Dunnage Bag Technical Metrics: AAR, Void and Pressure

Reviewed and updated by ICD - Week 44/2026

Five technical figures must be read on a dunnage bag data sheet: the load level (Level), working pressure and burst pressure, flat bag size together with maximum gap, shell and liner material, and valve type. For example, a Level 1 Kraft paper bag in the ICD Vietnam catalogue works at 0.2 bar (2.9 PSI), bursts at 9.1 PSI, and an 80 x 120 cm bag is for a maximum gap of 40 cm.

A dunnage bag is an air cushion placed in the gap between cargo units in a container, truck or rail car and inflated to press the load tight. I am Le Van Thang, and I have worked in packaging supplies at ICD Vietnam since 2011. What customers ask me most is not the price but “what do these numbers mean”: why is the inflation pressure so much lower than the burst pressure, what separates Level 1 from Level 3, why is a 100 x 180 cm bag only for a 60 cm gap. This article goes through each figure in turn, with the CTU Code method for calculating holding capacity so you can check for yourself whether a bag is enough to hold the cargo.

Which figures are on a dunnage bag data sheet?

A full data sheet has about ten lines, but only five groups decide whether the bag holds the cargo. The table below copies the lines in the ICD Vietnam Kraft paper and woven PP bag catalogue, with how to read each one.

The line most often misread is “air source pressure 2.0 to 3.5 kg”. This is the compressed-air pressure from the compressor into the gun, far higher than the level needed in the bag. If someone takes this number as the bag inflation pressure, a Level 1 paper bag will burst, because its burst pressure is only about 0.63 bar. A gun with a gauge and a pressure-reducing valve is exactly what brings the source pressure down to the working level.

What does the AAR strength level (Level) tell you?

The AAR level is the classification scale for air bags from the Association of American Railroads, set out in General Information Bulletin No. 9 on dunnage bags. The scale is for cargo in rail cars and intermodal cargo on North American railroads, where cars strike each other during shunting. Each level is tied to a type of gap and the total weight of cargo in the car, with a test pressure.

Many sources call AAR a mandatory standard for every mode of transport; that is too broad. AAR is mandatory for cargo moving by North American rail. For sea and road cargo, the common basis is the CTU Code of the IMO, ILO and UNECE, which calculates the holding capacity of the bag against the force the load applies (see section 4). AAR certification is still useful in that it shows the bag has been tested to a public standard.

Reading this table across, a pattern appears: at every level, the burst pressure is about three times the working pressure. That margin is for two things: the impulse when the container is struck, and the extra pressure when the container heats up. So choosing a higher level does not mean inflating higher to be safe; each level has its own inflation level, and going over it eats into the safety margin.

How do you read the standard void and the maximum gap (Max Void)?

Each bag size has a maximum gap set by the manufacturer, and the CTU Code requires following those instructions. The reason is geometry: a flat bag 80 cm wide is only about 50 to 55 cm wide once inflated, because the fabric or paper has to “give up” width to swell. The wider the gap, the rounder the bag swells, and the smaller the flat face against the cargo.

Table of bag sizes, width after inflation and maximum gap

The table above is taken from the ICD Vietnam Kraft paper air bag product page. The length also shrinks when inflated: an 80 x 120 cm bag is about 105 to 110 cm long, and a 120 x 240 cm bag about 220 to 225 cm. When measuring the gap, use the height after inflation to make sure the bag is not higher than the top edge of the cargo.

The standard void

A standard void is a gap of even width from top to bottom that lies within the maximum gap of the bag size you use. In such a gap the bag swells evenly and presses the largest flat area against the cargo, so the force is spread widely. The easiest way to get a standard void is to plan it from the loading plan: arrange the rows of units so the gap is gathered in one or two places instead of many small gaps spread through the container.

Non-standard voids and how to handle them

A non-standard void is a gap wider than the maximum gap, narrower than the bag can swell into, or an uneven, irregular gap. A gap that is too wide should be narrowed with rigid filler such as empty pallets, thick carton or honeycomb board before the bag is placed; do not stack several bags to fill it, because the place where bags meet is a weak point. The Union Pacific railroad loading guide does the same: a standard bag for gaps of 10 to 30 cm, with a square bag for gaps of 30 to 60 cm and honeycomb board on both sides.

