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Understanding Shipping Container Markings
Most ISO shipping containers carry identification markings on their doors and sides, together with a metal safety approval plate.
These markings identify the container's registered owner, equipment category, serial number, dimensions and type. The CSC plate records safety approval information used for international freight transport.
If you are buying a used shipping container, understanding these markings can help you check the unit's identity, age, specifications and suitability for your intended use.
Important:
A correctly marked container is not automatically
cargo worthy. Markings alone cannot establish
the present structural condition, weatherproofing
or current examination status of a particular unit.
The Four Things to Check
- Container identification number: An eleven-character ISO number.
- Size and type code: A four-character classification such as 22G1.
- CSC safety approval plate: Records design approval and safety-related ratings.
- Examination and condition: Confirms whether the required inspection arrangements and cargo-condition assessment are appropriate for freight use.
How to Read a Shipping Container Number
International freight containers use the ISO 6346 identification system . A standard container identification number contains four letters followed by seven digits.
For example:
CSQ
Owner Code
U
Equipment Category
305438
Serial Number
3
Check Digit
| Part | Characters | Example | Meaning |
|---|---|---|---|
| Owner Code | 3 letters | CSQ | Registered owner or principal operator identification. |
| Equipment Category | 1 letter | U | U = freight container; J = detachable container-related equipment; Z = trailers or chassis. |
| Serial Number | 6 digits | 305438 | Number allocated by the container owner. |
| Check Digit | 1 digit | 3 | Mathematical check calculated from the preceding characters. |
What Does the Letter U Mean?
The fourth character identifies the equipment category. U indicates a freight container, while J and Z apply to related equipment and chassis or trailers.
The owner code registration system is maintained by the Bureau International des Containers et du Transport Intermodal (BIC).
Identification is not proof of ownership:
A correctly formed container number does not
prove that a seller owns the container
or is authorised to sell it. Verify the
seller and supporting transaction documents.
How the ISO 6346 Check Digit Works
The check digit is calculated from the first ten characters of the container number. Its purpose is to detect transcription and identification errors.
Step 1: Convert Letters to Numbers
Each letter has a numerical value. A begins at 10, with multiples of eleven omitted from the sequence.
| Letters | Corresponding Values |
|---|---|
| A–M | A=10, B=12, C=13, D=14, E=15, F=16, G=17, H=18, I=19, J=20, K=21, L=23, M=24 |
| N–Z | N=25, O=26, P=27, Q=28, R=29, S=30, T=31, U=32, V=34, W=35, X=36, Y=37, Z=38 |
Step 2: Apply Positional Weights
Multiply each character's numerical value by a power of two. The positional weights run from 1 to 512 across the ten characters.
Step 3: Calculate the Remainder
Add the ten products and divide the result by eleven. The remainder is the check digit. If the remainder is 10, the printed digit is 0, although the standard recommends avoiding numbers with this result.
Worked Example: CSQU 305438 3
This example demonstrates how the final digit is verified.
| Position | Character | Value | Weight | Product |
|---|---|---|---|---|
| 1 | C | 13 | 1 | 13 |
| 2 | S | 30 | 2 | 60 |
| 3 | Q | 28 | 4 | 112 |
| 4 | U | 32 | 8 | 256 |
| 5 | 3 | 3 | 16 | 48 |
| 6 | 0 | 0 | 32 | 0 |
| 7 | 5 | 5 | 64 | 320 |
| 8 | 4 | 4 | 128 | 512 |
| 9 | 3 | 3 | 256 | 768 |
| 10 | 8 | 8 | 512 | 4,096 |
| Total | 6,185 | |||
6,185 ÷ 11 = 562 remainder 3
Check digit: 3 — matches CSQU 305438 3.
If the result differs from the printed check digit, recheck the number against the physical unit and ask the seller for clarification.
Shipping Container Size and Type Codes Explained
The four-character size and type code describes the container's physical classification and intended design.
For example, 22G1 identifies a standard-height 20ft general-purpose container, while 45G1 identifies a 40ft high-cube general-purpose unit.
| Code | Container Description |
|---|---|
| 22G1 | 20ft standard-height general-purpose container |
| 42G1 | 40ft standard-height general-purpose container |
| 45G1 | 40ft high-cube general-purpose container |
| 45R1 | 40ft high-cube refrigerated container |
What the Characters Mean
The first character identifies nominal length, the second represents the height and width category, and the final two indicate the design type.
| Position | Example Code | Meaning |
|---|---|---|
| Length | 1, 2, 3, 4, L | 10ft, 20ft, 30ft, 40ft and 45ft length classes |
| Height / Width | 0, 2, 5 | 2,438 mm height, 2,591 mm standard height, 2,896 mm high-cube height for the referenced width class |
| General Purpose | G1 | General-purpose freight container configuration |
| Refrigerated | R1 | Integral refrigeration machinery |
| Open Top | U1 | Open-top freight container type |
| Platform-Based | P3 | Platform-based container type |
| Tank | T* | Tank container family, with additional classification digit |
For exact measurements, see Palliat's Shipping Container Dimensions & Sizes Guide .
