Size in Roof Cargo Boxes: What It Is and Is It Worth It?

Roof Box Size Explained Simply

When you shop for a roof cargo box, the first spec you will see is size. This number tells you how much gear fits inside. It is the most important factor for planning your trips. You need to know if the box holds your skis, luggage, or camping gear. Manufacturers list this in cubic feet or by capacity labels. Common values include 15 cubic feet, 20 cubic feet, and 21 cubic feet. These numbers represent the internal volume. A 15 cubic foot box is compact. It fits small sedans well. A 20 or 21 cubic foot box offers more space. It suits larger SUVs or families with more gear. Some listings use vague terms like large or one size. These labels are less precise. You should always check the cubic footage for clarity. Weight capacity is another critical size metric. You may see a limit of 500 lbs. This is the maximum weight the box and your car roof can handle. Exceeding this limit is dangerous. It can damage your vehicle or affect driving safety. Specialized boxes exist for specific items. You might find a size labeled as 2-bike. These are designed to hold bicycles securely. They do not use standard cubic foot measurements. Instead, they focus on frame clearance and strap points. Understanding size explained helps you avoid buying the wrong box. If you buy too small, you leave gear behind. If you buy too large, you block your view or strain your car. Measure your roof space first. Then compare it to the box dimensions. Check the cubic feet against your gear volume. Always verify the weight limit. This ensures your load is safe and secure. The right size makes travel easier. It keeps your car interior free of clutter. It also improves aerodynamics when the box is empty. Choose a size that matches your typical load. Do not guess. Use the numbers provided by the manufacturer. This simple step saves time and money later.

Common Size Options Compared

15 Cubic Feet

This size offers a compact storage solution for weekend gear or light luggage. The MeeFar 15 cu ft Roof Bag demonstrates how soft shell designs can fit vehicles without racks. Its dimensions of 44 by 34 by 17 inches provide manageable volume without excessive wind noise. You should choose this if you travel light or have limited roof space. The trade-off is lower capacity, meaning you cannot fit large suitcases or bulky sports equipment. It is ideal for solo travelers or couples with minimal baggage. The HubPax set also uses this volume for basket coverage, showing versatility in storage methods. Consider your typical load size before selecting this tier.

Large

Large boxes prioritize aerodynamic efficiency and high capacity for long trips. The Thule Motion 3 L Large features an 18-inch height and 81-inch length, maximizing internal volume while maintaining a sleek profile. This size suits families or those moving significant amounts of gear. The Thule Pulse 2 L also falls into this category, offering dual-side access for easier loading. You will benefit from reduced wind drag compared to boxier models. However, the increased length requires compatible roof racks with sufficient bar spacing. Verify your vehicle’s rack limits before purchasing. This option balances volume with highway stability, making it a practical choice for extended road trips where space is at a premium.

20 Cubic Feet

This capacity provides substantial storage for heavy loads or extended vacations. The Asinking 20 cu ft Waterproof Roof Bag illustrates how soft bags can offer high volume without rigid structures. Its 52-inch length and 40-inch width allow for bulky items like camping tents or multiple large suitcases. You should select this if you need maximum flexibility for irregularly shaped gear. The waterproof design protects contents from rain, which is crucial for outdoor adventures. The trade-off involves potential wind resistance compared to hard shells, though modern designs mitigate this. This size is excellent for groups or those transporting sports equipment. Ensure your vehicle’s weight limit can handle the added load when fully packed.

One Size

This designation typically refers to a universal or fixed-size product that does not vary in volume options. The SportRack Sidekick Cargo Box is listed as One Size, indicating a specific, compact footprint. Its 25-inch width and 54-inch length make it suitable for small loads and compact roof racks. You should choose this if you have limited space or only need to carry a few items. It is not suitable for large cargo or heavy items, as noted in its verdict. The fixed nature simplifies the buying process by removing volume comparisons. However, it limits your ability to scale storage for larger trips. This option is best for urban drivers or those using roof boxes for occasional, light-duty use.

