UN certified packaging — Decoding the UN String: Anatomy of a Rating

Overview

UN certified packaging is the internationally standardized system for marking packagings that have been tested and approved to safely contain hazardous materials in transport. The permanent UN marking—commonly called the UN string—serves as a concise 'passport' for a container, telling a trained reader what type of packaging it is, the construction material, the performance level it met during testing, and the maximum mass or specific gravity it was certified to carry.

Why the UN marking matters

The UN string exists so shippers, carriers, warehouse staff, inspectors, and emergency responders can quickly verify that a packaging is suitable for the hazardous contents it will carry. Using properly certified packagings reduces the risk of spills, reactions, and transport incidents, and it is legally required for many hazardous shipments in most jurisdictions.

Typical structure and what to look for

A complete UN marking is usually stamped or printed permanently on the packaging and may be preceded by the UN symbol. Reading it left-to-right, the main logical parts are:

• Packaging type and material code — a short code (for example, 4G) that tells you the basic packaging construction and material. The first character is a numeric type and the second is a letter for material (see separate entry on types and materials).
• Performance level — one of X, Y, Z, indicating the tested performance and the Packing Groups for which the packaging is suitable (see separate entry on performance levels).
• Numeric value — a number such as 145 that represents the maximum gross mass in kilograms for solids, or the specific gravity used in testing for liquids. This is the tested limit; it is not an arbitrary label.
• Content-class indicator and additional identifiers — markings may include a letter indicating the intended contents type (for example, a letter denoting solids) and then the country authorizing the mark, year of manufacture or test, and an identifying manufacturer code or serial number.

Practical example

Consider a simplified marking: 4G X 145 S 20 XX 1234. A trained reader extracts: 4G = fiberboard box (type and material), X = top performance suitable for Packing Groups I–III, 145 = tested to 145 kg (or specific gravity of 1.45 for liquids), S = packaging tested for solids, followed by the year, country code and manufacturer identifier. Actual formats and elements vary slightly by packaging type and regional implementation, but the core components remain consistent.

How to use the UN string in a 3PL/warehouse environment

Staff handling hazardous shipments should be trained to read and verify UN markings as part of incoming inspections, storage assignment, and dispatch checks. Typical checks include:

• Confirm the packaging type and material are appropriate for the goods.
• Verify the performance level (X/Y/Z) matches the packing group required by the material’s classification.
• Ensure the numeric value meets or exceeds the gross mass or specific gravity requirements for the filled shipment.
• Check for a valid manufacturer code and year where applicable, and inspect the physical condition of the marking to ensure it’s legible and unaltered.

Best practices

Train staff on UN marking interpretation and integrate checks into standard operating procedures. Keep Safety Data Sheets (SDS) accessible so the correct Packing Group and compatibility requirements can be cross-checked. Maintain a register of common packagings and their limits used by your operation so packers and inspectors can quickly confirm compliance. If reconditioning or reusing packaging, follow applicable regulations and keep traceable records of tests or recertification.

Common mistakes to avoid

Do not assume a packaging that 'looks right' is compliant—always read the UN string. Avoid using a container with illegible or missing markings for hazardous loads. Don’t mix packing groups in a single packaging unless the combination packaging and UN marking explicitly permit it. Finally, never exceed the numeric limit stamped on the packaging; that limit is based on standardized tests and must be respected for safety and legal compliance.

Closing note

For a 3PL, the ability to read the UN string quickly and accurately is a foundational safety skill. The UN marking condenses critical test and suitability information into a compact code; paired with knowledge of Packing Groups and SDS content, it enables safe handling, storage, and transport of dangerous goods.