UN Rating for Drums: Understanding Types, Overpacks, and Safety

Key Takeaways

• UN rated drums are tested to meet strict standards for transporting hazardous materials, including drop, pressure, and vibration resistance based on the contents and hazard level.
• Choosing between an open head vs closed head drum depends on the material’s physical form, handling method, and required closure.
• Using an overpack container is required when drums are compromised, combined, or covered during transport, and the overpack must meet labeling and performance requirements.
• Common compliance failures include improper closures, using incorrect or unmarked packaging, and reusing drums without retesting—all of which can lead to fines, shipment rejection, or safety hazards.

What Is a UN Rated Drum?

A UN rated drum is a container that meets specific performance standards for holding hazardous materials during transport or storage. These standards come from the United Nations and are enforced in the U.S. through DOT regulations. Each drum is tested for impact, pressure, weight tolerance, and leak resistance to match it with the right type of hazardous material.

This testing exists for a reason. According to the Federal Highway Administration, accidents account for approximately 2% of hazmat transport incidents, but they cause 85% of the reported property damage. A large portion of those cases involve packaging failures that could have been prevented with proper UN rated containers.

While working on internal guidance related to flammable liquids, I came across an issue with drum ratings. A few units had been loaded using plastic drums that were rated for solids only—not liquids. If that load had made it onto the truck, it could’ve turned into a DOT violation or a spill if the liquid had shifted too much.

When to Use a UN Rating Container

After seeing firsthand how easily a misstep in container selection could have led to a spill, I started paying closer attention to which drums were used for what. If a drum looks sturdy but lacks the correct mark, it’s easy to miss—but the DOT won’t miss it during an inspection. Once you understand what the UN rating for drums actually stands for, it becomes clear why certain situations demand it.

You are required to use UN rated containers in the following cases:

• Transport by air, sea, rail, or highway: Any regulated hazmat must be placed in tested, marked containers. Without the right rating, you could be violating CFR Title 49 even if the material never leaves the pallet.
• On-site storage before disposal: Many states require even short-term, on-site storage of hazardous waste to use containers that meet DOT specifications—especially for flammables and corrosives.

That said, there are plenty of cases where it's not legally required, but it's still worth using a UN rated drum. I've seen storage rooms filled with aging chemicals—some in dusty paint cans, some in generic barrels. No leaks yet, but every time I see that setup, I start a conversation about risk and replacement.

Container type directly affects how well a drum matches the contents inside. If you're working with liquids, a closed head drum is designed to handle pressure changes without leaking. For solids, powders, or thick waste, an open head drum gives you easier access and safer closure.

I’ve found it helps to simplify the decision by focusing on how the container lines up with the material’s needs. Here are a few quick checks that will point you in the right direction:

• Is it pressurized, flammable, or reactive? Use a drum that’s rated for liquids and sealed tight.
• Is it a mix of old substances, heavy sludge, or dry goods? Go with an open head drum that’s been rated for solids and is easy to re-close.
• Are you unsure what’s inside? Don’t take chances. Treat it like it needs a full UN rated container.

Understanding UN Rating Codes

Reading the code on a UN rated drum isn’t complicated once you know what each part means. The code tells you exactly what the container is made of, what it’s rated to carry, and what level of hazard it’s approved for. If you’re responsible for handling or selecting drums, knowing how to read this code is the only way to confirm that a container meets the job requirements.

A typical UN marking looks like this: 1A1/X1.8/300/22/USA/M4567

Here’s what each part of that code means:

1. Container Type Code (1A1)

This part tells you what kind of container it is, what it’s made from, and whether it’s an open head drum or a closed head drum.

2. Performance Level and Specific Gravity or Mass (X1.8 or Y25)

This section shows the drum’s hazard level rating and the maximum load it can carry, depending on whether it's for liquids or solids.

• Hazard level (Packing Group compatibility)
• X = Suitable for Packing Groups I, II, and III (high, medium, low danger)
• Y = Suitable for Packing Groups II and III
• Z = Suitable for Packing Group III only

• For liquids: You'll see a number like 1.8, which means the container can hold liquids with a specific gravity up to 1.8
• For solids: You'll see a number like 25, meaning the container is approved for a gross mass of 25 kg

3. Pressure Test or Solids Designation (300 or S)

• For liquids: A number like 300 indicates the drum passed a hydrostatic pressure test at 300 kilopascals (kPa)
• For solids: The letter S stands for “Solids,” meaning no pressure test was done

4. Manufacture Year (22)

This is the last two digits of the year the drum was manufactured or certified (22 = Year 2022).

