It is one of the most common questions in fire safety, and one of the least satisfying to ask a fire inspector, because the answer always starts with "Well, it depends." It does depend. But it depends on only four things, and once you know them you can work out the answer yourself with a floor plan, a tape measure and a calculator. This guide walks through the rules, then does the math for five real-world buildings, from a small shop to an aircraft hangar.
One promise before we start: no one will ask you to solve for x. Mostly you will be dividing by 1,500.
The four things that decide the answer
- What could burn. Ordinary combustibles like wood, paper and cardboard are Class A. Flammable liquids and gases are Class B. Most buildings have both, and each class is counted separately.
- How much of it there is. The fire code sorts areas into light, ordinary and extra hazard. An office full of desks is light hazard. A warehouse or shop is usually ordinary. A hangar, a paint shop, or anywhere with five gallons or more of flammable liquid in play is extra hazard.
- How far someone has to walk. Every rule is built on travel distance: the distance a person actually walks to reach an extinguisher, around desks, racks, forklifts and that pallet nobody has moved since 2019. Not a straight line on the floor plan.
- How big the extinguisher is. The numbers on the label, like 4-A:80-B:C, tell you how much area a unit covers and how far apart units can be. Our guide to fire extinguisher classes and ratings explains what those numbers mean.
The Class A rule: divide by the area each unit covers
For ordinary combustibles, NFPA 10 and the International Fire Code use the same table.
| Light hazard | Ordinary hazard | Extra hazard | |
|---|---|---|---|
| Smallest extinguisher allowed | 2-A | 2-A | 4-A |
| Floor area covered per "A" on the label | 3,000 sq ft | 1,500 sq ft | 1,000 sq ft |
| Most floor area any one extinguisher can cover | 11,250 sq ft | 11,250 sq ft | 11,250 sq ft |
| Maximum travel distance to an extinguisher | 75 ft | 75 ft | 75 ft |
So a 4-A unit in an ordinary hazard warehouse covers 4 × 1,500 = 6,000 square feet. A 10-A unit would cover 15,000, except no single unit may cover more than 11,250, so it tops out there.
That 11,250 is not a random number someone liked the look of. It is exactly the area of the largest square that fits inside a circle with a 75-foot radius, about 106 feet on each side. In other words, it is the 75-foot travel rule wearing a different hat.
Two more rules apply. Even a tiny building smaller than the table's coverage needs at least one extinguisher of the minimum size. And a bigger extinguisher can cover more area, but never stretches the 75-foot travel distance.
The Class B rule: it's all about distance
For flammable liquids, area does not matter. The rule is purely a rating and a distance.
| Hazard level | Flammable liquid present | Minimum rating and maximum travel distance |
|---|---|---|
| Light | Up to 1 gallon | 5-B within 30 ft, or 10-B within 50 ft |
| Ordinary | 1 to 5 gallons | 10-B within 30 ft, or 20-B within 50 ft |
| Extra | 5 gallons or more | 40-B within 30 ft, or 80-B within 50 ft |
You get a choice: smaller extinguishers placed closer together, or bigger ones placed farther apart. OSHA keeps the outer limit at 50 feet for Class B hazards in any case. Where liquid is spraying, running or under pressure, NFPA 10 adds a separate requirement for high-flow units of at least 10 lb that discharge at least one pound per second.
Now let's put the rules to work.
Example 1: The small shop
The building: a 2,000 square foot repair shop with a small parts cleaner holding about 3 gallons of solvent.
Class A: the building is smaller than any single unit's coverage, so the rule for small buildings applies: at least one 2-A extinguisher.
Class B: 3 gallons of solvent puts the parts cleaner in the ordinary hazard row, so it needs 10-B within 30 feet or 20-B within 50 feet.
The answer: one multipurpose ABC extinguisher rated at least 2-A:20-B:C, mounted near the exit and within 50 feet walking distance of the parts cleaner, covers both rules. Many ABC units carry higher ratings than that; check the label. If the shop is long and narrow, or the parts cleaner hides behind a wall of shelving, the walk may exceed 50 feet and you will need a second unit. This is where the tape measure earns its keep.
Example 2: The warehouse, where bigger isn't always fewer
The building: a 200 by 150 foot warehouse, 30,000 square feet, ordinary hazard, no significant flammable liquids.
Here is the area math for three common sizes:
| Extinguisher | Area each covers | Units needed by area | Units needed by 75-ft distance | You need |
|---|---|---|---|---|
| 2-A | 3,000 sq ft | 10 | 4 | 10 |
| 4-A | 6,000 sq ft | 5 | 4 | 5 |
| 10-A | 11,250 sq ft (capped) | 3 | 4 | 4 |
The distance column comes from dividing the floor into four 100 by 75 foot zones with a unit near the center of each. The farthest corner of each zone is then 62.5 feet away in a straight line, leaving some slack for the walk around the racks. Three units cannot manage it in this building: no matter where you put them, some corner ends up more than 75 feet from all three.
The lesson: upgrading from 4-A to 10-A units saves you one extinguisher, not three, because the 75-foot rule takes over. Somewhere a procurement manager just felt a small, sharp pain.