What Is a CSC Safety Approval Plate?
A CSC plate is a metal safety approval plate fitted to a container approved for international transport under the International Convention for Safe Containers, 1972.
The convention is administered by the International Maritime Organization (IMO) .
The plate is generally found on or near the left-hand container door. Many units use a combined data plate carrying CSC, customs approval and operational information.
CSC Plate Fields Explained
| Field | What It Means |
|---|---|
| Approval Country & Reference | Identifies the national approval authority and approval reference. |
| Manufacture Date | Month and year of manufacture. Useful for identifying the unit's age. |
| Manufacturer Identification | Manufacturer's assigned identification or serial number. |
| Maximum Operating Gross Mass | Maximum permissible combined mass of the container and its contents. |
| Allowable Stacking Load | Stacking capacity under the specified test acceleration of 1.8g. |
| Transverse Racking Force | Lateral test force used in structural approval. |
| Wall Strength | Additional end-wall or side-wall markings where they differ from convention defaults. |
| NED or ACEP | Identifies the next examination date or an approved continuous examination programme. |
Always read the actual plate:
Common gross mass, stacking and racking
ratings are not universal. Manufacturer,
construction and design approval can
change the values shown.
ACEP vs Periodic CSC Examination
Under the CSC examination framework, containers used in international transport must remain subject to appropriate safety examinations.
The two principal schemes are periodic examination and the Approved Continuous Examination Programme (ACEP).
| Feature | Periodic Examination (PES) | Approved Continuous Examination (ACEP) |
|---|---|---|
| How It Works | The container is examined at specified intervals. | Examination forms part of an approved ongoing inspection programme. |
| First Examination | Within five years of manufacture. | Continuous examination arrangement under the approved programme. |
| Later Examinations | Intervals of no more than 30 months. | Programme requirements apply, with inspection frequency meeting the relevant CSC examination conditions. |
| Plate Marking | Next examination date, often abbreviated NED. | Approved ACEP identifier. |
| Buyer Check | Confirm examination status is current. | Confirm the programme identifier and relevant examination records. |
What If the Next Examination Date Has Expired?
An expired examination date does not automatically mean the container is damaged or unsuitable for static storage.
However, a container intended for international transport must satisfy the applicable CSC examination requirements before it is shipped.
Where examination status is uncertain, ask for current inspection documentation and seek a qualified assessment.
CSC Plate vs Cargo-Worthy Certificate
The terms CSC approved and cargo worthy are often used together, but they refer to different requirements.
CSC Safety Approval
Relates to the approved design, construction, safety ratings and required examination arrangements for freight use.
Cargo-Worthy Assessment
Relates to the container's condition at the time of inspection, including relevant structural and weatherproof requirements for carrying cargo.
A container may retain valid CSC approval markings but be damaged in a way that makes it unsuitable for freight.
Equally, a container in good structural condition may require updated examination documentation before returning to international service.
What Should Export Buyers Request?
- Current photographs of the actual unit.
- A legible CSC plate and identification number.
- Confirmation of examination status.
- An appropriate cargo-worthy survey or condition report.
- Confirmation that the unit satisfies requirements of the intended carrier, route and shipment.
For more information on condition descriptions, read our Container Grades Explained Guide .
Do Storage Containers Need a Valid CSC Plate in Australia?
A container used only for static storage on private land generally does not need to maintain the examination status required for international maritime transport.
That distinction does not remove other requirements. A container may still need planning or building approval, suitable structural condition and safe installation on the property.
The intended use also matters. A shipping container converted into an office, workshop or habitable building may need additional engineering and regulatory assessment.
See our Council Approval Guide for Australian planning considerations.
Australian Shipping Container Biosecurity Requirements
Containers arriving in Australia from overseas are subject to biosecurity controls administered by the Department of Agriculture, Fisheries and Forestry (DAFF).
DAFF explains these requirements in its Cargo Containers: Biosecurity Aspects and Procedures .