An uneven gap, such as when blocking machinery with many protruding parts, needs a flat surface made with plywood or stiff carton on both sides of the bag. This padding both bridges the hollow so the bag presses evenly on the load and covers sharp edges. The CTU Code says exactly this: rough cargo surfaces must be smoothed before the bag is placed. In my experience, a Kraft paper bag touching sharp metal parts directly with no padding is very easily cut while the ship is moving.

How do you calculate the holding capacity of a dunnage bag under the CTU Code?

Under the CTU Code, the holding capacity of an air bag equals the burst pressure times the contact area times a safety factor: 0.75 for single-use bags and 0.5 for reusable bags. The contact area is smaller than the bag size because the bag edges bulge round, and is calculated as (width minus 1.57 times the gap) times (height minus 1.57 times the gap). This formula shows clearly why a wide gap weakens the bag.

I calculated a few cases myself by that formula, using a Level 1 Kraft paper bag with a burst pressure of 9.1 PSI (about 63 kPa) and a factor of 0.75. The results are rounded and only illustrate how to read it; they do not replace the calculation for each shipment.

With the same 80 x 120 cm bag, widening the gap from 10 to 30 cm cuts the holding capacity to about one third, even though a 30 cm gap is still within the 40 cm maximum gap. Switching to a 100 x 180 cm bag for the 30 cm gap brings the holding capacity back to about 33 kN. That is why I advise choosing the bag size by the actual gap, not by the habit of one size for every container.

The holding capacity must exceed the force the cargo applies on the bag. That force equals the mass of the row of cargo behind the bag times the vehicle acceleration, less the friction with the floor. The CTU Code gives these design accelerations: road 0.8 g forward, 0.5 g rearward and sideways; rail 0.5 g (1.0 g for short impacts under 150 ms); sea sideways 0.5 g in area A and 0.7 to 0.8 g in areas B and C. For example, a 4-tonne row of cargo on the road, taking 0.5 g sideways, needs about 20 kN before friction; an 80 x 120 cm bag in a 30 cm gap (about 11 kN) is not enough, while in a 10 cm gap it is.

The kinds of force acting on a dunnage bag in a container

These four kinds of force do not act separately but add up over the whole journey. Prolonged vibration does not burst the bag at once but wears the bag shell where it touches the cargo edges, and then a hard brake later tears the bag. That is why the strength figure must be read together with the shell material, not on its own.

How do static and dynamic compression force differ?

Static compression force is the steady pressure the bag puts on the cargo when the container stands still or runs smoothly; dynamic compression force is the sudden, strong impulse when the vehicle is struck, brakes or sways. The bag must hold the static force for many weeks, and absorb the dynamic force in a moment. These two jobs are done by two different layers of the bag.

Static compression force and the role of the inner bag

Static compression force comes from the pressure in the bag pushing on the two cargo faces, plus the pressure of the surrounding blocks of cargo. To keep this force over a journey of many days, the bag must be airtight: a PA film liner laminated with PE and a tightly closing valve are the two main factors. PA makes the film tough and less gas-permeable than plain PE, so a bag with a PA liner holds pressure longer. A bag that gradually loses pressure through a leak at the valve or the film loses its static pressing force, the cargo has room to build speed and every impact is harder than the last.

The valve: a figure few people read

The ICD Vietnam catalogue lists two valve types: a standard valve of 10 mm and a quick-fill valve of 30 mm, with inflation time for the two in a ratio of about 3 to 1. The large valve shortens the inflation time per container but needs the right type of gun; a gun that does not fit the valve easily pierces the valve film and causes leaks. When ordering, state the valve type clearly so the warehouse prepares the matching gun.

Dynamic compression force and the role of the bag shell

When an impulse arrives, the bag yields part of the way to absorb it and then swells back, like a shock absorber. At that moment the outer shell is under the greatest tension, especially where it touches the cargo edges. A plastic-laminated Kraft paper shell suits cartons and palletised cargo with padding; a woven PP shell resists cutting and abrasion better, can be used many times, and has a range of levels from Level 0 to Level 5 in the catalogue.

