What do the numbers on a hazard placard mean?
Four digits on an orange plate, and they identify what is in the tank. 1203 is petrol. This one decodes, which is not something every page here can say.
The lookup is the Hazardous Materials Table at 49 CFR 172.101 — 2,480 entries, reproduced here in full because a work of the United States government carries no copyright at all.
This one runs on your own device. It is worked out in your browser, so there is nothing to send and nothing to log. Where a tool on this site does send something out, that page says so and names where it goes.
The two letters in front mean something
The table writes the number with a prefix, and 172.101(e) says what each one is for:
| Prefix | What 172.101(e) says |
|---|---|
UN | Those preceded by the letters “UN” are associated with proper shipping names considered appropriate for international transportation as well as domestic transportation. |
NA | Those preceded by the letters “NA” are associated with proper shipping names not recognized for transportation outside of the United States. |
ID | Those preceded by the letters “ID” are associated with proper shipping names recognized by the ICAO Technical Instructions |
So an NA number on a tank is a real fact
read out of two letters: the load is moving under a number the rest of the
world does not use. A plate showing bare digits gives no such clue, and the
tool above checks all three prefixes when you type four digits alone.
The same number can mean several things
This is the trap. UN1950 has five entries — aerosols, in different hazard classes with different labels — and so does NA1993. Fifty-nine numbers in the table carry more than one entry.
A lookup that showed the first match would be choosing one and presenting it as the answer. The tool above shows every entry, and says how many there are before it lists them, so nobody reads the first and stops.
The hazard class
Column 3 gives a class or division, and 49 CFR 173.2 says what each is:
| Class | Division | What it is |
|---|
Explosives carry a letter that table does not define
A class-1 entry reads 1.1D, not 1.1.
The letter is a compatibility group, and 173.2 does not
define it — 49 CFR 173.52
does, in a different subpart of a different part. There are thirteen of
them and thirty-five valid division-and-letter combinations.
That was found by a check rather than by reading: the page’s hazard classes were cross-checked against 173.2’s list, the class-1 codes came back undefined, and the honest response to a red check was to go and look for where they were defined rather than to loosen the check until it passed.
The packing group
“Packing Groups I, II, and III indicate the degree of danger presented by the material is great, medium, or minor, respectively.”
172.101(f), which also says: “Class 2, Class 7, and Division 6.2 do not have packing groups.” So a blank in that column is usually the rule working rather than a gap — though not always, and where an entry outside those classes has no packing group the tool says so instead of quoting a rule that does not cover it.
The table contains pointers as well as entries
412 rows have a name and no number: “Accellerene, see p-Nitrosodimethylaniline”. They are cross-references, and they are searchable here, because a name you can find in the table but cannot look up is a real thing to meet and a page that hid them would look as though the substance were missing.
What the number does not tell you
It gives a name, a class and a packing group. It does not tell you what to do. How much is in the tank, what condition it is in, and how to deal with a leak are not in this table — emergency response is a different document and a telephone number.
This page is for reading a plate you are curious about, not for standing next to one that is leaking.
The European plate is a different thing
On a European tanker the orange plate carries two numbers, one above the other. The lower one is the same four-digit substance number. The upper one is a hazard identification code — two or three digits, sometimes with an X in front — and it is defined by ADR rather than by 49 CFR.
This page does not read the upper number. It is a different source with different terms, and guessing at it from the American table would be inventing.
A note on where this came from
The table is the 2025 annual edition of the CFR, fetched from govinfo.gov as XML and extracted whole. The links above point at the eCFR, which is the version a person can navigate — but it renders in JavaScript and gives an automated reader no regulation text, so what a machine checks and what a reader opens are deliberately different.
One column is missing on purpose. Column 1 of the table
holds symbols — +, A, D,
G, I and W — which change how
an entry applies. Their meanings are in 172.101(b) and this page has not
read that paragraph yet, so the column is not shown. A column nobody can
explain is worse than an absent one.