Container Cleanliness
Containers entering Australia must be managed to minimise biosecurity risk. Soil, seeds, insects, timber pests and plant or animal material can lead to inspection, treatment or cleaning requirements.
Importers should ensure the container, packaging and cargo comply with the applicable conditions.
External and Rural Inspections
Some cargo containers are directed for external inspection, including checks for contamination on the underside and other surfaces. The specific procedures depend on the import risk assessment.
Sea Container Hygiene System
The Sea Container Hygiene System is an Australian biosecurity initiative intended to reduce contamination risks associated with certain sea container pathways.
Participation and inspection arrangements are governed by DAFF's applicable requirements.
Brown Marmorated Stink Bug (BMSB)
Australia's seasonal BMSB measures apply to specified cargo and transport pathways associated with nominated target risk countries.
For the 2026–27 season, targeted shipments fall within the window from 1 September 2026 to 30 April 2027, inclusive, subject to the applicable measures.
See the DAFF 2026–27 BMSB season notice .
Domestic resale:
These import controls should not
be confused with routine resale
of a container that is already
within Australia. Importing cargo
and purchasing an existing
domestic storage container
involve different requirements.
Five-Minute Container Markings Inspection Checklist
Use this checklist to review basic identification and condition details before buying.
Before You Purchase
Tick the checks you have completed.
Inspection reminder:
Reading the markings does not replace
examining the unit's current condition.
Do not enter an unsafe container or
close yourself inside to test for leaks.
Request appropriate inspection evidence
if access or safety is uncertain.
Frequently Asked Questions
How do I check if a shipping container number is valid?
Apply the ISO 6346 check-digit calculation to the first ten characters and compare the result with the eleventh digit. A matching digit checks the format but does not prove the container exists or belongs to the seller.
What is a CSC plate on a shipping container?
A CSC plate is a safety approval marking used for internationally transported containers. It records important identification, construction and load-rating information, together with relevant examination details.
Do I need a valid CSC plate for storage in Australia?
Static domestic storage generally does not require the examination status used for international freight transport. Other planning, building and safety requirements may still apply.
What does 22G1 mean on a container?
22G1 identifies a standard-height 20ft general-purpose ISO container. The characters encode its nominal length, height and equipment type.
What is the difference between ACEP and NED?
NED refers to the next examination date in the periodic scheme. ACEP refers to an approved continuous examination programme under which required inspections are incorporated into the operator's procedures.
Is a cargo-worthy certificate the same as CSC approval?
No. CSC approval relates to the container's design approval and examination regime. A cargo-worthy assessment evaluates the unit's condition for freight use at the time of inspection.
What biosecurity rules apply to containers entering Australia?
Incoming containers and cargo must meet DAFF biosecurity requirements, which can include cleanliness declarations, inspections, treatment and seasonal pest risk measures. Requirements depend on the shipment and current import rules.
How can I find out how old a shipping container is?
Check the manufacture date on the CSC plate or combined data plate and compare it with the current date. If the plate is missing or illegible, request documentary evidence from the seller.
Understanding the Limits of This Guide
This guide explains the identification structure specified by ISO 6346, the principal information recorded on CSC safety approval plates, examination arrangements and general Australian biosecurity requirements.
Individual container ratings, construction details and examination records can differ from the examples discussed.
The identification number and safety markings alone cannot establish that a particular unit is fit for international transport, free from contamination or suitable for a specific project.
Disclaimer:
This article provides general
technical information, not
shipping certification, customs,
engineering or biosecurity advice.
Obtain current documentation
and qualified assessment
for the individual container.
Official Technical References & Sources
The following sources support the technical topics covered in this guide. They include international standards, maritime safety guidance and Australian government biosecurity information.
01
ISO 6346:2022 — Freight Containers: Coding, Identification and Marking
Identification numbers, check digits and size/type coding. ISO indicates that a newer edition is being prepared.
02
ISO 668:2020 — Series 1 Freight Containers
Container classifications, external dimensions and ratings.
03
International Maritime Organization — International Convention for Safe Containers, 1972
CSC safety approval plates, examination arrangements, safety requirements and convention amendments.
04
Bureau International des Containers et du Transport Intermodal (BIC)
Information about container owner-code registration and intermodal identification.
05
DAFF — Cargo Containers: Biosecurity Aspects and Procedures
Australian import biosecurity controls and container cleanliness requirements.
06
DAFF — 2026–27 Brown Marmorated Stink Bug Risk Season Notice
Official notification of seasonal measures for targeted goods and affected shipping pathways.
Always refer to the latest version of an applicable standard, regulation or government procedure before arranging international freight.