How to Calculate the Correct Dunnage Bag Size and PSI for Ocean Freight

Ocean freight exposes cargo to constant movement, pressure changes, and challenging conditions that can lead to costly damage. Properly sized dunnage bags help stabilize loads, fill empty spaces, and keep goods secure throughout the journey. This overview explains the key factors to consider when selecting the right dunnage bag for your shipment.

Introduction

Ocean freight puts your cargo through severe stress. Unlike over-the-road transport, ocean vessels subject containers to six distinct dynamic motions: pitch, roll, heave, sway, surge, and yaw. The secret to damage-free delivery lies in securing the void spaces correctly so your goods cannot shift. Choosing the correct dunnage bag isn't just about grabbing any inflatable sack; it demands precise calculations of the gap size and the correct internal working pressure (PSI) to withstand weeks of turbulence.

In this comprehensive guide, we break down how to calculate the optimal size and PSI for your dunnage bags when shipping overseas, ensuring compliance with international cargo securing codes.

Measuring the Void Space Correctly

Before selecting a bag, you must know the exact dimensions of the empty space (the void) between your cargo pallets or between the cargo and the container walls.

• Width: Measure the gap at its widest point.
• Height: Measure the distance from the floor to the top of the load.
• Depth: Measure the length of the pallets running from the doors to the nose of the container.

Your bag’s height should roughly match the height of your cargo load to prevent cargo movement effectively and avoid top-heavy shifting. Leaving the top of a pallet unsupported allows the cargo to tilt, which often leads to catastrophic failure during a heavy ship roll. When measuring, also inspect the edges of the cargo—sharp wooden pallets will require corrugated edge protectors to prevent punctures.

Calculating the Right Dunnage Bag Size

The most critical rule in load securing: The dunnage bag must be wider than the void it is filling. If the bag is too small, it will bulge excessively, rounding out into a cylinder. A rounded bag loses surface contact with the cargo, which severely reduces its bracing power and friction grip.

A standard industry formula for finding the right bag width based on the void is:

(Void size × 2) + a slight buffer.

Standard Size Chart for Ocean Freight Voids

To take the guesswork out of your packaging, use this standard sizing matrix for optimal efficient space utilization:

Never try to stretch a small bag across a large gap, and never place two dunnage bags side-by-side to fill one large void—they will rub together and pop under pressure. These are common industrial packaging mistakes that cost businesses thousands in damaged goods.

Determining the Right PSI (Pressure) for Ocean Freight

PSI (Pounds per Square Inch) determines the bag’s bursting pressure and holding force. Ocean transit involves severe temperature fluctuations; cold sea air can cause the pressure inside the bag to drop, so calculating the correct starting PSI is critical. The Association of American Railroads (AAR) rating system classifies bags based on construction and pressure ratings.

• Level 0 & 1 (2.0 to 3.0 Working PSI): Suitable for lightweight truck loads (up to 45,000 lbs). Not robust enough for rough sea motion.
• Level 2 (Up to 5.0 Working PSI): Operates at higher pressures with burst pressures up to 17 PSI. Good for demanding transport and standard intermodal containers.
• Level 3 (Up to 8.0 Working PSI): The standard starting point for heavy ocean freight (loads between 75,000 and 160,000 lbs).
• Level 4 & 5 (Up to 10.0 Working PSI): Extreme heavy-duty applications (like paper rolls, steel coils, or heavy machinery, up to 240,000 lbs).

For ocean freight, Level 2 to Level 4 polywoven bags are heavily recommended depending on cargo weight. They handle the immense multi-directional forces of a cargo ship on open water.

The Impact of Materials: Why Polywoven is Best for Sea Transit

While calculations matter, the fabric of the bag is equally vital. Standard paper bags can absorb ocean humidity, causing the paper to weaken and tear over a long voyage.

Polywoven dunnage bags feature a multi-layer PA+PE (polyamide + polyethylene) inner bladder combined with a tough outer woven polypropylene shell. This layered construction provides exceptional elasticity, puncture resistance, and airtight inflation. Unlike traditional paper alternatives that lose structural integrity when exposed to sea moisture, woven polypropylene is waterproof and tested to endure temperatures from -20°C to 60°C.

Secure Your Next Shipment with Precision

Selecting the correct size and pressure guarantees that your cargo arrives exactly how you loaded it. By eliminating void spaces, you drastically lower the chances of transit damage, insurance claims, and delays, ultimately preventing product damage in storage & transit.

Designed for extreme durability, Bubna Polysack's Dunnage Bags offer unmatched moisture resistance, immense elasticity, and precise valving for secure, long-lasting inflation.

FAQ's

1. What happens if a dunnage bag is too small for the void?

Ans. If the bag is too small, it will over-expand to reach the pallets, turning into a round cylinder shape. A rounded bag has minimal surface contact with the cargo, severely reducing its frictional grip and its ability to absorb shocks.

2. Can I place two dunnage bags side by side to fill a large void?

Ans. No, placing two dunnage bags side-by-side or stacking them on top of one another creates an unstable contact point. As the cargo shifts, the bags will rub against each other, causing friction that can lead to popping and total failure.

3. What is the recommended PSI for ocean shipping?

Ans. For standard ocean freight, Level 2 bags (up to 5.0 working PSI) or Level 3/4 bags (up to 10.0 PSI) are recommended depending on load weight. Sea transit subjects containers to heavy, multi-directional forces like pitch and roll, requiring higher bursting strength than over-the-road transport.

4. How does temperature affect dunnage bag pressure during sea transit?

Ans. Air expands in heat and contracts in the cold. If a bag is inflated in a hot environment and ships to a cold climate, the internal PSI will naturally drop, potentially loosening the cargo block. A high-quality polywoven bag with a 30mm leak-proof valve maintains better pressure stability.

5. Are dunnage bags reusable for multiple international shipments?

Ans. Yes, high-quality polywoven dunnage bags are designed for multiple uses. They can be safely deflated via their built-in valves, inspected for wear and tear, and reused, making them a cost-effective load-securing option.