2-Bike

This size category refers to roof boxes designed specifically to carry bicycles. While the provided data focuses on cargo volume, 2-bike carriers have unique structural requirements. They are wider and often have a different shape to accommodate bike frames securely. You should choose this if your primary goal is transporting bicycles rather than general cargo. The trade-off is reduced versatility for other types of gear. Standard cargo boxes may not fit bikes safely due to lack of specific mounting points. This option is essential for cycling enthusiasts who need to transport their equipment on long trips. Verify compatibility with your bike type and rack system before buying.

500 lbs. Capacity

This specification refers to the maximum weight the roof box or rack system can support, not the volume. The data mentions 500 lbs. Capacity as a listed value. You must ensure your vehicle’s roof limit exceeds this weight to avoid damage. This capacity is crucial for heavy gear like tools or large equipment. Most standard roof boxes weigh significantly less, but the total load includes the box itself. Exceeding this limit can compromise vehicle handling and safety. Always check your car’s manual for roof load limits. This spec is a critical safety factor that outweighs volume considerations in some cases. Prioritize weight limits when transporting dense items.

Is Larger Size Worth The Cost

Who needs it

You should prioritize a larger capacity if you pack for extended trips or carry bulky equipment. A twenty-one cubic foot box provides ample room for multiple suitcases, camping gear, or sports equipment. This size suits families traveling together or individuals moving household items. The five-hundred-pound capacity rating ensures you can load heavy items without worrying about structural limits. If your vehicle has a high weight limit and you frequently transport large volumes, choosing a larger box prevents the need for multiple trips. It also keeps your cabin clear of clutter. The extra space allows you to organize gear efficiently, protecting fragile items from shifting during transit. For those who value convenience and space over aerodynamic efficiency, a larger footprint is a practical choice that simplifies your travel logistics significantly.

Who can skip it

You can ignore maximum size if you only carry light luggage or travel solo. A fifteen cubic foot option is often sufficient for weekend getaways or short business trips. Larger boxes add wind resistance, which can reduce your vehicle's fuel efficiency. They also increase the vehicle's height, potentially causing issues in low-clearance garages or parking structures. If you prioritize aerodynamics and fuel economy, a smaller, more streamlined box is a better fit. You should also consider the mounting system's weight limit before choosing a large box. If your roof racks have a lower dynamic weight limit, a smaller box ensures you stay within safe operating parameters. Focus on fit and secure attachment rather than raw volume if your needs are modest. A compact solution reduces drag and maintains better handling characteristics on the road.

Roof Box Size Glossary

Shell Type

The material construction of the carrier, such as hard plastic or soft fabric. Hard shells offer better weather protection, while soft bags are lighter and foldable for storage.

Capacity

The internal volume measured in cubic feet. This number indicates how much luggage or gear fits inside. Common sizes range from fifteen to twenty-one cubic feet for most vehicles.

Opening

The direction the lid swings open. Dual-sided openings allow access from either side of the vehicle, which is useful when parking in tight spaces between other cars.

Load Capacity

The maximum weight the box can safely hold, usually around one hundred sixty-five pounds. Exceeding this limit can damage the carrier or affect vehicle handling during travel.

Vehicle Type

The specific car models or roof rack styles compatible with the carrier. You must ensure the base or straps fit your existing roof bars or crossbars securely.

Water Resistance

The ability of the seal to keep rain out. Look for models with gaskets and drainage channels to protect your gear from moisture during heavy storms or car washes.

Security

Features that prevent theft, such as integrated locks or lockable hinges. These mechanisms secure the lid to the roof bars, keeping your valuables safe while you are away.

Quick picks

Our 6 picks

1

MeeFar 15 cu ft Roof Bag

Best for: Cars without roof racks

Size

15 Cubic Feet (what this guide is about)

Capacity

15 cu ft

Shell Type

Soft shell, 700D PVC tarpaulin

Water Resistance

100% waterproof

Security

Combination lock

Vehicle Compatibility

Vehicles with or without roof rack

View on Amazon: MeeFar 15 cu ft Roof Bag Affiliate link

This MeeFar cargo carrier offers 15 cubic feet of storage in a soft shell design. It measures 44 inches long by 34 inches wide. The 700D PVC tarpaulin material is rated 100% waterproof. You can install it quickly on cars with or without roof racks. The package includes a combination lock for zipper security. It fits 4 to 6 suitcases.

