ISO 6346 · in your browser
Container Check Digit Calculator
The check digit is the 11th character of a container number: each of the first 10 characters gets a value (letters A = 10 to Z = 38, digits as themselves), is multiplied by 2 to the power of its place, and the sum divided by 11 leaves the digit. For MSKU 307703 the sum is 3,352, which leaves 8, so the number is MSKU 307703-8.
Type the first ten characters to get the check digit, or the whole number to check one you were given. The working appears under the answer, a character at a time, the way ISO 6346 sets it out — so you can do it by hand next time, or show someone why a number cannot be right.
The second field reads the size and type code painted under the number: 22G1, 42G1, 45G1, L5G1. Nothing you type leaves your browser.
Letter values (A = 10 … Z = 38; 11, 22 and 33 are skipped)
A 10B 12C 13D 14E 15F 16G 17H 18I 19J 20K 21L 23M 24N 25O 26P 27Q 28R 29S 30T 31U 32V 34W 35X 36Y 37Z 38
Worked out in your browser; the number is not sent anywhere.
How to calculate a container check digit by hand
Write down the ten characters. Give each letter its value — A is 10, B 12, C 13 and so on to Z at 38, skipping 11, 22 and 33 — and keep each digit as it is. Multiply the first by 1, the second by 2, the third by 4, doubling each time to 512 for the tenth. Add the ten products, divide by 11 and keep the remainder: that is the check digit. A remainder of 10 gives 0.
For MSKU 307703: M 24 × 1, S 30 × 2, K 21 × 4, U 32 × 8, then 3 × 16, 0 × 32, 7 × 64, 7 × 128, 0 × 256 and 3 × 512. The products add to 3,352; 3,352 ÷ 11 is 304 remainder 8. The full number is MSKU 307703-8.
What a wrong check digit tells you
A check digit that does not match means one character was misread, mistyped or painted wrong — most often a letter for a digit that looks like it (O and 0, I and 1, B and 8, S and 5). The calculator reads those swaps for you and says what it read. The check catches every single wrong character and most swapped neighbours, which is why carriers and terminals refuse a number that fails it.
A number that passes is well-formed, not proof that the box exists or where it is. To find out who owns the prefix and where to track it, use the container tracking page.
The size and type code: 22G1, 45G1 and the rest
The four characters under the number describe the box. The first is the length (2 = 20 ft, 4 = 40 ft; 45 ft was L in the 1995 edition and is 5 since 2022), the second the height (2 = 8 ft 6 in, 5 = 9 ft 6 in, a high cube), and the last two the type: G1 is a general-purpose box with passive vents, R1 a refrigerated and heated reefer, U1 an open top, P1 a flat rack with fixed ends, T a tank. So 22G1 is a standard 20 ft dry container and 45G1 a 40 ft high cube.
The decoder gives the detail only where the two sources we read — the BIC’s code tables and the ISO 6346 article — agree; for the few codes where they differ it names the group and says so.
Frequently asked questions
How do you calculate a container check digit?
Give each of the first 10 characters its value (A = 10 to Z = 38, skipping multiples of 11; digits as they are), multiply by 1, 2, 4 … 512, add, and divide by 11: the remainder is the digit, with 10 counting as 0. MSKU 307703 sums to 3,352, remainder 8.
How do I check if a container number is valid?
A container number is valid when its 11th character equals the check digit worked out from the first 10 — type all 11 above and the calculator says whether it matches, and which digit it should be if not.
What does 45G1 mean on a container?
45G1 is a 40 ft high cube general-purpose container: 4 for 40 ft long, 5 for 9 ft 6 in high, G1 for a dry box with passive vents. A standard 20 ft dry box is 22G1, a standard 40 ft 42G1.
Why is the check digit sometimes 0 when the remainder is 10?
Because the check digit is one character, a remainder of 10 is written as 0 — the same digit a remainder of 0 gives. Because the two cannot be told apart, owners are advised to avoid serial numbers that give 10.