Class 3 flammable liquids checker.

The two definitions that matter

The federal hazmat regulations define a Class 3 flammable liquid in 49 CFR 173.120(a) as a liquid with a flash point of not more than 60 degrees C (140 degrees F). The definition also catches any liquid with a flash point at or above 37.8 degrees C (100 degrees F) that is intentionally heated and offered for transport, or transported, at or above its flash point in a bulk packaging. A combustible liquid in 173.120(b) is a liquid that fits no other hazard class and has a flash point above 60 degrees C (140 degrees F) and below 93 degrees C (200 degrees F). Everything turns on the flash point, which 173.120(c) defines as the minimum temperature at which a liquid gives off vapor in a test vessel in sufficient concentration to form an ignitable mixture with air near the surface.

The Hazardous Materials Table in 172.101 entry for a named material controls how that material is described and classed, so treat a flash-point result as a first check.

Packing group by flash point and boiling point

When the 172.101 table lists more than one packing group for a Class 3 material, 173.121(a) sets the criteria, using the closed-cup flash point and the initial boiling point.

Placards, names and shortcuts

A flammable liquid transport vehicle carries the red FLAMMABLE placard with the class number 3 (172.542). For gasoline moved by cargo tank or portable tank on a highway, the word GASOLINE may replace the word FLAMMABLE, in white letters. A combustible liquid gets a COMBUSTIBLE placard (172.544), and on a cargo tank carrying fuel oil that is not classed as a flammable liquid the words FUEL OIL may replace COMBUSTIBLE. On a cargo tank or portable tank, a FLAMMABLE placard may be used in place of COMBUSTIBLE, and the same is allowed on a compartmented tank car holding both (172.504(f)(2)).

Whether the vehicle needs a placard at all depends on quantity and packaging. Class 3 is a Table 2 category, so non-bulk loads under 1,001 pounds (454 kg) aggregate gross weight need no placard on a highway vehicle, while bulk packagings are placarded at any quantity (172.504(c)). The hazmat placard page on this site walks through that rule.

Exceptions you should know

Limited quantities of flammable and combustible liquids are excepted from labeling and from specification packaging when packed in combination packagings that follow 173.150(b), and the package may not exceed 30 kg (66 pounds) gross weight. Inner packagings are capped at 0.5 liter (0.1 gallon) for Packing Group I, 1.0 liter (0.3 gallon) for Packing Group II and 5.0 liters (1.3 gallons) for Packing Group III and combustible liquids. Limited quantity shipments are not subject to the placarding subpart. Exceptions apply only if 173.150 is referenced for the specific material in the 172.101 table. Alcoholic beverages containing 24 percent or less alcohol by volume, or in inner packagings of 5 liters (1.3 gallons) or less, are not subject to the regulations when moved by motor vehicle, vessel or rail (173.150(d)).

A flammable liquid with a flash point at or above 38 degrees C (100 degrees F) that meets no other hazard class may be reclassed as combustible (173.120(b)(2)). That option does not apply to transport by vessel or aircraft except where no other means is practicable, and it does not apply to an elevated-temperature material that is Class 3 only because it is heated at or above its flash point.

Worked example

A shop ships a solvent with a closed-cup flash point of 20 degrees F (about minus 6.7 degrees C) and an initial boiling point of 150 degrees F (about 65.6 degrees C). The flash point is under 60 degrees C, so it is Class 3. The boiling point is above 35 degrees C and the flash point is below 23 degrees C, so by 173.121 it falls in Packing Group II if the 172.101 table gives more than one group for the entry. By contrast a liquid with a flash point of 150 degrees F (65.6 degrees C) is above the flammable cut-off and below 200 degrees F, so it is a combustible liquid. A liquid at 210 degrees F is outside the combustible definition on flash point alone.

Limits

The calculator reads only the flash point and boiling point you enter. It cannot see exceptions for liquids that do not sustain combustion (173.120(a)(3) to (5)), mixtures where a 60 degrees C component makes up 99 percent of the volume, the Associate Administrator’s power to reclassify a material (173.120(d)), or the test method used, which must be a closed-cup method listed in 173.120(c). The 172.101 table and the shipper’s classification are what the carrier relies on. We read the eCFR text current through October 29, 2026.

Sources

Sources opened and read November 2, 2026.

• eCFR: 49 CFR 173.120, Class 3 definitions (current through October 29, 2026) (read November 2, 2026)
• eCFR: 49 CFR 173.121, Class 3 assignment of packing group (current through October 29, 2026) (read November 2, 2026)
• eCFR: 49 CFR 173.150, exceptions for Class 3 (current through October 29, 2026) (read November 2, 2026)
• eCFR: 49 CFR 172.542, FLAMMABLE placard (current through October 29, 2026) (read November 2, 2026)
• eCFR: 49 CFR 172.544, COMBUSTIBLE placard (current through October 29, 2026) (read November 2, 2026)
• eCFR: 49 CFR 172.504, general placarding requirements (current through October 29, 2026) (read November 2, 2026)