Fire Extinguisher Classes Explained

What do the fire extinguisher classes A, B, C, D, and K mean?

Published June 22, 2026 · Last updated July 26, 2026

Short answer: Fire extinguisher classes match an extinguisher to the kind of fuel that is burning: Class A is ordinary combustibles, Class B is flammable liquids, Class C is energized electrical, Class D is combustible metals, and Class K is cooking oils. NFPA 10 sets the classification and OSHA 29 CFR 1910.157 requires employers to select and distribute extinguishers by hazard. Using the wrong class — water on a grease or electrical fire — can make the fire worse.

Planning your whole home? See the four-layer home fire safety checklist that ties extinguishers, alarms, and escape together.

Every portable fire extinguisher is built and rated to fight specific fuels, and the letter classes printed on the label are the key to matching the unit to the hazard. The classification system comes from NFPA 10, Standard for Portable Fire Extinguishers, and OSHA 29 CFR 1910.157 makes employers responsible for selecting, mounting, and maintaining extinguishers based on the fire classes present in each work area. Get the fire extinguisher class wrong and the consequences are immediate: water conducts electricity, and a stream of water hitting burning cooking oil can erupt into a fireball.

This guide decodes all five fire extinguisher classes, explains the numerical ratings that appear in front of the A and B letters, and shows how the agent inside the cylinder determines which classes it can fight. Whether you are equipping an ABC dry-chemical extinguisher for a shop or a Class K extinguisher for a commercial kitchen, the class is the first decision you make.

Why this matters.
Choosing the wrong fire extinguisher class is not a paperwork error — it is a flash-fire risk. Discharging a water or standard ABC unit onto a Class K cooking-oil fire can splash burning grease and spread the fire, which is why commercial kitchens are required to carry wet-chemical Class K units. OSHA cites employers under 29 CFR 1910.157 when extinguishers are missing, mismatched to the hazard, or unmaintained, and the National Fire Protection Association reports that cooking equipment is the leading cause of structure fires.

What the fire extinguisher classes mean

Fires are grouped by fuel because each fuel reacts differently to an extinguishing agent. The five U.S. classes under NFPA 10 are A, B, C, D, and K. An extinguisher may be rated for one class or several — a typical home or shop unit is rated A, B, and C — and the rated classes are shown as letters and pictographs on the nameplate.

Class A — ordinary combustibles

Class A covers wood, paper, cloth, cardboard, trash, and most plastics. These fires are extinguished primarily by cooling with water or by smothering. Class A is the only class that carries a water-based rating tied to the cylinder's effective water content.

Class B — flammable liquids and gases

Class B covers gasoline, diesel, oil, grease, solvents, paints, and flammable gases. Water is ineffective and often dangerous on Class B fuels because it spreads the liquid. Class B agents work by smothering or interrupting the chemical chain reaction.

Class C — energized electrical equipment

Class C is any fire involving energized electrical equipment — panels, motors, wiring, and appliances. The agent must be non-conductive. Once power is removed, the fire usually reverts to a Class A or B fire.

Class D and Class K

Class D covers combustible metals such as magnesium, titanium, sodium, and lithium, and requires a specialized dry-powder agent. Class K covers cooking oils and animal/vegetable fats in commercial kitchens and uses a wet-chemical agent that saponifies the oil to smother it.

Source: NFPA 10, Standard for Portable Fire Extinguishers. Class C/D/K carry no numerical rating.

How to read the rating on the label

The letters tell you which fires a unit fights; the numbers tell you how much. A rating like 2-A:10-B:C packs three pieces of information into one line. Only Class A and Class B carry numbers — C, D, and K are letter-only because their effectiveness is about the agent, not a measured capacity.

The A number

The number before the A indicates relative Class A extinguishing capacity. Each unit of A is roughly equivalent to 1.25 gallons of water, so a 2-A unit has about the effective capacity of 2.5 gallons of water on ordinary combustibles.

The B number

The number before the B approximates the square footage of a flammable-liquid fire that a trained non-expert can extinguish — a 10-B rating corresponds to roughly 10 square feet. A higher B number means more flammable-liquid coverage.

How the agent decides the class

What is inside the cylinder determines the classes it is rated for. Matching agent to hazard is the practical core of NFPA 10 selection.

Multipurpose vs. ordinary dry chemical

Monoammonium-phosphate dry chemical is the multipurpose ABC agent rated for A, B, and C. Sodium- or potassium-bicarbonate dry chemical is rated B and C only. ABC powder is mildly corrosive, which is why server rooms and labs often prefer clean agents or CO2.

CO2, water, wet chemical, and dry powder

Carbon dioxide is rated B and C and leaves no residue. Plain water and water-based units are Class A only. Wet chemical is the Class K kitchen agent. Class D fires require a specific dry-powder agent matched to the metal.

Matching the class to the space

Most general workplaces and homes are covered by a multipurpose ABC unit, but specific hazards demand specific classes. A welding shop with magnesium needs Class D on hand; a restaurant line needs Class K next to the cooking equipment in addition to ABC coverage elsewhere; a paint-mixing room is a Class B priority. Stock a rechargeable extinguisher where reuse and annual service make sense, and consider an extinguisher cabinet to protect and mark the unit in public spaces.

