UN Markings for Tight-Head Steel Drums (Liquids)

Packaging Technical Library › Hazmat Packaging › Drums · Document TL-002

A liquids mark carries two numbers a solids mark doesn’t: the specific gravity the drum was drop-tested at and the hydrostatic pressure it held. This guide decodes the mark on a 1A1 tight-head steel drum, field by field, and lists the checks to make before you fill one.

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u/n11A12/Y31.44/1505/266/USA7/M123481.29

Example mark. Read it as: a steel tight-head drum, tested for Packing Group II and III liquids up to specific gravity 1.4 and a 150 kPa hydrostatic test, made in 2026, certified in the USA by registrant M1234, with a 1.2 mm body.

Reading the UN mark, field by field

Field · exampleWhat it means
1UN packaging symbol
u/n
Certifies that the drum design passed UN performance tests. Embossed metal drums may show the capital letters “UN” instead of the circled symbol.
2Packaging type code
1A1
1 = drum · A = steel · 1 = non-removable head (tight head), filled and emptied through bungs. A 1A2 is the removable-head version.
3Packing group rating
Y
The highest hazard level the design was tested for: X = PG I, II and III · Y = PG II and III · Z = PG III only.
4Specific gravity
1.4
The relative density of the liquid the design was drop-tested with, to one decimal. It may be left off when that value is 1.2 or less.
5Hydrostatic test pressure
150
The internal pressure the design held in testing, in kPa, rounded down to the nearest 10. It must cover the vapor pressure of what you fill.
6Year of manufacture
26
The last two digits of the year the drum was made.
7Country
USA
The country (state) that authorized the mark, shown with its international vehicle registration code.
8Manufacturer / certifier
M1234
The name or registered symbol of the manufacturer or approval agency. In the U.S., symbols are registered with PHMSA.
9Metal thickness
1.2
Nominal body thickness in mm, required on metal drums meant for reuse or reconditioning. When the heads are thinner than the body, all three are shown as top–body–bottom, e.g. 1.0-1.2-1.0.

Drum type codes

CodeDrum
1A1Steel, non-removable head (tight head)
1A2Steel, removable head (open head)
1B1 / 1B2Aluminum, non-removable / removable head
1N1 / 1N2Other metal, non-removable / removable head
1H1 / 1H2Plastic, non-removable / removable head
1GFiber
1DPlywood

Liquids mark vs. solids mark

Liquids: 1A1/Y1.4/150/26/… — max specific gravity + hydrostatic test pressure (kPa).

Solids: 1A2/Y400/S/26/… — max gross mass (kg) + “S”.

Rules to check for liquids

  • Specific gravity is a ceiling. Your liquid’s specific gravity must not exceed the marked value. If none is marked, the limit is 1.2.
  • Test pressure must cover vapor pressure. The marked kPa has to be high enough for the liquid’s vapor pressure at 50–55 °C. The calculation is in the UN Model Regulations, Part 4.1, and 49 CFR 173.24a.
  • Leave room to expand. Fill so the drum is not liquid-full at 55 °C (131 °F).
  • Leakproof by design. Liquid designs pass a leakproofness test at 20 kPa for PG II and III, or 30 kPa for PG I.
  • Bungs are part of the test. Use the specified 2-inch and ¾-inch bung closures and gaskets, tightened to the closure instructions.
  • Permanent on the bottom. Metal drums over 100 L must carry the mark on the bottom head in permanent form, such as embossing. A printed or labeled mark on the side or top is extra.
  • The rating is a ceiling. A Y drum may carry PG II or III material, never PG I. An X drum covers all three.

Before you fill: a four-step check

1
Find the packing group
Look up the UN number and PG in the Hazardous Materials Table (49 CFR 172.101) or Section 14 of the SDS.
2
Check specific gravity
Section 9 of the SDS gives the liquid’s specific gravity. It must be at or below the marked value (or 1.2 if none is shown).
3
Check vapor pressure
Compare the liquid’s vapor pressure at 50–55 °C with the marked test pressure, using the regulation’s formula.
4
Fill and close as tested
Leave ullage for expansion, then torque bungs with the specified gaskets per the closure instructions.

Frequently asked questions

What does 1A1 mean on a drum?

1A1 is the UN packaging code for a steel drum with a non-removable (tight) head. The 1 means drum, A means steel and the final 1 means non-removable head; it is filled and emptied through bung openings.

What do the numbers after the packing group letter mean on a liquids drum?

The first is the maximum specific gravity the design was tested at (for example 1.4). The second is the hydrostatic test pressure in kPa (for example 150).

What if no specific gravity is shown in the UN mark?

The specific gravity may be left off when the design was tested at 1.2 or less. Treat 1.2 as the limit.

Can a removable-head (1A2) drum carry liquids?

Only if it carries a liquids mark and the packing instruction for your material allows removable-head drums. Some liquids require a tight-head drum.

Sourcing UN-rated tight-head drums?

Compare steel and plastic drum suppliers in the Packaura directory, or send one RFQ to vetted manufacturers and reconditioners.

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More from the Technical Library

For reference only. This guide explains the marking format and does not replace the regulations or your packaging’s test report. Confirm against 49 CFR 178.503, 49 CFR 173.28 and the UN Recommendations on the Transport of Dangerous Goods, Model Regulations, Chapter 6.1. The mark shown is an example, not a real certification.