5. Country Code (USA)

Indicates the country where the drum was tested and certified. USA is common, but others like CAN, DEU, or CHN may appear depending on origin.

6. Manufacturer or Certifier ID (M4567)

This alphanumeric packaging identification code identifies the company or facility that produced or certified the drum. It’s traceable and required for recordkeeping.

Solid vs. Liquid Format: Know the Difference

UN ratings for drums are formatted differently depending on the contents. This distinction is critical when selecting or reusing a container.

Each part of this code carries weight in day-to-day operations. When selecting a drum for shipping hazardous materials, reviewing the full UN code is just as important as checking the label. It's the one clear signal that the container was built and tested for the material inside.

How Is Rating Calculated?

The UN rating for drums isn’t assigned arbitrarily. Each container goes through a series of performance tests based on what it’s designed to hold—solids or liquids—and the hazard level of the material. These tests are carried out according to protocols set in 49 CFR, Part 178.

Manufacturers submit containers for testing under worst-case scenarios. Ratings are based on the highest level the drum passes consistently, not the maximum it can survive once. This keeps the markings reliable for real-world use when shipping hazardous materials.

Here are the core tests that apply to UN rated containers:

Drop Test

This test simulates the impact of a fall during handling or transit. The drum is dropped in multiple orientations—on its corner, flat side, and flat end—onto a hard surface. The required drop height depends on the Packing Group:

• X (Packing Group I): 1.8 meters
• Y (Packing Group II): 1.2 meters
• Z (Packing Group III): 0.8 meters

Hydrostatic Pressure Test

This applies only to drums rated for liquids. The container is filled with water or another non-compressible fluid and pressurized internally to the kPa level stated on the drum’s marking. It must hold that pressure for five continuous minutes without leaking or deforming.

Leakproofness Test

Also specific to liquid drums, this test uses air pressure and either submersion or leak detection fluid to check for any air escaping. Any sign of bubbling or loss of pressure is considered a failure, even if it's minor.

Stack Compression Test

Both liquid and solid drums must withstand vertical pressure that mimics warehouse stacking or freight piling during long-haul shipping. The drum is loaded with weight and left under compression for at least 24 hours. Permanent deformation or structural failure during this time disqualifies the drum from certification.

Vibration Test

This test is mandatory for all UN rated containers. The drum is placed on a vibrating platform to simulate truck or rail movement. A flat metal plate must be able to slide underneath the drum freely throughout the test, showing it’s not bouncing or sticking due to poor stability.

Once a drum passes all required tests, the results determine its full UN marking—pressure tolerance, specific gravity (for liquids), or max gross mass (for solids), along with the allowed Packing Group. If the drum fails any part, it cannot be legally marked for use in hazardous materials transport. Once a design is certified, any change in material thickness, closure hardware, or even the shape of the drum means it must be re-tested before it can be sold as a UN rated drum again.

Open Head vs Closed Head Drum and Their UN Ratings

There are two main drum styles used in packaging hazardous goods: the open head drum and the closed head drum. Each is designed for specific types of materials and comes with its own testing requirements under the UN rating for drums system.

Closed Head Drum

A closed head drum (also called a tight head) has a permanently attached top with two threaded openings, usually a 2-inch bung and a 3/4-inch vent. These drums are designed for liquids, especially those that are flammable or reactive.

They are pressure-tested and must pass leakproofness testing, making them suitable for materials that need a sealed, secure environment during transport. If you're moving solvents, fuels, or corrosive liquids, this is the style that's most likely required.

Open Head Drum

An open head drum has a removable lid that's secured with a clamp or bolt ring. It's easier to fill and empty, which makes it better suited for solids, powders, pastes, and thick sludges.

These drums don’t go through pressure or leakproof testing unless they're intended for certain regulated liquids, which is rare. The UN rating for drums on an open head will usually reference solids and include a maximum gross mass instead of pressure specs.

