NFPA 704 Flammability (Red)
The red quadrant at the top of the NFPA 704 diamond is the flammability rating. It runs from 0 (will not burn) to 4 (rapidly vaporizes or is readily dispersed in air and burns readily), and for liquids it is set mainly by flash point and boiling point.
Where it sits: the red quadrant on the top of the diamond.
Sample diamond for illustration only. The red box is the one this page explains.
How to read it
For flammable and combustible liquids the rating is tied to two numbers you can find in Section 9 of the SDS: the flash point and the boiling point. The lower the flash point, the more easily the liquid gives off enough vapor to ignite, and the higher the number. Gases, dusts, mists and solids are rated by how readily they ignite and burn under ordinary conditions.
The flash point bands used for liquids are: below 23 °C (73 °F) with a boiling point below 38 °C (100 °F) is a 4; below 23 °C with a boiling point at or above 38 °C, or a flash point from 23 °C up to (but not including) 38 °C (100 °F), is a 3; from 38 °C up to 93 °C (200 °F) is a 2; at or above 93 °C (200 °F) is a 1. Published summaries differ on exactly which side of the 38 °C and 93 °C boundaries a value falls, so always confirm against the current edition of the standard, because the rules for gases, dusts and solids are separate and more detailed.
These bands do not line up with the GHS flammable liquid categories or with the DOT definition of a flammable liquid, even though they use similar temperatures. A liquid can be rated 3 under NFPA 704 and be GHS Category 3 or Category 2, depending on where its flash point and boiling point fall. See the NFPA 704 versus GHS comparison for the details.
What each rating means
| Rating | Description | Meaning | Criteria |
|---|---|---|---|
| 4 | Severe | Will rapidly or completely vaporize at normal pressure and temperature, or is readily dispersed in air and will burn readily. | Liquids: flash point below 23 °C (73 °F) and boiling point below 38 °C (100 °F). Includes flammable gases and cryogenic flammable liquids. |
| 3 | Serious | Liquids and solids that can be ignited under almost all ambient temperature conditions. | Liquids: flash point below 23 °C with a boiling point at or above 38 °C, or flash point from 23 °C to below 38 °C. |
| 2 | Moderate | Must be moderately heated, or exposed to relatively high ambient temperature, before ignition can occur. | Liquids: flash point from 38 °C (100 °F) up to 93 °C (200 °F). |
| 1 | Slight | Must be preheated before ignition can occur. | Liquids: flash point at or above 93 °C (200 °F). |
| 0 | Will not burn | Will not burn under typical fire conditions, including intrinsically noncombustible materials such as concrete, stone and sand. |
Paraphrased summary of the NFPA 704 criteria. Check the current edition of NFPA 704 for the authoritative text; ratings for specific products come from the supplier or the facility.
Points to remember
- Flash point and boiling point come from Section 9 of the SDS, and the GHS flammable liquid category is in Section 2.
- Water-based products with a very high water content often have no flash point and rate 0, but a product that leaves a flammable residue or contains solvent can still burn. Check the SDS.
- NFPA 704 numbers run the opposite way to GHS categories. See NFPA 704 versus GHS.
Where to find the information on an SDS
Section 9 lists the flash point, initial boiling point and flammability limits. Section 5 covers fire-fighting measures, and some SDSs give an NFPA flammability number in Section 16. See SDS Section 9 and Section 16 for optional NFPA and HMIS ratings.
Related GHS pages
Frequently asked questions
What does the red number on an NFPA diamond mean?
The red number is the flammability rating from 0 to 4. A 4 means the material vaporizes rapidly or is readily dispersed in air and burns readily, while 0 means it will not burn under typical fire conditions.
What flash point is NFPA flammability 3?
A liquid is rated 3 if its flash point is below 23 °C (73 °F) and its boiling point is at or above 38 °C (100 °F), or if its flash point is from 23 °C up to below 38 °C. Gasoline and acetone are commonly published at 3, though ratings vary by source.
Is NFPA flammability 4 the same as GHS flammable liquid Category 4?
No. NFPA 4 is the most flammable rating. GHS Category 4 flammable liquids are the least flammable of the four liquid categories. The two scales run in opposite directions.