In short: using a dunnage bag in a container takes six steps: measure the void after loading, pick a bag size that matches it, shield the bag from sharp cargo edges, place the uninflated bag upright in the gap, inflate it to the rated working pressure, and verify the brace is firm. Done right, the whole job takes a few minutes per container — and it is the difference between cargo that arrives intact and cargo that shifts, topples or collapses in transit.
Step 1: Measure the Void After Loading
Never size a dunnage bag from the container dimensions alone. Once cargo sits in its final position, measure the void width at several points along the container length — voids are rarely even, and the widest measurement decides. Also note the cargo stacking height: the bag should cover most of that height. If the void sits between two standard sizes, take the larger bag and run it at a moderate pressure rather than stretching a small bag.
Step 2: Pick the Right Bag Size and Material
Match the bag to the measured void using the size chart (see How to Choose Dunnage Bag Sizes), then let cargo weight and transport mode decide the pressure rating and material: heavier cargo and rail/intermodal legs call for higher-rated, thicker material; standard pallet cargo on ocean routes works fine with standard specifications. Our 5-step selection workflow walks through the full decision process.
Step 3: Shield the Bag From Sharp Edges
Before the bag goes in, scan the void for pallet nails, sharp carton corners, protruding bolts and rough deck boards. Cover or remove them — a dunnage bag is tough but not knife-proof, and most punctures in the field come from cargo edges, not from the cargo's weight. A sheet of cardboard between a rough surface and the bag is cheap insurance.
Step 4: Place the Uninflated Bag Upright
Unfold the bag and slide it flat into the void, positioned upright so it will cover the cargo's contact height once inflated. The usual placements are mid-container between two loaded stacks, and at the door end between the last stack and the doors. Make sure the bag spans the full width of the void — a bag that only covers part of the gap braces only part of the load. Keep the valve accessible; you will connect the inflation gun in the next step.
Step 5: Inflate to the Rated Working Pressure
Connect the inflation gun to the valve and inflate steadily. Any standard workshop air compressor or air pump works — large-valve bags inflate noticeably faster, which pays off when a dock crew secures many containers a day. Watch the bag expand into the void; inflate to the rated working pressure printed on the product, and stop there. Heavier cargo needs a higher-rated bag, not more pressure on a low-rated one — over-inflation is the fastest way to burst a bag.
Step 6: Verify the Brace Before Closing the Doors
Walk the brace before sealing the container: push the bag by hand — it should feel firm with no loose play; check that no sharp edge has found the bag during inflation; confirm the valve sealed when you removed the gun. On long ocean voyages, ask the receiving end to note any visible bag deflation when doors open — it flags handling issues early. More on safe pressures in Dunnage Bag Inflation Pressure: What's Right.
Common Mistakes to Avoid
- Guessing the void size instead of measuring after loading — the single most common reason bags fail to brace.
- Inflating past the rated pressure to compensate for a bag that is too small — it bursts the bag instead.
- Placing the bag against exposed nails or sharp edges — punctures come from cargo contact, not cargo weight.
- Using one bag across a void wider than about 50 cm — use two bags or add lashing straps instead.
Frequently Asked Questions
Where exactly should I place the dunnage bag in a container?
In the void between two loaded cargo stacks (or between cargo and the container wall) where shifting is most likely — typically at mid-container and near the door end. The bag must sit upright, cover the cargo's contact height, and span the full width of the void.
What pressure should I inflate a dunnage bag to?
Inflate to the bag's rated working pressure printed on the product — never above it. Under-inflation won't brace the cargo; over-inflation risks bursting. Heavier cargo calls for higher-rated bags, not higher pressure on a low-rated bag.
Do I need special equipment to inflate a dunnage bag?
No special equipment — a standard workshop air compressor or air pump plus an inflation gun that matches the valve size is enough. Large-valve bags inflate faster and suit operations loading many containers per day.
Can I use a dunnage bag between pallets?
Yes, voids between pallets are a classic dunnage bag application. Choose a bag height that covers most of the pallet stack height, and protect the bag from exposed pallet nails or sharp deck boards before inflating.
What if the void is bigger than 50 cm?
Standard dunnage bags cover voids up to about 50 cm. For wider voids, use two bags side by side or combine with lashing straps to lock cargo lengthwise. Never stretch one bag across a void it wasn't sized for.
Related reading: start with what a dunnage bag is and which types exist, then fine-tune size choice with the 5-step selection workflow.