How to Choose Dunnage Bags for Freight Loads

A dunnage bag only works when it matches the gap, the load and the journey. Choosing an undersized bag or the wrong strength level can leave pallets free to shift, while an oversized bag may not inflate correctly or deliver even pressure across the load. Knowing how to choose dunnage bags starts with assessing the cargo space, not simply ordering the largest bag available.

For Australian freight operators, warehouse teams and exporters, the right bag can reduce movement between palletised goods in containers, rail wagons and enclosed road freight. It is one part of a complete load restraint approach, used alongside sound palletisation, blocking, bracing, lashing and suitable packaging.

Start with the transport mode and journey

The forces acting on freight vary considerably between a local road run and an international sea shipment. Road freight is exposed to braking, acceleration and cornering. Rail freight can create sustained vibration and shunting forces. Sea containers may experience rolling, pitching and extended periods of vibration, particularly when cargo is moving through several handling points.

Select a dunnage bag rated for the transport environment it will face. Light-duty bags may suit controlled, low-risk loads with short transport distances, but they are not a substitute for higher-strength products in export containers or demanding freight lanes. Where a load is travelling by more than one mode, choose for the most demanding part of the journey.

Also consider how long the cargo will remain packed. A container that is loaded, moved to port, shipped offshore, unpacked and transferred again can be exposed to movement for weeks. The restraint method needs to remain effective for that full period, not just while the container is being loaded.

Measure the void before choosing a dunnage bag

Dunnage bags are designed to fill voids between stable cargo units. They do not correct poor pallet stacking, damaged cartons or uneven loads. Before selecting a size, measure the width and height of the gap where the bag will sit, then check the available contact area on either side.

A properly sized bag should fill the void at its recommended inflation pressure without being stretched excessively. It should make broad, even contact with the load. If the bag is too narrow, it may press into a small area and provide limited restraint. If it is too wide for the void, it can fold, pinch or inflate unevenly, reducing its effectiveness.

Bag length matters as well. The bag should cover enough of the load face to distribute force, but it must not extend into spaces where it can be damaged by pallet corners, protruding product or container fittings. In many palletised loads, positioning the bag around the centre of the load face provides the most useful support, though the final position depends on the cargo's centre of gravity and the shape of the void.

Check whether the gap is suitable

Very large gaps often need more than a dunnage bag. Blocking, timber bracing, load bars or other engineered restraint may be required before a bag is added. Likewise, a narrow gap may not provide enough room for a bag to inflate properly.

The load faces must also be capable of taking the pressure. Soft cartons, unstable drums, irregular machinery and stacked goods with weak outer packaging can deform under inflation. Use edge protection, slip sheets or load spreaders where needed, and do not inflate against sharp corners without suitable protection.

Match bag construction to the application

Dunnage bags are available in different materials and performance levels. Paper or kraft outer bags are commonly used for dry freight applications where a cost-effective void-fill solution is suitable. Woven polypropylene bags offer improved resistance to moisture and handling damage, making them a practical option for many container and transport applications. Higher-duty bags may use stronger outer construction and internal liners for demanding export or industrial loads.

The right choice depends on more than bag material. Check the manufacturer's rated application, maximum working pressure, recommended inflation range and any transport or industry performance classification. A burst pressure alone is not enough to determine suitability. The relevant figure is the pressure the bag can safely hold in the intended application, with an appropriate margin for the load and journey.

For export freight, confirm that the selected bag is suitable for container use and aligns with the cargo restraint requirements of the carrier, consignee and destination market. Some operations work to specific rail or maritime requirements, while others have site standards based on previous damage experience. Procurement should compare those requirements against the product specification before standardising a bag across a freight program.

Consider the cargo, not just the pallet count

Two containers with the same number of pallets may need very different restraint methods. Dense, rigid pallet loads usually provide a better contact surface for a dunnage bag than lightweight cartons or irregular product. A load of bagged agricultural material, for example, can settle during transport. Machinery components may have hard edges and uneven profiles. Drums can move independently unless they are secured as a unit.

Assess the cargo's weight, shape, packaging strength and ability to settle. Heavier cargo creates greater force when it shifts. Higher centres of gravity increase the risk of tipping. Products that are sensitive to crushing may need load spreaders or another restraint method rather than direct dunnage bag pressure.

A dunnage bag should restrain movement between loads, not carry the full weight of a cargo block. It is not designed to replace proper pallet condition, shrink wrap, strapping or blocking. If goods can move on the pallet, resolve that issue before using a bag to secure the space between pallets.

Select the right valve and inflation method

The valve affects loading speed, pressure control and repeatability. For occasional use, a simple manual inflation arrangement may be adequate. For regular container loading, a compatible inflation tool can improve consistency and reduce time at the loading bay.

Use clean, dry compressed air and follow the stated inflation pressure for the bag. Overinflation can damage the bag, crush packaging or create excessive pressure against container walls. Underinflation leaves the load unsupported and allows movement to build during transit.

Teams should use a pressure gauge where the bag specification calls for one. Estimating pressure by feel is not reliable, particularly when operators are working across different bag sizes and load types. Ensure valve caps are fitted after inflation and that the bag is protected from forklift tynes, pallet nails and sharp packaging during final loading.

Review the full load plan

Dunnage bags perform best when they are included in the load plan from the start. Consider where voids will occur as pallets are loaded, how access will be maintained for inflation, and whether the final row of cargo needs separate blocking or bracing. A bag cannot be positioned correctly if there is no access to the void after loading.

Before dispatch, inspect the completed load. Check that pallets are stable, bags are centred in the intended voids, valves are accessible but protected, and no bag is rubbing against a sharp edge. Confirm that the restraint system accounts for forward, rearward and lateral movement appropriate to the transport method.

For repeated freight lanes, record the container layout, bag size, bag type and inflation procedure that worked. This creates a practical reference for warehouse staff and helps purchasing teams order the right product in volume. It also makes it easier to investigate and correct any damage pattern that appears after delivery.

When to use a higher-duty bag

A higher-duty dunnage bag is generally justified where loads are heavy, travel long distances, move by rail or sea, or have a higher consequence if they shift. Export containers, mixed pallets, dense industrial products and freight with unavoidable voids are common examples.

There is a cost trade-off. A lighter bag may have a lower unit cost, but it can be false economy if it is unsuitable for the route or needs additional labour to manage. Conversely, specifying the highest performance level for every local load may add cost without improving outcomes. The useful comparison is total freight risk: product value, likelihood of movement, transport duration, damage cost and loading time.

The best dunnage bag selection is the one that fits the actual void, holds the expected load forces and can be used consistently by the people packing the freight. Where the application is high value, unfamiliar or repeated at volume, trial the selected size and construction on a representative load before making it the standard.