Fire Extinguisher Calculator
Extinguisher A-units and count from floor area, NFPA 10.
What it does
Fire extinguishers answer to two rules at once: enough rating for the area and short enough travel from every point. NFPA 10 grants 1-A per 3,000 sq ft in light hazard with 2-A minimum singles, while OSHA caps Class A travel at 75 ft. A 10,000 sq ft office needs 4 A-units in at least 2 extinguishers.
The tool above runs the rating math with the travel cap beside. Enter floor area and hazard class for A-units and the 2-A count, then walk the 75 ft rule across the plan. Ordinary hazard halves coverage to 1,500 sq ft per A-unit.
The paper trail is public. OSHA 1910.157 states the 75 ft Class A distribution rule word for word, NFPA 10 publishes the coverage table universities reproduce, and Cornell mirrors the regulation text readably. Class B travel runs tighter at 50 ft for flammable liquids.
Rating math rarely finishes the job alone. Long narrow floors fail travel with two units at opposite ends, so space more smaller placements across the 75 ft grid. Mount visible, sign above, inspect yearly, and train hands before smoke.
How to use
- Enter the floor area in sq ft.
- Pick light or ordinary hazard.
- Read A-units first, then extinguisher count.
- Check 75 ft travel from every point.
- Add units for travel before rating.
- Mount and inspect per NFPA 10.
Formula
- area
- Floor area in sq ft.
- coverage
- Sq ft per A-unit: 3000 light, 1500 ordinary.
- 2-A
- Minimum single unit rating.
NFPA 10 table via university EHS reproductions; 75 ft Class A travel verbatim in OSHA 1910.157 (Cornell mirror).
Worked example
Example: 10,000 sq ft light hazard
Defaults: 10,000 sq ft, light hazard.
- Inputs: 10,000 sq ft, light hazard.
- Coverage: 1-A per 3,000 sq ft (NFPA 10).
- A-units: ceil(10,000 / 3,000) = 4.
- Extinguishers: ceil(4 / 2) = 2 x 2-A minimum.
- Travel cap 75 ft. Tool shows 4 A-units, 2 x 2-A minimum, 75 ft max travel.
Reference table
| Hazard | Per A-unit | Min single | Travel |
|---|---|---|---|
| Light | 3,000 sq ft | 2-A | 75 ft |
| Ordinary | 1,500 sq ft | 2-A | 75 ft |
| Extra | 1,000 sq ft | 4-A | 75 ft |
Limits
- Class A rating math only. B/C/K classes separate.
- Travel needs a floor walk, not just counts.
- Extra hazard uses 4-A minimums, simplified here.
- Local AHJ rules override this page.
Common mistakes
- Counting rating while ignoring 75 ft travel.
- One unit for a long floor.
- Light coverage on ordinary hazards.
- Mounting behind doors and clutter.
- Skipping yearly inspections.
Try it yourself
Quick checks. Answers sit tucked under each question.
10,000 sq ft ordinary hazard?
A-units ceil(10,000/1,500) = 7. Extinguishers ceil(7/2) = 4 x 2-A.
5,000 sq ft light hazard?
A-units ceil(5,000/3,000) = 2. Extinguishers ceil(2/2) = 1 x 2-A; add placements for travel.
Questions
How are A-units figured?
Ceiling of area over coverage: 1-A per 3,000 sq ft light, 1,500 ordinary. Here ceil(10,000/3,000) = 4.
How many extinguishers?
Ceiling of A-units over 2, since 2-A is the minimum single unit. Four A-units need 2 extinguishers.
How far apart can they be?
75 ft maximum travel to Class A units (OSHA 1910.157); Class B runs 50 ft.
What minimum rating?
2-A minimum single extinguisher for light and ordinary; 4-A for extra hazard.
Is rating math enough?
Yes. Travel distance usually demands more units than rating math alone; space them per both rules.
Does this cover all fires?
No. This page covers Class A spacing; kitchens, flammables, and electrical need their own classes.