It is not the case that heavy cargo can only use woven PP; the key point is that in the ICD Vietnam catalogue the Kraft paper bag currently comes only in Level 1, so for a higher level you choose a woven PP bag. For high-value or fragile cargo, my advice is to calculate the holding capacity by section 4 for the hardest route of the shipment, and then choose the level and bag size from that result, instead of choosing by feel.

How does temperature affect dunnage bag pressure?

When the air in the bag warms up, the pressure rises. I calculated by the gas law, assuming the bag does not expand further: a bag inflated to 0.2 bar at 20 degrees C reaches about 0.32 bar when the container heats to 50 degrees C and about 0.37 bar at 60 degrees C. In reality the bag swells a little more so the rise is lower, but that is the trend.

For a Level 1 paper bag with a burst pressure of about 0.63 bar, 0.37 bar is still far from the bursting point. If the packer over-inflates to 0.4 bar to be safe, the same container heating to 60 degrees C would push the pressure to about 0.6 bar, nearly touching the burst threshold. That is why each level should be inflated only to its own working level; the margin between working pressure and burst pressure is kept for temperature and impact.

Frequently asked questions about dunnage bag technical figures

How do working pressure and burst pressure of a dunnage bag differ?

Working pressure is the recommended inflation level when the bag is placed in the gap; burst pressure is the level at which the bag fails in testing. According to the ICD Vietnam catalogue, a Level 1 Kraft paper bag works at 0.2 bar (2.9 PSI) and bursts at 9.1 PSI, about three times higher. That margin is for impact and temperature, not for inflating further.

Which AAR level of dunnage bag suits sea cargo?

AAR is the North American railroad scale and sets no minimum level for sea cargo. For sea transport, calculate the holding capacity under the CTU Code: burst pressure times contact area times a safety factor (0.75 for single-use bags, 0.5 for reusable bags), and compare it with the force the row of cargo applies on the bag.

What maximum gap is a 100 x 180 cm dunnage bag used for?

According to the ICD Vietnam Kraft paper air bag product page, a 100 x 180 cm bag is about 70 to 75 cm wide once inflated and is used for a maximum gap of 60 cm. An 80 x 120 cm bag is for a maximum gap of 40 cm and a 120 x 240 cm bag for a maximum gap of 70 cm.

Is the 2.0 to 3.5 kg figure on the data sheet the bag inflation pressure?

No. That is the compressed-air pressure the compressor supplies to the gun. The pressure in the bag must follow the working level of each bag level, for example 0.2 bar for a Level 1 paper bag. A gun with a gauge helps stop at this level.

Why does a wider gap make the bag hold the cargo worse?

Because the bag swells rounder and the flat face against the cargo shrinks. By the CTU Code method, each bag dimension is reduced by 1.57 times the gap width. I calculated: an 80 x 120 cm bag in a 10 cm gap has a holding capacity of about 32 kN, and in a 30 cm gap only about 11 kN.

How do PA and PE liners of a dunnage bag differ?

The liner is the air-holding film inside the shell. A PA film laminated with PE is tougher and less gas-permeable than plain PE film, so the bag holds pressure longer on a long journey. The ICD Vietnam catalogue lists a PA and PE liner for both Kraft paper bags and woven PP bags.

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References

• Technical figures of the dunnage bag - International Safe Transit Association (ISTA).
• Association of American Railroads (AAR) - General Information Bulletin No. 9 on dunnage bags.
• IMO/ILO/UNECE Code of Practice for Packing of Cargo Transport Units (CTU Code), MSC.1/Circ.1497 - design accelerations, Annex 7 section 2.3 (air bags), Appendix 4 (safety factor, contact area).
• Union Pacific - Air Bag Placement and Usage Guidelines.
• ICD Vietnam dunnage bag catalogue (level table, Kraft paper and woven PP bag specifications, 10 mm and 30 mm valves) and the maximum gap table on the ICD Vietnam Kraft paper air bag product page, consulted October 2026.