Pros

• 15 cubic feet capacity
• 100% waterproof 700D PVC material
• Universal fit for cars with or without racks
• Includes combination lock

Cons

• Soft shell lacks rigid protection
• 100 kilograms weight is high
• No dual-side opening access

Good fit if

• You need extra storage for a short trip
• You have a car without roof racks
• You want a waterproof soft shell

Skip if

• You need rigid shell protection
• You require dual-side access
• You want a lighter carrier

Verdict: MeeFar 15 Cu Ft Soft Shell Roof Bag provides flexible storage for travelers without roof racks. It suits those prioritizing waterproof soft shell capacity over rigid protection.

2

Thule Motion 3 L Large

Best for: drivers needing large capacity and secure mounting

Size

Large (what this guide is about)

Security

SlideLock system

Installation

PowerClick quick-mount system

Dimensions

81 x 38 x 18 in

Weight

59.5 pounds

View on Amazon: Thule Motion 3 L Large Affiliate link

This is a large roof cargo box measuring 81 by 38 by 18 inches. It weighs 59.5 pounds and features a wing-shaped profile for reduced drag. The PowerClick quick-mount system includes a torque indicator for secure installation. You get separate locking and opening functions with the SlideLock system. This box suits you if you need substantial storage capacity and reliable mounting security.

Pros

• PowerClick quick-mount system with integrated torque indicator
• SlideLock system separates locking and opening functions
• Wing-shaped profile reduces drag and fuel consumption
• Wide handle allows one-handed lid operation

Cons

• Large dimensions may not fit smaller roof racks
• Heavy at 59.5 pounds for solo installation
• No mention of interior lighting or liner included

Good fit if

• You have a roof rack with sufficient width and load capacity
• You prioritize aerodynamic efficiency for highway driving
• You want a secure locking mechanism separate from opening

Skip if

• You need a low-profile box under 16 inches in height
• You prefer a lighter carrier under 50 pounds
• You require a one-size-fits-all universal bag

Verdict: Thule Motion 3 L Large is a robust option for drivers needing high capacity and secure mounting. It fits you best if you value aerodynamic design and easy installation on compatible roof racks.

3

Asinking 20 cu ft Waterproof Roof Bag

Best for: waterproof storage on cars without roof racks

Size

20 Cubic Feet (what this guide is about)

Capacity

20 cu ft

Shell Type

900D PVC tarpaulin

Water Resistance

100% waterproof

Security

Lock

Installation

Metal ring strap design

View on Amazon: Asinking 20 cu ft Waterproof Roof Bag Affiliate link

This is a soft-sided roof bag with 20 cubic feet of capacity. It uses 900D PVC tarpaulin and triple-reinforced seams to stay waterproof. The bag fits vehicles with or without roof racks. You get a lock for security and metal ring straps for easy installation. The shape lowers at the front to reduce drag. It is a flexible option for road trips where you need extra space without a hard shell.

Pros

• 20 cubic feet capacity
• 100% waterproof 900D PVC construction
• Fits cars with or without roof racks
• Includes a lock for security
• Metal ring strap design for easy setup

Cons

• Soft shell offers less impact protection than hard plastic boxes
• Aerodynamic shape may create more wind noise than rigid boxes
• Requires manual folding and storage when not in use

Good fit if

• You need flexible storage that fits cars without roof racks
• You want a waterproof solution for rainy weather trips
• You prefer a lower profile design for fuel efficiency

Skip if

• You need maximum security against theft or impact
• You want a rigid shell for easier loading
• You prefer a permanent mount that stays on the roof

Verdict: Asinking 20 Cu Ft Rooftop Cargo Bag is a versatile choice for drivers who need extra space but lack roof rails. Its waterproof build and rack-free design make it ideal for spontaneous road trips where flexibility matters more than rigid protection.

4

HubPax 15 Cu Ft Roof Basket & Bag Set

Best for: Drivers needing waterproof basket coverage

Size

15 Cubic Feet (what this guide is about)

Capacity

15 cu ft

Shell Type

soft shell

Load Capacity

200 lbs

Water Resistance

waterproof

Installation

U-bolt installation

View on Amazon: HubPax 15 Cu Ft Roof Basket & Bag Set Affiliate link

This HubPax set includes a 51 x 36 x 5 inch basket and a 15 cubic foot soft shell bag. It mounts to cross bars using U-bolts and holds 200 lbs. The waterproof bag protects gear from rain. This combination offers flexible storage for smaller vehicles. You get a rigid base and a soft cover. It suits road trips where you need adaptable cargo space.

