A liquid is considered flammable if its flash point is at or below 199.4 °F (93 °C) under current OSHA regulations. However, the answer depends on which regulatory system you’re looking at, because different agencies draw the line at different temperatures. The most commonly cited threshold in the United States is 100 °F (37.8 °C), which is the traditional dividing line between “flammable” and “combustible” liquids.
The Key Thresholds by Regulatory System
There is no single universal flash point that defines “flammable.” Three major systems in the U.S. each use slightly different cutoffs:
- OSHA (workplace safety): Any liquid with a flash point at or below 199.4 °F (93 °C) is classified as a flammable liquid. This is the broadest definition, broken into four categories based on severity.
- NFPA 30 (fire codes): Liquids with a flash point below 100 °F (37.8 °C) are “flammable” (Class I). Liquids with flash points from 100 °F up to 200 °F are “combustible” (Class II and III). This is the traditional distinction most people encounter.
- DOT (transportation): A liquid is classified as a Class 3 flammable material for shipping purposes if its flash point is 140 °F (60 °C) or below.
If you’re filling out a safety data sheet, labeling a container, or figuring out storage requirements, the system that applies depends on the context. Workplace handling falls under OSHA. Fire code compliance follows NFPA. Shipping and transport follow DOT rules.
OSHA’s Four Flammable Categories
OSHA’s system, aligned with the Globally Harmonized System (GHS), groups flammable liquids into four categories based on flash point and, for the most dangerous ones, boiling point. Lower categories are more hazardous.
Category 1 covers liquids with flash points below 73.4 °F (23 °C) and boiling points at or below 95 °F (35 °C). These are extremely volatile. Gasoline falls into this range, with a flash point around minus 45 °F. It releases ignitable vapors well below room temperature, which is why a spark near an open gas can is so dangerous.
Category 2 includes liquids with flash points below 73.4 °F (23 °C) but boiling points above 95 °F (35 °C). Ethanol and acetone are typical examples. They still ignite easily at room temperature but don’t evaporate quite as aggressively as Category 1 liquids.
Category 3 covers flash points from 73.4 °F to 140 °F (23 °C to 60 °C). Diesel fuel and kerosene generally fall in this range. These liquids won’t ignite at typical room temperature, but they become dangerous when warmed. OSHA adds an important rule here: if you heat a Category 3 liquid to within 30 °F of its flash point, you must handle it as if it were more hazardous.
Category 4 includes flash points from 140 °F to 199.4 °F (60 °C to 93 °C). These are the least hazardous flammable liquids under OSHA’s system. Some heavier oils and certain solvents fall here. The same 30 °F heating rule applies: if you warm a Category 4 liquid close to its flash point, stricter handling rules kick in.
Why the 100 °F Line Still Matters
Before OSHA adopted the GHS system, the dividing line between “flammable” and “combustible” in U.S. workplaces was 100 °F (37.8 °C). NFPA 30 still uses this distinction. Liquids below 100 °F are Class I flammable. Liquids from 100 °F to 140 °F are Class II combustible. Liquids at 140 °F and above are Class III combustible.
This older system is still deeply embedded in fire codes, building standards, and storage regulations. Many local fire departments and insurance companies reference NFPA classifications when determining how much of a given liquid you can store in a building, what type of cabinet is required, and how far containers must be from ignition sources. For example, OSHA limits storage of Category 1 and 2 liquids (and Category 3 liquids with flash points below 100 °F) to 60 gallons in closed containers inside a service station building. Category 3 and 4 liquids with higher flash points get a more generous 120-gallon limit per tank.
How Flash Point Is Tested
Flash point is measured using a closed-cup test, where a small sample of liquid is gradually heated in a sealed container. At set temperature intervals, a flame is introduced to the vapors above the liquid’s surface. The lowest temperature at which those vapors ignite briefly is the flash point. The most widely referenced method is the Pensky-Martens closed cup test, which can measure flash points up to 370 °C (698 °F).
Closed-cup testing is the standard for regulatory classification because it simulates real-world conditions better than open-cup methods. In a closed container, vapors concentrate above the liquid rather than dissipating into the air, giving a more conservative (and safer) measurement.
Flash Points Shift With Altitude and Pressure
The flash point listed on a safety data sheet is measured at standard atmospheric pressure, 101.3 kPa (sea level). At higher altitudes or lower pressures, flash points drop. This means a liquid that’s technically “combustible” at sea level could behave more like a flammable liquid in Denver or inside an aircraft fuel tank during flight.
Standard test methods include a linear pressure correction formula to account for this, but research has shown the relationship between pressure and flash point is actually more complex and nonlinear. For most everyday situations on the ground, the published flash point is reliable. But if you’re working with flammable liquids at high elevation or in reduced-pressure environments, the actual ignition risk may be higher than the label suggests. In some cases, a liquid’s fire hazard classification could effectively move up a category under reduced pressure.
Quick Reference for Common Liquids
To put these thresholds in practical terms, here’s where familiar liquids fall:
- Gasoline: Flash point around minus 45 °F (minus 43 °C). Category 1, extremely flammable.
- Ethanol: Flash point around 55 °F (13 °C). Category 2, flammable at room temperature.
- Diesel fuel: Flash point typically 125 °F to 180 °F (52 °C to 82 °C), varying by grade. Category 3 or 4.
- Kerosene: Flash point around 100 °F to 150 °F (38 °C to 66 °C). Category 3, sometimes Category 4.
The lower the flash point, the more easily a liquid catches fire. A liquid with a flash point below room temperature is releasing ignitable vapors right now, wherever it’s sitting. A liquid with a flash point of 150 °F needs to be heated significantly before it poses the same risk, but it’s still classified as flammable under OSHA’s system and requires proper labeling, storage, and handling.