Somewhere between the fire marshal saying "you need more extinguishers" and a salesperson quoting one for every wall, there's the actual answer — and it comes from NFPA 10, the standard every U.S. fire code adopts for portable extinguishers. The count depends on three inputs: what kind of fire your space can produce (hazard classification), how big the floor is (area coverage), and how far someone has to walk to reach a unit (travel distance).
This guide walks the logic with worked examples. For your own building, our free extinguisher placement calculator runs the same rules in about a minute.
Step 1 — classify your hazard level
- Light (low) hazard: offices, classrooms, churches, hotel guest areas — small amounts of ordinary combustibles.
- Ordinary (moderate) hazard: retail, light manufacturing, warehouses of ordinary goods, parking garages, restaurant dining rooms.
- Extra (high) hazard: woodworking, vehicle repair, kitchens' service areas, manufacturing with flammable liquids, warehouses of high-piled or plastic-heavy stock.
Most buildings mix zones — an office (light) attached to a shop floor (ordinary or extra) is classified zone by zone, not by the front door.
Step 2 — apply the Class A coverage rules
For the ordinary-combustible (Class A) risk present in nearly every space, NFPA 10 works from the extinguisher's numeric rating (2-A, 4-A, and so on):
| Hazard level | Minimum rating per unit | Max floor area per unit of A | Max travel distance |
|---|---|---|---|
| Light | 2-A | 3,000 sq ft | 75 ft |
| Ordinary | 2-A | 1,500 sq ft | 75 ft |
| Extra | 4-A | 1,000 sq ft | 75 ft |
Two caps sit on top of the table: no single extinguisher can be credited with more than 11,250 sq ft regardless of rating, and the 75-foot travel rule wins whenever it demands more units than the area math does. In practice, travel distance — measured along real walking paths, around walls and racking — drives the count in most subdivided buildings.
Step 3 — add coverage for special hazards
- Class B (flammable liquids): shorter travel distances — 30 or 50 feet depending on the unit's B rating — wherever fuels, solvents or paints are stored or used.
- Class K (commercial kitchens): a K-rated extinguisher within 30 feet of the cooking appliances, as the required backup to the hood suppression system.
- Class C (energized electrical): not a separate count — choose multipurpose units (the standard 2-A:10-B:C dry chemical covers it) near electrical rooms and server areas.
Worked examples
A 5,000 sq ft single-floor office (light hazard). Area math: 5,000 ÷ 3,000 = 2 units at 2-A. Travel check: a typical corridor layout keeps everything within 75 ft of two well-placed units near the exits. Answer: 2 extinguishers, one at each exit path.
A 20,000 sq ft warehouse (ordinary hazard). Area math with 4-A units: each covers 4 × 1,500 = 6,000 sq ft, so four units. Travel check: racking forces walking paths, and hitting 75 ft everywhere across a 100 × 200 ft floor typically takes 4–6 positions. Answer: 4–6 extinguishers depending on rack layout — the travel rule, not the area rule, decides.
A restaurant. Dining room (ordinary): usually 1–2 units by area and travel. Kitchen: one Class K within 30 ft of the line, plus the hood system. Storage and mechanical areas: multipurpose ABC coverage. Answer: typically 3–4 units across the occupancy.
Mounting, signage, and the mistakes inspectors flag
Placement rules are simple but violations are constant: units ≤40 lb mount with the carry handle no more than 5 feet up, heavier units no more than 3.5 feet, always with 4 inches of floor clearance; every unit stays visible, signed where sightlines are blocked, and unobstructed — the classic citation is an extinguisher behind a stack of boxes. Every unit also needs its annual professional service and tag, plus a quick monthly gauge-and-pin check your own staff can log.
Run your building through the placement calculator for a count, then have a licensed extinguisher company confirm placement on their first service visit — here's what that service covers, and you can compare licensed companies near you. Your AHJ has the final word on count and placement.