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Short answer
Under OSHA’s Hazard Communication Standard and 29 CFR 1910.106, a flammable liquid in Category 3 has a flash point at or above 23 C (73.4 F) and at or below 60 C (140 F). Its label carries the flame pictogram, the signal word “Warning” and the statement “Flammable liquid and vapor”. Category 3 liquids may be kept in a flammable liquid cabinet up to 60 gallons per cabinet.
Definition and scope
OSHA sorts flammable liquids into four categories using the flash point, and for the two most hazardous categories also the boiling point. The same four-category scheme appears in the Hazard Communication Standard (29 CFR 1910.1200, Appendix B.6) and in the flammable liquids standard (29 CFR 1910.106). This guide covers that scheme, how Category 3 differs from Category 2, and what the standard says about cabinet storage. It is general regulatory information, not site-specific safety advice. For the wider hazard-class picture see GHS classification of chemicals.
How classification works: flash point and boiling point
The category is decided by test properties of the liquid, not by its product name or use. The two properties that matter are the flash point and, for Categories 1 and 2, the initial boiling point (measured at 14.7 psia under 1910.106(a)(5)). A liquid’s safety data sheet (SDS) reports its flash point under physical and chemical properties; see how to read a safety data sheet. Matching that value to the bands below gives the category.
Flammable liquid Categories 1 to 4 compared
| Category | Flash point | Boiling point | Pictogram | Signal word | Hazard statement |
|---|---|---|---|---|---|
| 1 | Below 23 C (73.4 F) | At or below 35 C (95 F) | Flame | Danger | Extremely flammable liquid and vapor |
| 2 | Below 23 C (73.4 F) | Above 35 C (95 F) | Flame | Danger | Highly flammable liquid and vapor |
| 3 | At or above 23 C and at or below 60 C (73.4 F to 140 F) | No boiling point criterion | Flame | Warning | Flammable liquid and vapor |
| 4 | Above 60 C and at or below 93 C (140 F to 199.4 F) | No boiling point criterion | None | Warning | Combustible liquid |
Sources: 29 CFR 1910.1200 Appendix B.6 (classification) and Appendix C.4.19 (label elements), and 29 CFR 1910.106(a), read 2026-11-08.
Category 3 compared with Category 2
Both carry the flame pictogram, but they differ in three ways. Category 2 liquids flash below 23 C, while Category 3 liquids flash from 23 C to 60 C. Category 2 is also defined by a boiling point above 35 C, which Category 3 does not use. On the label, Category 2 reads “Danger” and “Highly flammable liquid and vapor”, where Category 3 reads “Warning” and “Flammable liquid and vapor”.
Combustible liquid or flammable liquid?
The two words overlap in OSHA’s own text. In 29 CFR 1910.106(a)(19), a flammable liquid is any liquid with a flash point at or below 199.4 F (93 C), which takes in all four categories. In the Hazard Communication labelling tables, Category 4 (above 60 C up to 93 C) is the one labelled “Combustible liquid”, and it is the only category with no flame pictogram. Categories 1 to 3 are the ones labelled “flammable liquid” in the hazard statement. So whether a liquid is “flammable” or “combustible” depends on which text you are reading; the flash point band tells you which label and storage rule applies.
“Class 3” hazmat versus “Category 3”
These are different systems that share a number. “Category 3” is the OSHA/GHS flash point band described above, used for workplace labels, SDSs and storage. “Class 3” in a hazmat context is the Department of Transportation hazard class for flammable and combustible liquids in shipping, with its own placards and packaging rules; the question “what is a Class 3 substance hazmat?” refers to that. Our Hazard Class 3 guide covers the shipping side. The older lettered terms (Class I, II, III liquids) come from NFPA 30 and are not used in the current 1910.106 text, which speaks only of Categories 1 to 4.
Storing Category 3 liquids in cabinets
29 CFR 1910.106(d)(3) limits a storage cabinet to no more than 60 gallons of Category 1, 2 or 3 liquids, and no more than 120 gallons of Category 4 liquids. For cabinet construction the standard sets a fire-test limit: the interior temperature may not exceed 325 F when the cabinet is exposed to a 10-minute fire test. It also gives construction minimums. Metal cabinets must be at least No. 18 gage sheet iron, double walled with a 1.5 inch air space; wood cabinets must be at least 1 inch plywood. For the fuller storage picture see flammable liquid storage cabinet requirements and chemical storage compatibility and segregation.
A four-step category check
- Find the flash point in the liquid’s current SDS.
- Match it to the band in the table: below 23 C is Category 1 or 2 (split by boiling point at 35 C), 23 to 60 C is Category 3, above 60 C up to 93 C is Category 4.
- Compare the container label with the table: flame pictogram, “Warning” and “Flammable liquid and vapor” for Category 3.
- Count the volume of each category stored in a cabinet against the 60 and 120 gallon limits.
Aerosols and chemicals under pressure should not be classified as flammable liquids (Appendix B.3), so a flammable spray product follows the aerosol rules instead.
Common misunderstandings
- “Category 3 means it will not catch fire.” Category 3 liquids are labelled flammable, with a flame pictogram. The category reflects how readily the liquid gives off ignitable vapor, not whether it burns.
- “A higher number is a bigger hazard.” The reverse holds: Category 1 is the most hazardous and Category 4 the least.
- “The product name tells the category.” Only the flash point (and boiling point for Categories 1 and 2) decides it. That is why petroleum products and solvents appear in different categories depending on the product. Check the SDS for the actual product.
- “Hazard Class 3 on a shipping paper is the same as Category 3.” It is not; see the section above.
Frequently asked questions
What is a Class 3 substance hazmat?
It is a DOT hazard class 3 material, meaning a flammable or combustible liquid for transport. It is a shipping classification, separate from the OSHA Category 3 flash point band. See the Hazard Class 3 guide.
What are the OSHA requirements for flammable liquids stored in cabinets?
Under 29 CFR 1910.106(d)(3), no more than 60 gallons of Category 1, 2 or 3 and no more than 120 gallons of Category 4 per cabinet, with the construction and fire-test limits described above.
What are some examples of Class 3 petroleum products?
The category turns on measured flash point, not on the product family, so no product type can be assigned to Category 3 by name alone. Read the flash point on the product’s SDS and match it to the 23 to 60 C band.
What is the difference between a combustible liquid and a flammable liquid?
Under the Hazard Communication labelling, Category 4 liquids (flash point above 60 C up to 93 C) are labelled “Combustible liquid” with no flame pictogram, while Categories 1 to 3 are labelled “flammable liquid”. Note that 1910.106(a)(19) uses “flammable liquid” for any liquid at or below 93 C.
Where does Category 2 sit relative to Category 3?
Category 2 has a flash point below 23 C and a boiling point above 35 C, so it ignites at lower temperatures and carries the “Danger” signal word.
Sources
- OSHA, 29 CFR 1910.106, Flammable liquids: osha.gov/laws-regs/regulations/standardnumber/1910/1910.106 (read 2026-11-08).
- OSHA, 29 CFR 1910.1200 Appendix B (B.6 flammable liquids): 1910.1200 Appendix B (read 2026-11-08).
- OSHA, 29 CFR 1910.1200 Appendix C (C.4.19 label elements): 1910.1200 Appendix C (read 2026-11-08).
Related reading: common lab chemical hazard classes and GHS labels, pictograms and signal words.
