helps ensure your equipment fits comfortably.

Practical Example

Imagine a family of four travelling from the Midlands to Cornwall for a week-long summer holiday. With the boot already occupied by a pushchair, cooler box and everyday essentials, a 420–450 litre roof box provides additional space for clothing, beach gear and soft luggage without overcrowding the cabin. The result is a more comfortable journey and easier access to essential items during the trip.

Roof Box Size Comparison Table

If you’re still unsure which roof box size is right for your vehicle, this quick-reference table can help narrow down your options.

Roof Box Size	Capacity	Typical Luggage	Best For	Suitable Vehicles
Small	250–350L	2–3 cabin suitcases, sports gear	Weekend breaks, couples	Hatchbacks, city cars
Medium	350–450L	3–4 medium suitcases, pushchairs, holiday luggage	Small families, road trips	Hatchbacks, estates, crossovers
Large	450–550L	4–5 suitcases, camping gear	Family holidays	Estates, SUVs, MPVs
Extra Large	550L+	Bulky outdoor equipment, skis, extended holiday luggage	Large families, camping and winter sports	Large SUVs, MPVs, estate cars

Remember that these figures are only a guide. Always check the roof box’s dimensions, your vehicle’s roof load limit and the compatibility of your roof bars before purchasing.

Roof Load Limits and Safety Considerations

Choosing the correct roof box size isn’t just about storage—it also affects safety, handling and vehicle performance.

Never Exceed Your Vehicle’s Roof Load

Every vehicle manufacturer specifies a maximum dynamic roof load. This includes:

• Roof bars
• Roof box
• All luggage inside the roof box

The lowest-rated component—whether it’s the vehicle roof, roof bars or the roof box itself—determines the maximum safe carrying weight.

Overloading your roof can:

• Reduce vehicle stability
• Increase braking distances
• Affect steering performance
• Put unnecessary stress on the roof structure

Always consult your vehicle handbook before loading a roof box.

Pack Smart

For safer driving:

• Place lighter, bulkier items in the roof box.
• Keep heavier items inside the vehicle boot where possible.
• Distribute weight evenly from side to side.
• Secure all luggage to prevent movement during braking or cornering.

Consider Vehicle Height

Adding a roof box increases your vehicle’s overall height.

Before travelling, remember to check:

• Multi-storey car parks
• Height barriers
• Garage entrances
• Ferry restrictions where applicable

A quick reminder inside the vehicle can help prevent expensive accidents.

Common Roof Box Buying Mistakes

Avoiding a few common mistakes can save time, money and frustration.

Buying the Largest Roof Box Available

Bigger isn’t always better.

An oversized roof box may:

• Reduce fuel efficiency
• Create more wind noise
• Make parking more difficult
• Interfere with boot opening

Choose the smallest size that comfortably meets your usual travel requirements.

Ignoring Roof Bar Compatibility

Not all roof boxes fit every roof bar system.

Before purchasing, check:

• Bar profile
• Crossbar spacing
• Mounting system compatibility
• Weight ratings

Buying compatible roof bars and a roof box together often simplifies installation.

Forgetting About Internal Dimensions

Manufacturers usually advertise external dimensions and litre capacity, but internal measurements are equally important.

For example, if you’re carrying skis or fishing rods, internal length matters far more than total volume.

Focusing Only on Capacity

A 450-litre roof box isn’t automatically better than a 400-litre model.

Shape, opening mechanism, internal layout and ease of loading all influence how much usable storage you actually gain.

Which Roof Box Size Is Right for You?

If you’re still deciding, use the following guide.

Choose a Small Roof Box (250–350L) if you:

• Travel mainly as a couple
• Need occasional extra storage
• Own a small hatchback
• Carry sports equipment
• Want easier storage when not in use
• Prefer better fuel efficiency
• Rarely need space for bulky luggage

Choose a Medium Roof Box (350–450L) if you:

• Take regular family holidays
• Need room for several suitcases
• Own an estate, crossover or SUV
• Travel throughout the year
• Want the best all-round option
• Carry children’s equipment
• Occasionally transport outdoor gear

Choose a Large Roof Box (450L+) if you:

• Have a larger family
• Frequently go camping
• Carry bulky outdoor equipment
• Travel long distances
• Need maximum luggage capacity
• Take winter sports holidays
• Have sufficient roof space and roof load capacity

Choose the Right Roof Box with Confidence

Finding the ideal roof box size doesn’t have to be complicated. By considering your vehicle’s dimensions, roof load limit, travel habits and the type of luggage you carry most often, you can choose a roof box that offers the right balance of storage, safety and convenience.

Whether you’re planning a weekend getaway, a family holiday or an outdoor adventure, selecting the right size helps maximise available space without compromising comfort or driving performance.

If you need expert guidance, A&S Towbars is here to help with quality roof boxes, roof bars and vehicle carrying solutions for a wide range of makes and models. Explore our products at https://www.astowbars.co.uk/, call 0800 211 8076, or email sales@astowbars.co.uk for personalised advice on choosing the right roof box for your next journey. 

 

Frequently Asked Questions

What roof box size do I need?

The right roof box size depends on your vehicle, the number of passengers and the type of luggage you carry. Most families find a 400–500 litre roof box provides an excellent balance of storage and practicality.

Is a 320-litre roof box big enough?

Yes. A 320-litre roof box is suitable for couples, weekend breaks or carrying sports equipment, although it may be too small for longer family holidays.

Does every roof box fit every car?

No. You must ensure the roof box is compatible with your vehicle’s roof bars, crossbar spacing and maximum roof load.

Does roof box size affect fuel economy?

Generally, yes. Larger roof boxes create more aerodynamic drag, which can increase fuel consumption, particularly at motorway speeds. Choosing a streamlined, appropriately sized model can help minimise this effect.

Can I still open my boot with a roof box fitted?

Usually, yes—but only if the roof box is correctly positioned. Always check tailgate clearance before purchasing, especially if you drive a hatchback or SUV.

How much weight can I put in a roof box?

This depends on the manufacturer’s payload limit and your vehicle’s maximum roof load. Always use the lower of the two limits and include the weight of the roof bars and roof box in your calculations.

Should I choose a longer roof box for skis?

Yes. Internal length is often more important than total capacity when transporting skis, snowboards or other long equipment.

Are roof boxes waterproof?

Most quality roof boxes are designed to protect luggage from rain and road spray. However, using waterproof bags for valuable or sensitive items provides an extra layer of protection during severe weather.