Container Check Digit Calculator (ISO 6346)
A container check digit calculator computes the 11th character of an ISO 6346 container number from the three-letter owner code, the equipment category letter, and the six-digit serial number.
Calculate or verify the ISO 6346 Mod-11 check digit for a shipping container number.
Calculate a container check digit
Enter a value and run the check. Everything runs locally in this page.
Run up to 10,000 validations/mo at <5ms edge latency with zero data retention and 99.9% availability.
curl -X POST https://api.stanzaapi.com/container-validator/api/v1/container/validate \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{"containerNumber": "CSQU3054383"}'Runs entirely in your browser. The value you type is never sent to a server.
How to calculate a container check digit
- Enter the container number — Type the ten-character prefix (owner code, category letter, six-digit serial) to calculate the check digit, or paste the full eleven-character number to verify it.
- Apply the ISO 6346 letter values — Letters A to Z map to 10 through 38, skipping the multiples of 11 (11, 22, and 33). Digits keep their face value.
- Sum the weighted values — Multiply each of the first ten characters by two to the power of its position (2^0 to 2^9), sum the products, then take the sum modulo 11 and that remainder modulo 10.
What the check verifies
The check verifies the ISO 6346 check digit, the 11th character of a container number. It is calculated by weighting the first ten characters with powers of two, summing the products, and reducing the total modulo 11, then modulo 10.
The calculator assigns each letter its ISO 6346 numeric value (10 to 38, skipping 11, 22, and 33), keeps digits at face value, and multiplies each of the first ten characters by two raised to its position. The products are summed, reduced modulo 11, and reduced again modulo 10 to produce the check digit.
Enter the ten-character base to compute the missing check digit, or paste the complete eleven-character number to confirm that its last digit matches the ISO 6346 calculation.
ISO 6346 letter values and weights
| Character | Value |
|---|---|
| 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 |
Worked example: C=13, S=30, Q=28, U=32, then 3, 0, 5, 4, 3, 8. Weighted sum 13x1 + 30x2 + 28x4 + 32x8 + 3x16 + 0x32 + 5x64 + 4x128 + 3x256 + 8x512 = 6185. Then 6185 mod 11 = 3 and 3 mod 10 = 3, so CSQU3054383 is valid.
Frequently Asked Questions
Each of the first ten characters is multiplied by 2 raised to its zero-based position, the products are summed, and the check digit is (sum mod 11) mod 10.
The fourth character is the equipment category identifier: U for a freight container, J for detachable cargo equipment, and Z for a trailer or chassis.
No. The check digit is the only integrity check in the number, so a mismatch means at least one character was mistyped or the number was not issued under ISO 6346.
The free calculator processes one number at a time. The Stanza Container Validator API is the server-side option for integrations and also decodes the container size and type code.
Free browser tool vs Stanza API
| Capability | Free browser tool | Stanza API |
|---|---|---|
| Where it runs | Your browser | Cloudflare edge |
| Authentication | None | Bearer API key |
| Rate limit | None | Published per-tier limits |
| Data handling | Never leaves your device | In-memory, zero retention |
| Size and type code decoding through the Container Validator API | No | Yes |
| Typed JSON envelope with latency metadata for server-side integrations | No | Yes |