# ISO 6346 Modulo-11 Shipping Container Check Digit Formula & Step-by-Step
> Step-by-step Modulo-11 check digit calculation for 11-character intermodal container numbers.

- **Canonical URL:** https://stanzaapi.com/tools/container-validator/iso-6346/iso-6346-check-digit-calculation
- **Governing Standard:** ISO 6346 / BIC Container Prefix Coding (Bureau International des Conteneurs (BIC) & ISO)
- **Latency:** <1ms Edge
- **Privacy:** In-memory execution only (zero data stored or logged)

## Technical Overview & Governing Rules
The 11th digit is a check digit calculated using powers of 2 weights (2^0 to 2^9) with letters converted to values where multiples of 11 (11, 22, 33) are excluded.

## Specification Matrix
| Letters | Assigned Values | Rule |
| --- | --- | --- |
| A - J | 10, 12, 13, 14, 15, 16, 17, 18, 19, 20 | 11 is skipped |
| K - U | 21, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32 | 22 is skipped |
| V - Z | 34, 35, 36, 37, 38 | 33 is skipped |

## Standard JSON Response Model
```json
{
  "containerNumber": "CSQU3054383",
  "checkDigit": 3,
  "checkDigitCalculated": 3,
  "valid": true
}
```

## Frequently Asked Questions & Edge Cases
### Q: What happens if the modulo 11 remainder is 10?
**A:** According to ISO 6346, if the remainder is 10, the check digit is designated as 0.