ISO 6346 · BIC

Container number check digit validator

Catch the typo in the container number before the broker files.

Type a container number and get the ISO 6346 check digit verdict, plus the owner code, equipment category and serial number explained.

Last updated 2026-09-27. Math runs in your browser, no data leaves your computer.

Copy the number exactly as it appears on the container door, the bill of lading or the carrier's booking. Ten characters work too, and the tool then prints the check digit that belongs on the end.

Four letters then seven digits, and spaces or dashes are ignored, because the door marking often splits the number across lines.

Before the booking, screen the name on the invoice. Shipment pre-check is on the paid plans, from USD 59 a month.

General guidance only. Verify against the cited primary sources before you commit to a shipment, declaration, or contract.

How a container number is built

A container number has four parts, as BIC sets them out under ISO 6346. Three capital letters form the owner prefix for the owner or principal operator, one letter is the equipment category, six digits are the serial number of the owner's choosing, and one digit is the check digit.

Category letter Meaning per BIC
UAll freight containers
JDetachable freight container related equipment
ZTrailers and chassis

The check digit comes from the first ten characters. Letters take the values A = 10, B = 12, C = 13 and so on to Z = 38, skipping the multiples of 11, and digits keep their face value. Each value is multiplied by 2 to the power of its position, from 1 on the first character to 512 on the tenth, the products are added, and the remainder after dividing by 11 is the check digit. A remainder of 10 is written as 0.

BIC does not print the formula on its site, so this tool was tested against 252 results from the BIC check digit calculator across five owner codes, including every remainder of 10 in that range, and matched all of them.

Worked example. One transposed digit on the packing list

The booking. A supplier emails the packing list with container CSQU 305483 3 and the broker copies it into the entry. The owner code is right, the category letter is U and the serial has six digits, which looks fine on paper.

The failure. The container on the vessel is CSQU 305438 3, and the supplier swapped the last two digits of the serial when typing it. The check digit for CSQU 305483 is 0, not 3, so the eleven characters cannot all be right. A container number that does not match the bill of lading means the paperwork and the box disagree, and the broker has to chase the supplier while the demurrage clock can run.

The fix. Pasting the number into this tool flags the mismatch in a second. The buyer asks the supplier to read the number off the door or the seal photo, gets CSQU 305438 3 back, the tool confirms check digit 3, and the corrected packing list goes to the broker before filing.

Planning what goes inside that box? The CBM calculator works out how full a 20 ft, 40 ft or 40 ft high cube gets from your carton sizes.

Frequently asked

What is a container check digit?

It is the eleventh character of a container number, one digit that validates the owner code and serial number in front of it. BIC, which registers owner codes under ISO 6346, describes it as a means of validating the recording and transmission accuracies of the owner code and serial number.

How is the ISO 6346 check digit calculated?

Each of the first ten characters gets a value: digits keep their own value, and letters run from A = 10 upward, skipping 11, 22 and 33, so B = 12, K = 21, L = 23 and Z = 38. Each value is multiplied by 2 raised to its position, 1 for the first character up to 512 for the tenth. The sum is divided by 11, and the remainder is the check digit, with a remainder of 10 written as 0.

What do the four letters mean?

The first three letters are the owner code, registered with BIC for the owner or principal operator of the container. The fourth letter is the equipment category identifier: U for freight containers, J for detachable freight container related equipment, and Z for trailers and chassis.

The check digit matches. Is the container number real?

A match proves only that the eleven characters are consistent with each other. It does not prove the owner code is registered or that the container exists, so look the owner code up in the BIC database and track the number on the carrier site before you rely on it.

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Related tools

Sourzi shows what official files say. It is not customs or legal advice. For a binding answer, ask a licensed customs broker, a CBP ruling or an Australian Border Force Tariff Advice.

File dates and links are on Data sources.