Example 3: The fuel island, where the answer is surprisingly small
The site: a fleet fueling island with four dispensers in a row spanning 60 feet, and an underground tank fill pipe 40 feet off to one side.
The International Fire Code requires an extinguisher rated at least 2-A:20-B:C within 75 feet of every pump, dispenser and tank fill-pipe opening. One unit mounted near the middle of the island is within about 32 feet of the farthest dispenser and 30 feet of the fill pipe.
The answer: one 2-A:20-B:C unit meets the code minimum. That is the minimum, not a recommendation. A single extinguisher in the middle of the island is no use if the fire is between you and it, so many sites put one at each end of the island, plus a wheeled unit where trucks load. Our fuel storage fire extinguisher guide covers tank farms and loading racks.
Example 4: The aircraft hangar, where Class B runs the show
The building: a 120 by 80 foot hangar, 9,600 square feet. Aircraft storage and servicing areas are treated as extra hazard.
Class A: extra hazard means at least 4-A per unit and 1,000 square feet per A. A single 10-A unit covers 10,000 square feet, enough for the whole floor, and every point is within 72 feet of the center. On paper, one unit.
Class B: extra hazard means 80-B within 50 feet. Units can't stand in the middle of the floor where the aircraft go, so they go on the long walls. Three on each wall, 20, 60 and 100 feet from one end, puts every spot on the floor within about 45 feet in a straight line. Six units rated 80-B or more.
The Class B rule needs six times as many units as the Class A rule. In a hangar, the fuel decides the count.
One geometric curiosity worth knowing: if the hangar were 100 feet deep instead of 80, wall-mounted units alone could never get the center of the floor within 50 feet, because the center would be 50 feet from both walls before you even walk sideways. That is one reason hangars rely on wheeled units that can be parked closer to the aircraft. Agent matters too, since ABC dry chemical corrodes aircraft. Our aviation fire extinguisher guide explains which agents belong near aircraft.
Example 5: The marina fuel dock, counted by the float
The site: a fuel dock with three floats, reached by a gangway from the pier.
The fire code doesn't use area or hazard tables here. It counts locations: one extinguisher rated at least 20-B:C on each float, plus one on the pier within 25 feet of the head of the gangway. If the fuel dock office sits within 25 feet of the gangway, the pier unit can be skipped.
The answer: four units, or three with a nearby office. If the pier has standpipe hose connections, each one gets its own 2-A:20-B:C unit on top of that. Salt air is hard on extinguishers, so our marina fire extinguisher guide covers corrosion-resistant models.
Where to put them once you know how many
Getting the count right is half the job. Where the units go matters just as much.
- On the way out. Put extinguishers along exit routes, so the person grabbing one is also moving toward safety. Nobody should have to walk toward a fire to reach the extinguisher meant to fight it.
- At the right height. NFPA 10 limits mounting height by weight: hand units of 40 lb or less are mounted with the top no higher than 5 feet, and heavier hand units no higher than 3½ feet. Wheeled units stand on the floor in a marked spot.
- Visible and unblocked. An extinguisher behind a stack of boxes is decor. Use signs where units are out of direct sight, and cabinets or covers where they are exposed to weather or damage.
- Walk it. The final test is the simplest: stand at the farthest corner of each area, walk to the nearest extinguisher the way a person actually would, and count the steps. A normal step is about 2½ feet, so 30 steps is roughly 75 feet.
Common mistakes
- "One per floor." The rules are based on area and distance, not floors. A large floor may need ten.
- Measuring in straight lines. Travel distance follows the walking path around racks, walls and equipment.
- Counting only Class A. Flammable liquids have their own distance rule, and it often drives the total.
- Counting a Purple K unit toward Class A. Purple K is rated B:C only. It is excellent on fuel and does nothing for the Class A count.
- Putting the only extinguisher next to the hazard. If the fire starts there, so does the problem.
- Forgetting it's the minimum. The code tells you the fewest you can have. Nobody has ever stood in front of a fire wishing they owned fewer extinguishers.
Before you buy
Count Class A by area, then check the 75-foot distance. Count Class B by distance. Take whichever needs more units for each area, and remember a single multipurpose ABC unit can satisfy both rules at once where its ratings meet both. Then walk the floor and adjust for real walls and real aisles. Finally, run the plan past your fire marshal or fire protection contractor, because the local code and the local inspector always have the last word.
When the count is done, browse our fire extinguishers by type, or start with ABC fire extinguishers for general coverage.
References
- NFPA 10, Standard for Portable Fire Extinguishers, Class A size and placement table, as adopted in California Code of Regulations Title 19, § 568, and summarized for International Fire Code Table 906.3(1) by EVstudio.
- NFPA 10 sections 5.4 and 5.5.4, Class B hazard levels and high-flow requirements, as summarized in Amerex Corporation Tech Tip 47.
- 29 CFR 1910.157, portable fire extinguishers.
- International Fire Code motor fuel-dispensing and marina provisions, as adopted in the Seattle Fire Code (2015 and 2018 editions).
- City of San Diego Fire-Rescue Department Policy E-09-8, mounting heights under the California Fire Code and NFPA 10.
This guide is educational. Requirements vary by jurisdiction and code edition. Confirm your extinguisher count and placement with your fire marshal or a licensed fire protection contractor.
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