Container Check Digit Calculator (ISO 6346)
Validate any container number or compute its missing check digit — exact ISO 6346 math, shown step by step.
Format: 3-letter owner code + U/J/Z + 6 digits + check digit (ISO 6346). Letters map to values skipping multiples of 11; each of the first 10 characters is weighted 2position; the sum mod 11 mod 10 is the check digit.
Weighted sum = 5560 → 5560 mod 11 = 5
Sources & references
- ISO 6346:2022 — coding, identification and marking
- BIC — check digit calculation guidance
This tool runs entirely in your browser — nothing you enter is uploaded. Information is provided for operational convenience; verify regulated or contractual matters against the official source.
Every container number ends in a single digit that exists to catch typos: the ISO 6346 check digit, computed from the ten characters before it. Booking systems, terminal gates and EDI processors all validate it — a transposed digit in MSKU1234565 bounces the transaction hours or days after someone typed it. This calculator runs the exact algorithm both ways: paste 11 characters to validate, or 10 to get the missing digit.
About Container Check Digit Calculator (ISO 6346)
The math, transparently: each letter maps to a value from 10 to 38 (skipping multiples of 11 — there's no 11, 22 or 33, which is why A=10 but K=21 and L=23), digits keep face value, each of the ten characters is multiplied by 2 raised to its position (1, 2, 4 … 512), the products are summed, and the sum mod 11 mod 10 is the check digit. The tool shows the weighted sum so you can audit it by hand. Worth knowing the algorithm's one wrinkle: when the sum mod 11 equals 10, the check digit wraps to 0 — meaning some numbers differing only in that wrap can both 'look' plausible. The BIC recommends owners avoid issuing serials whose check digit derives from the 10-case, but boxes with them exist; validation still works, it's manual transcription that deserves extra care on those.
How to use Container Check Digit Calculator (ISO 6346)
- 1Type the container number — 10 characters to compute the check digit, 11 to validate one.
- 2The correct check digit appears instantly, with the weighted-sum math shown underneath.
- 3A green badge confirms a valid 11-character number; red shows the expected digit instead.
- 4Use it to catch typos in bookings, EDI files and gate paperwork before systems reject them.
Why use Container Check Digit Calculator (ISO 6346)?
- ✓Exact ISO 6346 algorithm — letter values, 2^n weights, mod-11 check
- ✓Validates full numbers and computes missing check digits
- ✓Shows the math, so you can verify it by hand if you want
- ✓Catches container-number typos before bookings and EDI reject them
Frequently asked questions
How is a container check digit calculated?+
Map the four letters to numbers (A=10, B=12 … Z=38, skipping 11, 22 and 33), keep the six digits as-is, multiply each of the ten values by 2^position (first character ×1, second ×2, … tenth ×512), sum everything, take mod 11, then mod 10. The result is the eleventh character. This page runs exactly that and displays the intermediate sum.
Why does my correctly-typed container number fail validation?+
Three usual causes: a transcription error upstream (the document you copied from is itself wrong — surprisingly common on manually-typed packing lists), confusion between similar glyphs (0/O, 1/I, 5/S) somewhere in the chain, or a genuinely misprinted door decal (rare but documented). When a physical box and its paperwork disagree, the box wins — and the carrier should be told.
Why do letter values skip 11, 22 and 33?+
To keep the mod-11 arithmetic unambiguous: values that are multiples of 11 would vanish in the modulo step, making different letters indistinguishable to the checksum. Skipping them preserves each letter's contribution. It's also why the mapping looks irregular (K=21, L=23) — the gaps are deliberate, not typos.
Where does check-digit validation actually matter day to day?+
Anywhere a container number is keyed: bookings, shipping instructions, VGM submissions, EDI 301/315 messages, gate systems and yard inventories. Validating at entry costs nothing; an invalid number discovered downstream means rejected transactions, mismatched moves and boxes that 'disappear' from tracking because two systems hold two different numbers for one box.
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