Every boat built or imported for sale in the United States since 1972 carries a twelve-character hull identification number, and the characters are not arbitrary. Four of them are a date. Two more are the model year, stamped into the hull by the manufacturer, which is a different thing from the year in a Marketplace advert and does not always agree with it.
Paste the number in below. The decoder splits it the way the regulation does and tells you what each piece means. It runs entirely in your browser: nothing is uploaded, nothing is stored, and there is no lookup happening in the background.
Decode a hull number
What does each character mean?
The layout is fixed by 33 CFR 181.25, which requires twelve characters with no slashes, hyphens or spaces between them.
| Characters | What they are | How they are written |
|---|---|---|
| 1 to 3 | Manufacturer identification code, or an importer designation | Assigned by the Coast Guard, not chosen by the builder |
| 4 to 8 | Serial number the manufacturer assigned to that hull | Letters or numerals, but never I, O or Q |
| 9 | Month of certification or manufacture | A letter: A is January, B is February, on to L for December |
| 10 | Year of certification or manufacture | The last digit of that year only, as a numeral |
| 11 and 12 | Model year | The last two digits, so 82 means 1982 |
Character ten being a single digit is the part that trips people up, and it is why a decoder that reads the hull number alone cannot hand you a certification year on its own. It can with characters eleven and twelve alongside it, which is what the tool above does: a hull whose model year is 2017 and whose tenth character is 6 was certified in 2016, because that is the only recent year ending in 6 that a 2017 model could have been built in.
Why do the two years on the hull disagree?
They usually do, and it is not a problem. Boats are built ahead of the model year they are sold as, the same way cars are, so a hull certified in September 2016 is commonly a 2017 model. One year of daylight between the certification date and the model year is ordinary.
What is worth a question is a gap of two years or more, or a listing whose stated year does not match the model year on the hull at all. Neither is proof of anything. Sellers inherit a boat and repeat what they were told, paperwork gets copied wrong, and a hull can genuinely sit unsold for a season. But it is a specific, checkable thing to ask about, and the answer usually arrives fast.
Where is the hull number on the boat?
There are two of them and they must be identical. 33 CFR 181.29 puts the primary number on the starboard outboard side of the transom, within two inches of the top of the transom, gunwale or hull-to-deck joint, whichever sits lowest. The duplicate goes somewhere unexposed inside the boat or under a fitting, and it is not advertised.
The same section requires the characters to be at least a quarter of an inch high, and requires the number to be permanently affixed so that altering, removing or replacing it would be obvious. If it is on a separate plate, the plate has to be fastened so that pulling it off scars the hull around it. A transom plate sitting on fresh sealant with clean surrounding gelcoat is worth asking about, and so is a seller who can produce the transom number but not the hidden one.
The rest of the walkthrough, in the order that saves you the drive home, is in the used boat inspection guide. Which paperwork that number has to match is a separate question with fifty-one answers: what a used boat needs in each state covers it.
What can this decoder not tell you?
It cannot name the manufacturer. Characters one to three are a code the Coast Guard assigns, and turning a code into a company means searching the Coast Guard's own register of more than sixteen thousand records. That database is not embedded here, because a partial copy would silently fail on exactly the obscure builder you are trying to identify. Search it directly at the Coast Guard's manufacturer identification lookup, which takes either a code or a company name.
It cannot tell you the boat is what the seller says it is. A hull number decodes whether or not it belongs to that hull. The check that matters is that both numbers on the boat match each other and match the title in front of you.
It cannot read a number that does not follow the current rule. The format above is the one in force. Hulls numbered in the earliest years after 1972 do not all follow it, and neither do imports that were never certified for US sale. Where the characters do not fit, the decoder says which position failed rather than inventing a date to fill the field.
Cite this page
The decoder implements one source: 33 CFR 181.25, the Coast Guard regulation that fixes the twelve-character format, linked in full above. The placement and marking rules quoted on this page come from 33 CFR 181.29 and the country-of-origin note from 33 CFR 181.27, both also linked. Nothing here is derived from DockSnipe's own data and no figure has been carried over from a secondary source. The tool holds no manufacturer database and does not guess at one: the code in the first three characters resolves only through the Coast Guard's own lookup, which is linked rather than copied. Everything runs in the reader's browser, so no hull number typed here is transmitted or stored. Last checked September 8, 2026.
You may republish these figures with attribution and a link to https://docksnipe.com/tools/hin-decoder.