Pros

• Includes both a hard basket and a 15 cu ft waterproof bag
• 200 lbs load capacity for heavy gear
• Soft shell bag measures 44 x 34 x 17 inches
• Mounts securely with U-bolts to existing cross bars

Cons

• 15 cubic feet capacity is smaller than the 20 to 21 cubic foot options on this list
• Bag is soft shell, offering less rigid protection than hard plastic boxes
• Cross bars are required but not included in the set

Good fit if

• You need a waterproof cover for your roof basket
• You have a smaller car with limited roof space
• You want a 200 lbs load capacity for camping gear

Skip if

• You require a hard-shell box for maximum security
• You need more than 15 cubic feet of storage

Verdict: HubPax 15 Cu Ft Roof Basket & Bag provides a practical two-in-one solution for drivers who already have cross bars. It fits buyers who want to protect items in a basket with a waterproof bag rather than buying a separate hard box.

5

Thule Pulse 2 L 16 cu ft Dual-Side Box

Best for: dual-side access and durable construction

Size

Large (what this guide is about)

Capacity

16 cu ft (450 L)

Shell Type

ASA-ABS plastic

Opening

Dual-side

Load Capacity

75 kg

Water Resistance

Weather-resistant

View on Amazon: Thule Pulse 2 L 16 cu ft Dual-Side Box Affiliate link

This Thule box offers 16 cubic feet of space and a 75 kg load capacity. Its dual-side opening lets you access gear from either side of your vehicle. The streamlined 81 x 17 x 37 inch shell is made from durable ASA-ABS plastic. It uses a tool-free installation system with a torque indicator. This design suits commuters and weekend travelers who need secure, easy access to luggage or skis.

Pros

• 16 cubic feet of cargo space
• 75 kg load capacity
• Dual-side opening for easy access
• Tool-free installation with torque indicator
• ASA-ABS plastic shell resists scratches

Cons

• 16 cubic feet is smaller than the 21 cubic feet option
• 75 kg capacity is lower than the 500 lbs option
• Fixed 37 inch height limits compatibility with some roof racks

Good fit if

• You need side access to your cargo
• You carry skis or snowboards
• You prefer a streamlined, durable shell

Skip if

• You require over 500 lbs capacity
• You need more than 16 cubic feet
• You want a multi-size flexible system

Verdict: Thule Pulse 2 L 16 Cu Ft Dual-Side provides reliable dual-side access and a durable shell for users who prioritize convenience and material strength over maximum volume or weight limits.

6

SportRack Sidekick Cargo Box

Best for: small loads and compact roof racks

Size

One Size (what this guide is about)

Capacity

8 cubic foot

Opening

passenger-side opening

Load Capacity

75 pounds

Security

lock to secure contents

Dimensions

54 x 25 x 15.5 inches

View on Amazon: SportRack Sidekick Cargo Box Affiliate link

The SportRack Sidekick offers 8 cubic feet of cargo space with a 75-pound load capacity. Its compact 54-inch length and 25-inch width leave room for other items on your roof rack. The passenger-side opening simplifies loading and unloading. It includes a lock for security and mounts securely with U-bolts. This box suits drivers with small loads or limited roof space.

Pros

• 8 cubic foot capacity for small gear
• Passenger-side opening for easier access
• Includes lock for security
• Compact 54-inch length saves roof space

Cons

• Smaller 8 cubic foot capacity compared to 15-21 cubic foot competitors
• Lower 75-pound load limit restricts heavy items
• Single-side opening limits access from driver side

Good fit if

• You need a compact box for small loads
• You prioritize easy passenger-side access
• You want to save roof rack space

Skip if

• You require more than 8 cubic feet of space
• You need to carry heavy loads over 75 pounds
• You prefer dual-side opening for driver access

Verdict: SportRack Sidekick 8 Cu Ft Roof Box fits drivers with small loads who need a compact, secure, and easy-to-access option. It is not suitable for large cargo or heavy items.

Specifications side by side

Frequently asked questions