How to Read a Container Number, Check Digit and ISO Type Code

The container number, part by part

The identification number has eleven characters in four parts. Taking CSQU 305438 3 as the example:

• CSQ — owner code. Three capital letters identifying the owner or principal operator of the container.
• U — equipment category identifier. U for all freight containers, J for detachable freight container-related equipment, Z for trailers and chassis. A container you are buying should read U.
• 305438 — serial number. Six digits assigned by the owner, unique within that owner's fleet.
• 3 — check digit. One digit calculated from the ten characters before it, usually painted inside a box so it is not read as part of the serial number.

Who registers owner codes: BIC

Owner codes are registered with the Bureau International des Containers (BIC) in Paris. BIC has kept the register since 1970, and ISO appointed it as the global container prefix registry in 1972. Under ISO 6346, only owner codes registered with BIC may be used as the unique identity marking of containers in international transport and customs documents — which is why the four-letter prefix, owner code plus U, is widely called the "BIC code".

The public BIC code register shows which company a prefix is registered to. A used container that has left a shipping line or leasing fleet often still carries the previous owner's prefix. That makes no difference to a container used for storage; if you intend to ship it as your own, read shipper-owned containers before you book.

How the check digit is calculated

The check digit is arithmetic, not part of the serial, and takes a minute on a phone calculator:

• Step 1 — give every letter a value. Start at A = 10 and count upwards, skipping 11, 22 and 33 (the multiples of 11): 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 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. Digits keep their own value.
• Step 2 — weight each position. Multiply the ten values, left to right, by 1, 2, 4, 8, 16, 32, 64, 128, 256 and 512 — each weight double the one before.
• Step 3 — divide by 11. Add the ten results and divide the total by 11. The remainder is the check digit. If the remainder is 10, the check digit is 0; ISO 6346 recommends that owners avoid serial numbers that produce this, but containers numbered that way are in service.

Worked example: CSQU 305438

• C: 13 × 1 = 13
• S: 30 × 2 = 60
• Q: 28 × 4 = 112
• U: 32 × 8 = 256
• 3: 3 × 16 = 48
• 0: 0 × 32 = 0
• 5: 5 × 64 = 320
• 4: 4 × 128 = 512
• 3: 3 × 256 = 768
• 8: 8 × 512 = 4 096

The ten results add up to 6 185. Dividing by 11 gives 562 with a remainder of 3 (562 × 11 = 6 182), so the check digit is 3 and the full number is CSQU 305438 3.

Swap two digits, as happens when a number is copied by hand, and CSQU 350438 gives 8, not 3. Because every position carries a different weight, a wrong or transposed character almost always changes the result, so a number that does not validate has been misread somewhere. Check it against the doors before it goes on an invoice, a customs declaration or a booking. BIC also publishes a free check digit calculator.

The size and type code

The four-character code sits below or beside the identification number. The first character gives the length, the second the height (and the width, where a container is wider than the standard 2 438 mm), and the last two the type.

• Length (first character): 1 = 10 ft (2 991 mm), 2 = 20 ft (6 058 mm), 4 = 40 ft (12 192 mm). For 45 ft (13 716 mm), containers marked to the 1995 edition of ISO 6346 carry L; the current code table published by BIC assigns 5.
• Height (second character): 0 = 2 438 mm (8 ft), 2 = 2 591 mm (8 ft 6 in, standard height), 5 = 2 896 mm (9 ft 6 in, high cube).
• Type group (third character): G = general purpose, R = refrigerated, U = open top, P = platform-based containers such as flat racks and platforms.
• Type detail (fourth character): G1 = general purpose with passive vents at the upper part of the cargo space; R1 = mechanically refrigerated and heated; U1 = open top with removable top members in the end frames; P1 = two complete, fixed ends; P3 = folding end structure.

One detail is worth a second look. If the fourth character is a letter rather than a digit — 22GB instead of 22G1 — the container was designed and tested with reduced stacking or racking strength. That matters if the unit will be stacked or shipped, so ask before you buy.

Common codes decoded

The codes below cover the standard dry, high cube, reefer and open top sizes. The one that causes expensive mistakes is 45G1: it describes a 40ft high cube, not a 45ft container. If an offer calls a unit "45ft" and its doors say 45G1, it is a 40ft box.

• 22G1 — 20 ft long, 8 ft 6 in high, general purpose: the standard 20ft container.
• 25G1 — 20 ft long, 9 ft 6 in high: a 20ft High Cube.
• 42G1 — 40 ft long, 8 ft 6 in high, general purpose: the standard 40ft container.
• 45G1 — 40 ft long, 9 ft 6 in high: a 40ft High Cube. The 5 is the height, not the length.
• L5G1 — a 45ft High Cube marked to the 1995 table; under the current table the same container reads 55G1.
• 22R1 and 45R1 — a 20ft reefer and a 40ft high cube reefer, both mechanically refrigerated and heated. See reefer containers.
• 22U1 and 42U1 — 20ft and 40ft open top containers.

Weights, capacity and warning marks

Besides the two codes, the door end carries operational marks. ISO 6346 makes the weight marks mandatory, and they are the ones a buyer actually uses.

• MAX GROSS — the maximum permitted mass of container and cargo together. It must be identical to the maximum gross mass on the CSC plate.
• TARE — the empty weight of the container. Maximum gross and tare are both mandatory, in characters at least 50 mm high.
• NET or PAYLOAD — the cargo weight available, maximum gross minus tare. Optional: some owners paint it under the tare, some do not.
• Capacity (often stencilled CU. CAP.) — the internal volume, usually in m³ and cubic feet. A common marking rather than a mandatory one.
• Height mark — mandatory on containers taller than 2.6 m (8 ft 6 in): black figures on a yellow background, near the top right-hand corner of each side below the identification number, giving the height in metres to one decimal place and in feet and inches. A high cube reads 2.9 m and 9’6”.
• Overhead electrical danger sign — a black lightning flash on yellow, mandatory next to the ladder on any container fitted with one.

Painted weights and spec-sheet figures do not always agree — tare varies between builders, and older units are often plated lower — so for any single unit the CSC safety approval plate governs. It is normally riveted to the outside of the left-hand door and records the approval reference, the month and year of manufacture, the weight ratings, and either the next examination date or an ACEP scheme reference. See the CSC plate guide and, for typical figures by size, shipping container weight.

Checks to make before you accept a container

• The number on the doors matches the quotation, delivery note or invoice, and its check digit validates.
• The size/type code matches what you ordered. The common mix-ups are 45G1 against a genuine 45ft unit, and 22G1 against a 20ft high cube (25G1).
• MAX GROSS on the doors matches the CSC plate.
• The fourth character of the type code is a digit, unless you have knowingly accepted a reduced-strength unit.

For the physical condition, work through the container inspection checklist. And when you request a quote, giving the size/type code you need removes any doubt about which container is meant.