ISO 6346

What ISO 6346 actually covers

There are two ISO standards every freight container operator runs into. They get confused constantly. They cover different things:

• ISO 1496 — physical specification. Dimensions, racking strength, stacking loads, test methods. This is what container manufacturers build to.
• ISO 6346 — identification and marking. The 11-character ID, the check digit, the size & type code, the operational and safety stencils. This is what’s painted on the doors and side panels.

Both apply to every standard ISO container. If you’re inspecting equipment, ISO 1496 is the one that tells you whether the box is structurally sound; ISO 6346 is the one that tells you what to write down. This guide is about the second.

Anatomy of a container number

Every ISO container carries an 11-character identifier, painted in block stencil on both door panels and on at least one side. Here’s a real OWLU^® container broken apart:

The first three letters are the owner’s BIC-registered identifier — see the BIC Codes guide. The fourth letter is the equipment category: U for freight containers (which is almost everything), J for detachable equipment, Z for trailers and chassis. The six-digit serial is whatever the owner has assigned, sequential per fleet. The final digit is computed.

The check digit

ISO 6346 specifies a modulo-11 algorithm that produces a single check digit from the first 10 characters. The point isn’t security — it’s catching transcription errors when someone reads a container number off a worn door panel or types it into a manifest. The math is straightforward but the letter values are not.

The algorithm

1. Convert each of the first 10 characters to its numeric value. Digits are themselves; letters use a special table (next section).
2. Multiply each value by 2 to the power of its position — that’s weights 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 from left to right.
3. Sum the ten products.
4. Take the sum modulo 11. The result is the check digit.
5. Edge case: if the remainder is 10, it’s printed as 0. ISO 6346 recommends not allocating those serials, since single-character errors can mutate one valid number into another without detection.

Letter values — the table that bites people

Letters start at A = 10 and skip every multiple of 11 (so 11, 22, 33 are unused). The skips fall before L and before V. This is the part most amateur implementations get wrong:

A	B	C	D	E	F	G	H	I	J
10	12	13	14	15	16	17	18	19	20

K	L	M	N	O	P	Q	R	S	T
21	23	24	25	26	27	28	29	30	31

U	V	W	X	Y	Z
32	34	35	36	37	38

The skips exist for a real reason. The algorithm divides by 11 at the end, so any two characters with values congruent mod 11 would contribute identically — meaning a single-character substitution between them would slip past the check digit. Skipping the multiples guarantees every letter is unique mod 11. Implementations that compute letter_value = ord(c) - ord('A') + 10 without skipping silently produce wrong check digits for any container with L, V, W, X, Y, or Z in it. This is the classic bug.

Worked example

The same OWLU^® container from the anatomy section above — OWLU209148 — run through the algorithm:

Pos	Char	Value	×	Weight	=	Product
0	O	26	×	1	=	26
1	W	35	×	2	=	70
2	L	23	×	4	=	92
3	U	32	×	8	=	256
4	2	2	×	16	=	32
5	0	0	×	32	=	0
6	9	9	×	64	=	576
7	1	1	×	128	=	128
8	4	4	×	256	=	1024
9	8	8	×	512	=	4096
Sum						6300
6300 mod 11						8

Check digit = 8. The full number OWLU2091488 validates — and note the letter values at work: O, W, and L are 26, 35, and 23, with L landing past the first skipped multiple of 11. Try it yourself in the check-digit calculator.

Size & type codes

Below the container number you’ll see another four-character code — usually painted smaller. That’s the size and type code, also defined in ISO 6346. It tells you, in four characters, what kind of container you’re looking at.

Structure:

Position	What it encodes
1	Length
2	Width × Height (combined)
3	Primary container type
4	Subtype / detail

Decode one as you scroll

The most common code in the OWLU^® fleet, taken apart one position at a time — 45G1:

Length codes (position 1)

2	20ʹ
4	40ʹ
L	45ʹ
M	48ʹ
N	49ʹ

Width × Height codes (position 2)

0	8ʹ wide × 8ʹ high
2	8ʹ × 8ʹ6ʺ (standard)
5	8ʹ × 9ʹ6ʺ (high cube)
6	8ʹ × >9ʹ6ʺ

Primary type codes (position 3)

G	General purpose, no ventilation
V	General purpose with ventilation
B	Dry bulk
S	Named cargo (livestock, autos, fish)
R	Reefer (integrated refrigeration)
H	Reefer or heated, removable equipment
U	Open top
T	Tank
P	Platform or flat rack

Common full codes you’ll see at a depot

Code	What it is
22G1	20ʹ standard GP
25G1	20ʹ high cube GP
42G1	40ʹ standard GP
45G1	40ʹ high cube GP
L5G1	45ʹ high cube GP
22R1	20ʹ reefer
42R1	40ʹ reefer
45R1	40ʹ high cube reefer
22U1	20ʹ open top
42U1	40ʹ open top
22P1	20ʹ flat rack, fixed ends
22P3	20ʹ flat rack, collapsible
22T0–T6	20ʹ tank, varying pressure ratings
22B0	20ʹ dry bulk

For the full historical lookup, BIC publishes the size & type code reference ↗ reflecting ISO 6346:2022.

The other markings on a container

ISO 6346 also specifies a set of legal and operational stencils that must appear alongside the container number on every box. The most important:

• Max Gross Weight — the maximum allowed combined weight of the container plus contents. Typically around 30,480 kg / 67,200 lbs for a standard 20ʹ or 40ʹ.
• Tare — the empty weight of the container itself. Typically 2,200 kg (~4,850 lbs) for a 20ʹ GP, 3,800 kg (~8,380 lbs) for a 40ʹ GP.
• Net / Payload — Max Gross minus Tare. The cargo the container can actually carry.
• Cubic Capacity — interior volume in cubic feet or cubic meters.
• CSC Safety Approval Plate — a separate ISO/CSC requirement (not ISO 6346 itself) but mounted alongside. Indicates the container has passed the structural inspection required by the Convention for Safe Containers and lists the next inspection due date.
• Owner’s logo or brand stencil — Maersk’s "Maersk," Hapag-Lloyd’s "HL," etc. Convention, not requirement.

Why this matters for operators

ISO 6346 is the thread that runs through every system that touches a container:

• Bills of lading use the 11-character ID as the primary key.
• Terminal and depot management systems (Navis, CRM 1Stop, every TOS in the industry) index everything off the same number.
• Equipment interchange receipts (EIR) — the documents that handle handoffs between depots, truckers, and shippers — carry the ID and the size/type code as the only two facts that matter for legal liability.
• Customs filings, CSC records, OCR systems on terminal gates — everything that processes containers at scale assumes ISO 6346 is intact and validates the check digit.

A container without a valid ISO 6346 identifier is, operationally, invisible. It can’t be tracked, can’t be insured, can’t be moved through a major port. The standard is mundane — eleven characters, a modulo-11 algorithm — but it’s the operational backbone of the entire industry.