The Fire Extinguisher Capacity Calculator helps safety officers, facility managers, and homeowners determine the right size and type of fire extinguisher needed for a given space or fire hazard. By estimating the correct capacity, you can ensure effective fire suppression while complying with safety codes. This tool falls under the Fire Safety Calculator category.
Formula of Fire Extinguisher Capacity Calculator
1. Basic Agent Mass or Volume
A simple approach to extinguisher sizing is to calculate the amount of extinguishing agent required to control the expected fire size.
Extinguisher Capacity = Agent Mass or Volume Needed
2. Using Fire Class Rating
For general estimation, use the fire load and extinguishing factor:
Required Agent Mass = Fire Load × Extinguishing Factor
Where:
- Fire Load = total combustible material energy (in MJ or BTU)
- Extinguishing Factor = kg of agent needed per unit energy (from standard fire safety tables)
3. For Dry Chemical (ABC) Extinguishers
A practical rule for Class A hazards:
Minimum Extinguisher Rating = (Area in ft² × Fire Hazard Factor) / 3
Example:
A 2A extinguisher typically covers about 3,000 square feet of light hazard.
4. For CO₂ or Clean Agent Systems
For gaseous systems protecting an enclosed space:
Required Agent Weight = Protected Volume × Design Concentration × Gas Density
Where:
- Protected Volume = room or enclosure volume (m³ or ft³)
- Design Concentration = desired percentage by volume for effective suppression
- Gas Density = kg/m³ at storage pressure and temperature
Common Sizing Table
| Fire Class | Typical Extinguisher Size | Typical Coverage |
|---|---|---|
| Class A (Wood, Paper) | 2A, 4A, 10A | Up to 3,000 ft² (light hazard) |
| Class B (Flammable Liquids) | 10B, 20B, 40B | Based on square feet of liquid fire |
| Class C (Electrical) | Use agent rated for A or B with non-conductive media | Same as A or B |
| CO₂ Extinguisher | 5 lb, 10 lb, 15 lb | Small to medium equipment rooms |
Example of Fire Extinguisher Capacity Calculator
Scenario:
Office area of 2,400 ft², light hazard.
- Fire Hazard Factor for light hazard: use 1 (typical).
- Minimum Rating = (2,400 × 1) / 3 = 800
Divide by 100 to match the standard UL “A” rating:
800 / 100 = 8A
Choose one 8A extinguisher or multiple smaller ones (for example, two 4A units).
Most Common FAQs
This calculator provides practical estimates based on standard rules and guidelines. Always verify with local fire codes and consult a certified fire safety professional for critical applications.
Yes. Local fire safety regulations and building codes can vary. Always check with local authorities to meet legal and insurance requirements.
Yes. Many modern extinguishers are multi-class (ABC), meaning they can be used for ordinary combustibles, flammable liquids, and electrical fires. Always check the label to ensure compatibility with your fire hazards.