The fire triangle is a model of the three things a fire needs: heat, fuel and oxygen (or another oxidizing agent). Remove any one and the fire cannot continue. NFPA describes the triangle as fuel (something that will burn), heat (enough to make the fuel burn) and air (oxygen), and says a fire burns until one or more of these is removed.
This page explains what fire is, what each side of the triangle does, how a fourth element turns the triangle into the fire tetrahedron, and what that means for putting fires out and preventing them at work. Sources were read on 1 Nov 2026.
What is fire?
Fire is combustion: a chemical process of oxidation that happens fast enough to produce heat and usually light, as a glow or a flame. Many materials oxidize slowly without being on fire. Rusting is a slow oxidation that releases so little heat that it is not combustion. Combustion is the fast version, where heat is produced faster than it escapes, so the fuel reaches its ignition temperature and keeps burning until the fuel or the oxygen is used up.
The three sides of the fire triangle
| Side | What it does | Workplace examples | How to control it |
|---|---|---|---|
| Heat | Raises the fuel to its ignition temperature and keeps the reaction going | Welding sparks, hot surfaces, friction, electrical faults, smoking, open flames | Hot work permits, electrical maintenance, no smoking areas, keeping ignition sources away from fuel |
| Fuel | Anything that can burn: solids, liquids or gases | Paper, wood, packaging, oils, solvents, fuel gases, dust | Housekeeping, safe storage of flammable liquids and gases, removing combustibles from work areas |
| Oxygen (oxidizing agent) | Combines with the fuel. In air it is almost always present | Atmospheric air, oxygen cylinders, some chemical oxidizers | Rarely controlled in general workplaces. Control oxygen enrichment and oxidizing chemicals where they are used |
Oxygen from the air is available almost everywhere, so fire prevention at work mostly means controlling fuel and heat. The Nevada State Fire Marshal’s fire prevention material makes the same point: the job of an inspector is to keep fuel and heat from meeting in an atmosphere of oxygen. Oxidizers other than oxygen exist, and some react with fuels more readily and violently than oxygen does.
The fourth element: the fire tetrahedron
NFPA adds a fourth component, the uninhibited chain reaction. This is the feedback of heat to the fuel that produces the gaseous fuel burning in the flame, which provides the heat needed to keep the fire going. With this fourth side the model is called the fire tetrahedron. NFPA 921 explains that the triangle on its own accounts for a flash of flame or for glowing combustion in a solid, such as embers, and that the chain reaction is what sustains flames.
The tetrahedron also explains how some extinguishing agents work: clean agents that replaced halon break up the chain reaction. For the full model, see the fire tetrahedron.
Four ways to put out a fire
NFPA lists four ways to put out a fire. Each takes away one side of the model.
| Method | Side removed | In practice |
|---|---|---|
| Cool the burning material | Heat | Water, applied to ordinary combustibles |
| Exclude oxygen | Oxygen | Smothering, for example with foam or a fire blanket on a small fire, or by closing a door on a room fire |
| Remove the fuel | Fuel | Shut off the gas or liquid supply, move combustibles away, let a fire burn out when it is safe to do so |
| Break the chemical reaction | Chain reaction | Some extinguishing agents interrupt the chain reaction, such as the clean agents that replaced halon |
The right agent depends on the fuel. Water on a cooking oil fire or on live electrical equipment can make things worse. See how many types of fire extinguisher there are, what a Class A fire is and how to use a fire extinguisher with PASS. To plan the response to a fire, see what RACE stands for.
Using the triangle to prevent fires at work
The triangle is a quick way to review any task. Ask which of the three sides is present, and which can be taken away. Examples:
- Welding and cutting. OSHA’s welding standard, 29 CFR 1910.252(a), takes away fuel first: if the object cannot be moved, all movable fire hazards nearby are moved to a safe place. If the hazards cannot be removed, guards confine heat, sparks and slag. If neither can be done, welding or cutting must not be performed. Fire watchers are required where appreciable combustible material is closer than 35 feet to the point of operation, or other listed conditions exist, and a fire watch is kept for at least half an hour after the work. See the hot work permit checklist.