Using an extinguisher safely (the PASS method)

Even the correct class is only useful if it is discharged correctly and early. The PASS method — Pull, Aim, Squeeze, Sweep — is the standard discharge technique. Fight only small, incipient-stage fires with a clear exit behind you; if the fire is spreading, the room is filling with smoke, or you are unsure of the class, evacuate and call the fire department instead.

Common dangerous mismatches

The classes exist precisely because some agent-fuel combinations are hazardous. Never use water or a standard ABC unit on a Class K grease fire, never use water on a Class C electrical fire, and never use a water or ABC unit on burning combustible metal. When in doubt about an electrical fire, de-energize the equipment first if it is safe to do so, which converts it to an ordinary fire.

Worked example: equipping a home workshop and kitchen

Here is how the fire extinguisher classes drive a real buying decision for a household with an attached garage workshop and a kitchen, using units listed on this site:

1. Identify the fire classes in each space. The workshop has ordinary combustibles (Class A), fuels and solvents (Class B), and energized power tools and chargers (Class C). The kitchen adds cooking-oil risk (Class K). That maps to ABC coverage everywhere plus a dedicated kitchen unit.
2. Choose a multipurpose ABC unit for general coverage. A rechargeable First Alert HOME2PRO 2-A:10-B:C rechargeable extinguisher covers the workshop's A, B, and C hazards. Browse the full ABC dry-chemical extinguisher range to size up or down.
3. Add a dedicated kitchen unit. Mount a kitchen-rated unit such as the Kidde 711A kitchen fire extinguisher near (but not directly over) the range. For commercial cooking volumes, step up to a Class K extinguisher.
4. Mount each unit on a clear escape path. Place extinguishers near exits so you are never trapped behind the fire to reach one. An extinguisher cabinet protects and marks the unit in shared spaces.
5. Train everyone on PASS and limits. Pull the pin, Aim at the base of the flames, Squeeze the handle, and Sweep side to side. Fight only small fires with a clear exit; otherwise evacuate and call 911.

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The same class-first logic scales from a single home to a multi-building facility. Start from the fire extinguishers catalog and the best fire extinguishers guide to match each rated class to the hazards you actually have.

The class letter is half the label — the numbers are the other half

Knowing an extinguisher is rated Class A or B answers what it can fight; the numeric prefix (the "2A" and "10B" in a 2A:10BC label) answers how much fire. The two-part decode has its own reference: what 2A:10BC actually means — size equivalence for A ratings, square-feet coverage for B. Buy the class for the hazard, then size the number for the space.

Frequently asked questions

Where should fire extinguishers be mounted?

Extinguishers should be mounted on a clear escape path, near exits, and within the travel distances set by NFPA 10 — generally no more than 75 feet to a Class A unit and 30 to 50 feet to a Class B unit, depending on rating. Keep them visible, unobstructed, and at a height anyone can lift safely.

What do the numbers before the class letters mean?

Capacity. The A-number is a water-equivalence multiple (each 1A ≈ the quenching power of 1.25 gallons of water) and the B-number approximates the square footage of a flammable-liquid fire a trained operator can handle. Class picks the chemistry; the number sizes it to the room — the full decode lives in our ratings reference.

Further reading on this site

• Fire extinguishers — the full catalog across every rated class.
• ABC dry-chemical extinguishers — multipurpose A:B:C units for general workplaces and homes.
• Class K kitchen extinguishers — wet-chemical units for cooking-oil fires.
• Rechargeable extinguishers — refillable units worth servicing annually.
• Best fire extinguishers (2026) — editor picks by use case and rated class.
• OSHA 1910.157 explained — the employer requirements for portable fire extinguishers.
• OSHA flammable cabinet requirements — storing the Class B fuels that drive extinguisher selection.
• First Alert HOME1 rechargeable extinguisher — a compact 1-A:10-B:C unit for apartments and small spaces.
• Kidde 3-A:40-B:C high-capacity extinguisher — more reach and capacity for garages and workshops.
• OSHA 1910.132 PPE requirements — the general PPE rule that frames hazard assessment.
• Best smoke detectors (2026) — pair early detection with the right extinguisher class.

Related reference

Closely related on this site: fire extinguisher inspection guide, fire extinguisher placement requirements, fire extinguisher types explained, how to use a fire extinguisher (pass method), home fire safety: complete checklist, equipment & escape plan (2026, and what is osha 29 cfr 1910.157? portable fire extinguisher requirements.

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WC Safety is an independent, affiliate-supported review site. It is not a retailer: it holds no inventory, takes no orders, and earns only from qualifying purchases through clearly marked links — which never changes what a product is rated to do. Standards language is taken from the regulation text directly, and ratings are reported as the manufacturer publishes them. We run no laboratory and perform no testing of our own. Where published sources disagree, we say so and plan on the conservative figure rather than the flattering one.

Our methodology

Figures come from the regulation and the published specification, in that order. Derated numbers are calculated, not estimated. Nothing here is presented as a measured result, because we measure nothing.

Researched and written by Steven Eaton, editor of WC Safety. Steven holds no safety certification and does not test products; this page compares what manufacturers and regulators publish, with the gaps in that record marked. Last reviewed August 2026.

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