Material Types

Both open and closed head drums are available in different materials. Each has its own impact on compatibility, cost, and UN rated industrial container classification:

• Carbon steel: Common choice for oils, paints, and flammable liquids. Used for both drum types.
• Stainless steel: Handles corrosive or high-purity materials. Often used in specialty applications.
• Plastic (HDPE): Used for acids, detergents, or food-related materials. Found in both open and closed head styles.
• Fiberboard: Limited to dry, low-risk solids. Typically used for lightweight shipments or temporary storage.

Construction and Fittings

The construction of a drum—especially wall thickness and seam design—directly affects its ability to pass UN testing for drop, stack, and compression resistance. Drums rated for Packing Group I typically have thicker steel or plastic walls, tighter seam tolerances, and reinforced chimes to absorb impact. Lighter-duty drums may pass for Group II or III but would fail under higher drop or stacking requirements.

Fittings are another area that’s often overlooked during repacking or reuse. Threaded bungs and vent caps must match the original test configuration, including gasket material and closure torque. Substituting caps or rings—even if they seem compatible—can invalidate the drum as a UN rated packaging.

Drum selection should match the material, the handling method, and the test the container was certified under. Different substances require different access, closure, and pressure control. Use this as a quick guide:

Choose a closed head drum for:

• Liquids that require sealed transport
• Flammable or volatile materials
• Anything that needs pressure or leakproof testing

Choose an open head drum for:

• Solids, powders, or thick sludge
• Materials that require easy access
• Substances that don’t build internal pressure

Both styles can carry a UN mark, but they’re rated under different testing methods. Always verify the full marking code before selecting a drum for shipping hazardous materials.

Overpack Containers and When to Use Them

An overpack container is used when you need to consolidate multiple packages or provide additional protection during transport. It doesn't replace the inner container's UN rating—it adds a layer of security around it. This is used for intact and undamaged packages.

DOT regulations define an overpack as any outer packaging used to hold one or more UN rated containers for easier handling. The inner containers must stay fully marked, properly closed, and meet hazmat packaging requirements on their own. An overpack never corrects an unqualified or improperly closed drum.

Overpacks are used in situations like:

• Combining multiple UN rated drums into one pallet-sized container
• Securing containers that don't stack well or have awkward shapes
• Providing additional protection for fragile but undamaged packages

If labels on the inner packages are no longer visible after overpacking, you must duplicate them on the outside. This includes hazard labels, shipping names, and the full UN code.

You'll also need to add the “OVERPACK” marking clearly on the outer container. The overpack label meaning is straightforward: it tells handlers that the container is not the original tested unit but holds one inside. This marking must be in English, in bold, and easy to see similar to the image below.

You don't always need an overpack container, but certain conditions make it appropriate. Consider these questions:

Are you preparing multiple compliant packages that would be easier to handle as a single unit?

• Yes: Use an overpack to consolidate multiple packages into one handling unit, improving efficiency and reducing individual package handling.
• No: Individual handling is more appropriate when shipping a few packages or when consolidation offers no logistical advantage.

Are your compliant packages at risk of damage from normal transportation handling?

• Yes: Use an overpack as a protective barrier against impacts, compression, or handling issues that could compromise otherwise compliant packages.
• No: Packages specifically tested and certified to withstand transportation conditions for their hazard class don't require additional protection.

Would your shipment benefit from weather protection during outdoor transfers?

• Yes: An overpack can provide weather resistance for packages that might be exposed to rain, snow, or extreme temperatures during loading/unloading.
• No: Packages with inherent weather resistance or shipments that remain in climate-controlled environments don't need this additional protection.

Do you need to secure multiple small packages that might shift or separate during transport?

• Yes: An overpack creates a stable unit that prevents small packages from shifting in transit, reducing damage risk.
• No: Well-secured individual packages or those already configured for stability don't gain significant benefits from overpacking.

Does your shipping process frequently involve repalletizing or cross-docking?

• Yes: Overpacks maintain package groupings during multiple handling phases, preserving hazard communication and reducing handling errors.
• No: Direct point-to-point shipments with minimal handling may not warrant the additional packaging step.

Common Compliance Pitfalls (and How to Avoid Them)

Even when you’re using a UN rated drum, small oversights can still lead to violations. Most of the issues I’ve seen don’t come from ignoring the rules—they come from assuming something was “probably fine.” These are the mistakes that show up again and again during inspections or transport reviews.