- Storage. Keep flammable liquids and gases separate from ignition sources, and keep combustible stock away from heat sources.
- Housekeeping. Remove waste paper, packaging and dust, which are fuel.
- Electrical equipment. Maintain it and do not overload circuits, which are a heat source.
- Planning for a site. See fire safety planning in construction areas, the industrial fire safety checklist and the fire safety risk assessment.
The fire triangle in a UK fire risk assessment
In Great Britain the fire triangle is the first step of a workplace fire risk assessment. GOV.UK’s fire safety risk assessment guides tell the assessor to identify sources of ignition, sources of fuel and sources of oxygen, then remove or reduce them. Ignition sources include smokers’ materials, naked flames, heaters, hot processes such as welding, cooking equipment and faulty electrical equipment. Fuels include flammable liquids and solvents, packaging, stationery, plastics, textiles, LPG and waste such as shredded paper. Air is the usual source of oxygen. The guides add oxidizing chemicals and oxygen cylinders as extra sources. See the fire safety risk assessment guide.
Common misunderstandings
- “The fire triangle explains everything.” It explains the three conditions, but the tetrahedron is the better model for flaming fires.
- “Oxygen is easy to remove.” In normal workplaces it is not, so rely on controlling fuel and heat.
- “Removing one side always means the fire is safe.” A fire that has gone out can restart if all three sides come back, for example when a hot surface meets fuel again. Keep checking after a fire is out.
- “Any extinguisher will do.” The agent must suit the type of fuel.
Frequently asked questions
1. What are the three parts of the fire triangle?
Heat, fuel and oxygen. NFPA describes them as heat enough to make the fuel burn, fuel, and air (oxygen).
2. What is the fire tetrahedron?
The fire triangle plus a fourth side, the uninhibited chemical chain reaction. It explains how flames are sustained.
3. Can a fire burn without oxygen?
Not without an oxidizing agent. Oxygen from the air is the most common one, but other oxidizing agents can support combustion, and some react more violently than oxygen.
4. Which side of the fire triangle is easiest to control at work?
Usually fuel and heat, because oxygen is present in the air almost everywhere.
5. How do extinguishers use the triangle?
Different agents cool the fire, exclude oxygen, or interrupt the chain reaction. Which one to use depends on the type of fuel.
6. How is the fire triangle used in a fire risk assessment?
The assessor looks for sources of ignition, fuel and oxygen, then removes or reduces the ignition and fuel sources so they cannot meet. GOV.UK’s fire risk assessment guides start with exactly this step.
Related guides
- Fire tetrahedron
- Class A fires
- Types of fire extinguisher
- What PASS stands for in fire safety
- What RACE stands for in fire safety
- Fire safety in the workplace
- Fire extinguisher placement calculator
- Fire safety planning in construction areas
Sources
- NFPA, All About Fire: A Guide for Reporters (fire triangle, tetrahedron, four ways to put out a fire), read 1 Nov 2026.
- NFPA 921, section 5.5 Fire Tetrahedron, as quoted in the National Highway Fire Academy module Fire Behavior (2025 draft), read 1 Nov 2026.
- US Department of Energy, DOE-HDBK-1081-2014, Primer on Spontaneous Heating and Pyrophoricity (combustion, oxidizing agents, the fire tetrahedron), read 1 Nov 2026.
- Nevada State Fire Marshal, The Behavior of Fire and Fire Prevention (fire triangle, tetrahedron, prevention focus on heat and fuel), read 1 Nov 2026.
- eCFR, 29 CFR 1910.252(a) (fire prevention in welding and cutting), current to 29 Oct 2026, read 1 Nov 2026.
- GOV.UK, Fire safety risk assessment: offices and shops (step 1, identify fire hazards: sources of ignition, fuel and oxygen), read 1 Nov 2026.
- NIST glossary, Fire Triangle (source: NFPA 921, 2017), read 1 Nov 2026.