Change one letter in a container number and the check digit usually catches it. Usually. I fed every single-character mistake in the first ten characters of TGBU 2545411 through the formula, 154 wrong numbers in all, and 6 came out looking valid. One of the six turns B into V. Letter names get misheard on radio and phone lines, which is what the spelling alphabet is for: “B” is always Bravo, “V” is always Victor.
The Alpha Bravo Charlie phonetic alphabet is also called the NATO phonetic alphabet. The official name is the International Radiotelephony Spelling Alphabet. People use it to read out serial numbers and reference codes, and a container number is one of those.
How to read a container number aloud
A container number is four letters and seven digits. The fourth letter U marks a freight container. The last digit is a check digit calculated under ISO 6346.
Take TGBU 2545411. Apply the ISO 6346 formula to the first ten characters and the result is 1, the same as the last digit. On 8 November 2026 the Navo24 container tracking API returned this number as a CMA CGM 20GP (ISO type 22G1), in transit from New Orleans to Laem Chabang on CMA CGM HOPE, with an estimated ETA of 16 December 2026.
Over the phone it goes out like this: “Tango Golf Bravo Uniform, two, fife, fower, fife, fower, one, check digit one.”
Digits go one at a time. ICAO, NATO and the FAA also change how a few are said: three is “tree”, four is “fower”, five is “fife”, nine is “niner”. The ITU and IMO use a different set of digit words, so agree with the other end which one you are using. Either way, 2545 is read as four digits, not “twenty-five forty-five”.
What the check digit catches, and what it misses
I wanted to know how often a container number in real use fails its own check digit. On 8 November 2026 I listed everything saved under our Navo24 tracking key: 4,175 records, 3,892 of them container numbers. 3,891 had the usual shape, four letters and seven digits. 3,859 passed the check. 32 did not.
At least 8 of the 32 look like test input, among them ABCU 1234567 and ZZZU 1234567, so this is no error rate for real freight. The formula rejected all 32, and the API flagged 21 of them as invalid_number.
Where does the check digit fail? In the letters. B counts as 12 and V as 34, because the table starts at 10 and skips 11, 22 and 33. That puts them 22 apart. The formula divides the weighted sum by 11, so any two letters a multiple of 11 apart leave the same remainder, and the swap slips through. I tested it on every position in the first ten characters of TGBU 2545411. That gives 154 wrong numbers. 148 are caught. Six pass: J for T, Q for G, L or V for B, A or K for U.
B and V are both on that list. The same blind spot covers 22 letter pairs in all, among them D and N, C and M, E and O. The arithmetic is mine, not a rule from the standard, and it applies only to a substitution in one position.
The 26 code words
Here is the full Alpha Bravo Charlie phonetic alphabet, letter by letter.
| Letter | Code word | Letter | Code word |
|---|---|---|---|
| A | Alfa | N | November |
| B | Bravo | O | Oscar |
| C | Charlie | P | Papa |
| D | Delta | Q | Quebec |
| E | Echo | R | Romeo |
| F | Foxtrot | S | Sierra |
| G | Golf | T | Tango |
| H | Hotel | U | Uniform |
| I | India | V | Victor |
| J | Juliett | W | Whiskey |
| K | Kilo | X | Xray |
| L | Lima | Y | Yankee |
| M | Mike | Z | Zulu |
Alfa and Juliett look like typos. They are spelled that way so that people who do not know English spelling rules say them right. Xray lost its hyphen for another reason: to be said as one word, not two. The table follows NATO. ICAO keeps X-ray.
Where it came from
The first non-military international spelling alphabet came from the CCIR, a radio advisory committee, in 1927. ICAO’s 1951 version had word pairs that got mixed up (Delta and Extra, Nectar and Victor) and some words that were hard to catch on a weak signal. New tests with speakers from 31 nations replaced five words, for C, M, N, U and X. Charlie took the place of Coca. ICAO implemented the final list on 1 March 1956.
One pair of words travels on its own: Bravo Zulu, letter code BZ, means “well done”.
Before you book anything against a number you only heard, look it up on container tracking. The fields on the container marking rules page will tell you whether the owner code and type look right, and the container prefixes list names the owner behind the four letters. A matching check digit only means the number is plausible, as the six swaps above show.
Once the number is confirmed, the vessel schedules show where the ship is bound, the freight rates price the next move and the Load Calculator plans what goes into the box. An AI assistant connected to live freight data can answer the same questions in plain language, Nav builds custom AI agents for the routine checks, and a one-off job can go to the Navo Mesh task exchange. The full list is on the all tools page. A tracking problem goes to [email protected], a lane or a quote to [email protected], and anything else through the contact page. For why a box slips off its schedule, read 30 most common reasons of cargo delays.
Frequently asked questions
What is the Alpha Bravo Charlie phonetic alphabet?
It is the International Radiotelephony Spelling Alphabet, also called the NATO, ICAO or ITU phonetic alphabet. It assigns one code word to each of the 26 letters, from Alfa to Zulu, so letters stay clear over radio and phone.
Why did Charlie replace Coca in the phonetic alphabet?
ICAO's 1951 list did not work well in practice. After new tests with speakers from 31 nations, five words for C, M, N, U and X were replaced in the version that took effect on 1 March 1956.
How do you read a container number with the phonetic alphabet?
Say each of the four letters as a code word, then the seven digits one at a time. TGBU 2545411 becomes Tango Golf Bravo Uniform, two, fife, fower, fife, fower, one.
Is the NATO phonetic alphabet the same as the ICAO alphabet?
The letter code words are the same. The IMO and ITU define additional compound words for digits, but the 26 letter words are shared.