Avoid these common compliance failures:

• Using the wrong drum for the material or packaging group: A drum rated for solids cannot be used for liquids unless it has passed the required liquid tests. Always confirm the full UN rating for drums matches both the physical state and hazard level of the contents.
• Missing or incorrect markings: If labels are worn off, covered, or handwritten, the container may be flagged as noncompliant. You’re required to display all UN markings, hazard labels, and shipping info clearly and in the right format.
• Ignoring closure instructions: UN-tested drums are certified with specific closures, including torque specs for bungs and rings. Replacing a bolt ring or hand-tightening a bung without checking specs can void the container’s rating.
• Improper drum reconditioning or reuse: Reusing drums without proper cleaning or certification introduces unknowns into the system. If you can’t confirm the container’s past contents or its current condition, it should not be used for shipping hazardous materials.
• Overfilling or exceeding weight limits: Filling past the tested gross mass or liquid volume can cause deformation, leaks, or pressure failures. Always reference the original code (e.g., Y25) to know the max fill weight.
• Poor documentation or incomplete shipping papers: If the paperwork doesn’t match what’s inside or what’s marked on the drum, the entire load can be rejected or delayed. Carriers and inspectors check for consistency between labeling, placards, and manifest entries.
• Lack of training or awareness among handlers: I've reviewed operations where staff were unaware that a closed head drum had specific closure torque values, or that an overpack container still had to be labeled. These aren't technical errors—they’re training gaps.
• Using drums past their service life: Some drums degrade due to corrosion, UV exposure, or stress fatigue over time. Plastic drums, in particular, can become brittle, especially if storing hazardous materials outside or under fluctuating temperatures.

One way to prevent unauthorized or unsafe use of damaged, expired, or incorrectly labeled drums is through a drum lockout system. These devices physically block access to bung openings or drum lids and are used to isolate containers that are pending inspection, reclassification, or repair. In facility and transport settings, lockouts help reduce the risk of non-compliant drums being used in active shipments or mistakenly repurposed.

What Happens If You’re Not Compliant?

I reviewed multiple enforcement cases from PHMSA, and the patterns are always the same. Fines were issued for shipping hazardous materials in packaging that wasn't UN rated when required. In one example, drums intended for high-hazard contents were packed in containers with no performance rating at all. That led to a penalty of nearly $7,000 for a single violation—no incident, no spill, just non-compliant packaging.

Another case involved drums that were rated correctly, but the closures weren’t applied using the manufacturer’s instructions. That included using incorrect torque or swapping out parts like bung caps or ring bolts. The penalty for that one came in over $1,400, all because the container’s original test conditions weren’t maintained during use.

There was also a case where drums had been reused without proper reconditioning. They were filled and shipped without leakproof testing, and the original markings hadn’t been updated. That one raised several violations at once—improper reuse, missing marking, and failure to confirm performance integrity—and the penalties stacked up quickly.

Aside from penalties, there’s the issue no one wants to face—actual harm. A drum that leaks, collapses, or reacts because it wasn’t tested for the contents inside can endanger drivers, warehouse staff, and response teams. And even if nothing goes wrong this time, non-compliant packaging increases the chance something will happen down the line.

FAQs

What does the first number in a packaging code indicate?

It shows the general type of packaging being used. For example, “1” refers to drums, “3” means jerricans, and “4” represents boxes. This number helps identify the container’s base form before material or design details are added.

What is a 1H1 drum?

A 1H1 drum is a plastic, closed head drum designed for liquids. The top is non-removable and typically includes two bungs for filling and venting. It’s pressure-tested and must meet specific standards for transporting hazardous liquids.

What is the difference between 3H1 and 3H2?

Both are plastic jerricans used for hazardous materials. 3H1 has a closed head and is intended for liquids, while 3H2 has a removable head and is used more often for solids or thick pastes. Their testing requirements also differ based on the intended contents.

What is the difference between an overpack and a salvage drum?

An overpack is used to enclose one or more intact, compliant containers for easier handling or added protection. A salvage drum is used when the original drum is damaged, leaking, or no longer meets shipping standards. Salvage drums must meet stricter performance oriented packaging requirements, including pressure testing, and are typically marked with the word “SALVAGE.”

How is the lid on an open-head drum fastened?

An open-head drum uses a removable lid secured with a locking ring. The ring can be tightened with a bolt, lever, or clamp depending on the drum type. This closure must match the original tested configuration to maintain its UN rating.