Container Check Digit Calculator
Calculate the missing check digit for a 10-character ISO 6346 container prefix, or validate an existing 11-character container number against the international standard modulus-11 check digit algorithm.
Spaces and hyphens are automatically ignored; letters are capitalized.
Complete 11-character container number:
Container Number Structure Breakdown
ISO 6346 Check Digit Calculation Steps
Each character is assigned a numeric value, multiplied by 2position (starting at 20 = 1), and summed.
| Position | Character | Component Role | ISO Value | Weight (2pos) | Product |
|---|---|---|---|---|---|
| 0 | M | Owner code | 24 | 20 = 1 | 24 |
| 1 | S | Owner code | 30 | 21 = 2 | 60 |
| 2 | C | Owner code | 13 | 22 = 4 | 52 |
| 3 | U | Equipment category | 32 | 23 = 8 | 256 |
| 4 | 6 | Serial number | 6 | 24 = 16 | 96 |
| 5 | 6 | Serial number | 6 | 25 = 32 | 192 |
| 6 | 3 | Serial number | 3 | 26 = 64 | 192 |
| 7 | 9 | Serial number | 9 | 27 = 128 | 1152 |
| 8 | 8 | Serial number | 8 | 28 = 256 | 2048 |
| 9 | 7 | Serial number | 7 | 29 = 512 | 3584 |
| Total Sum of Weighted Products: | 7656 | ||||
| Modulo 11 Remainder (7656 ÷ 11 = 696 with remainder): | 0 | ||||
| Final Check Digit (equals remainder 0): | 0 | ||||
Important Scope & Verification Notice:
This calculator implements the ISO 6346 check-digit algorithm (ISO 6346:2022). A successful validation confirms solely that the alphanumeric string satisfies the standard formatting and weighted checksum rules. It does not prove that the container physically exists, that the 3-letter owner code is currently registered with the Bureau International des Containers (BIC), actual container ownership, or current customs or cargo status.
What Is a Container Check Digit?
An intermodal shipping container number is an 11-character alphanumeric identifier displayed prominently on container door panels, side walls, and roof sections. The final character (the 11th digit, usually enclosed in a box or brackets) is the check digit.
Standardized internationally under the ISO 6346 check-digit algorithm (ISO 6346:2022), the check digit provides a mathematical integrity check. Its primary purpose is to guard against transmission and manual data-entry errors, such as mistyping a single letter or transposing two adjacent numbers during bill of lading creation, booking confirmation, customs entry, or terminal gate-in operations.
Structure of an ISO Container Number
Every standard shipping container marking is segmented into four distinct elements across its 11 characters:
The first three characters are capital Latin letters indicating the container owner or principal operator (for example, MSC for Mediterranean Shipping Company, CMA for CMA CGM, or CSQ for COSCO). Authorized owner codes are officially registered with the International Container Bureau (Bureau International des Containers, BIC).
The 4th character denotes the equipment category according to ISO 6346:
- U: Freight containers (all standard ISO freight containers)
- J: Detachable freight container-related equipment (e.g. clip-on diesel generator sets)
- Z: Trailers and chassis
Characters 5 through 10 comprise a 6-digit numeric sequence assigned by the owner or operator to uniquely identify the container within their fleet. If an owner assigns fewer than 6 digits, leading zeros are prefixed (e.g. 003450).
The 11th character is a single numeric digit computed by weighting the previous 10 characters with powers of 2 modulo 11. It validates that the entire identification sequence was entered without transcription errors.
How the ISO 6346 Check Digit Calculation Works
The calculation follows a strict mathematical procedure that converts alphanumeric characters into numerical values, weights them by exponential factors of 2, sums the products, and applies modulo 11 division:
- Assign Numeric Values: Digits
0–9keep their face value (0 to 9). LettersA–Zare assigned values from 10 to 38, with all multiples of 11 (11, 22, and 33) explicitly omitted: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 - Apply Positional Multipliers: Each of the first 10 characters (at index positions 0 through 9 from left to right) is multiplied by
2position:Position 0: ×1 | Position 1: ×2 | Position 2: ×4 | Position 3: ×8 | Position 4: ×16 | Position 5: ×32 | Position 6: ×64 | Position 7: ×128 | Position 8: ×256 | Position 9: ×512 - Sum the Products: All 10 weighted products are added together to produce a total sum.
- Calculate Remainder Modulo 11: Divide the sum by 11 to determine the remainder:remainder = sum mod 11
- Determine Check Digit: If
remainder = 10, the check digit is designated as 0. Otherwise, the check digit equals the remainder (0 through 9).
What a Successful Validation Does and Does Not Prove
When managing ocean shipments, it is vital to recognize the operational boundary of an algorithm-based checksum:
- The string conforms strictly to the ISO 6346 11-character identification structure.
- The owner prefix contains 3 alphabetic characters.
- The category identifier is an approved ISO code (U, J, or Z).
- The serial portion consists of exactly 6 numeric digits.
- The supplied check digit matches the mathematical checksum, eliminating common typos and digit transpositions.
- No proof of physical existence: Anyone can generate a mathematically valid ISO number for a container that does not exist.
- No proof of active BIC registration: It does not query the official BIC database to verify that the 3-letter owner code is registered or currently active.
- No proof of ownership or leasing: It does not show current asset ownership, leasing agreements, or steamship line affiliation.
- No cargo or tracking data: It contains no real-time data regarding gate movements, seal numbers, customs clearance, or cargo contents.
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