What is Fire Flow?
Fire flow is the rate of water flow (measured in gallons per minute, or GPM) required to suppress a fire in a specific building. It is a critical factor in fire protection engineering, municipal water system design, and building code compliance. Fire departments, water utilities, and insurance companies use fire flow calculations to ensure adequate water supply is available for firefighting operations in every area of a community.
The needed fire flow (NFF) depends on the building's size, construction type, occupancy classification, and the presence of fire suppression systems such as sprinklers. Larger buildings, combustible construction, and high-hazard occupancies require higher fire flows. Buildings with approved automatic sprinkler systems can receive significant reductions in the required fire flow, as sprinklers control or suppress fires before they fully develop.
Fire Flow Formula (ISO Method)
Where:
- Ci = Construction coefficient (based on construction class, ranges from 1.5 for fire-resistive to 8.0 for frame construction)
- A = Effective area of the building in square feet (floor area times number of stories, capped per code)
- Oi = Occupancy factor (0.75 to 1.25 depending on hazard level)
The result is rounded to the nearest 250 GPM. Minimum NFF is 500 GPM; maximum is 12,000 GPM for any single building. The duration over which this flow must be maintained ranges from 2 to 4 hours depending on the calculated NFF.
Construction Classes
| Class | Description | Coefficient (Ci) | Examples |
|---|---|---|---|
| 1 | Frame | 8.0 | Wood-frame homes, barns |
| 2 | Joisted Masonry | 6.0 | Brick walls, wood roof/floor |
| 3 | Noncombustible | 4.0 | Metal building, steel frame |
| 4 | Masonry Noncombustible | 3.0 | Concrete block, steel roof |
| 5 | Modified Fire Resistive | 2.0 | Protected steel, limited hours |
| 6 | Fire Resistive | 1.5 | Reinforced concrete, 2+ hr rated |
Duration Requirements
The duration for which the calculated fire flow must be sustained depends on the NFF value:
| NFF (GPM) | Duration (hours) | Total Water (gallons) |
|---|---|---|
| 500 - 2,500 | 2 | 60,000 - 300,000 |
| 3,000 - 3,500 | 3 | 540,000 - 630,000 |
| 4,000+ | 4 | 960,000+ |
These duration requirements ensure that the water supply system (municipal mains, storage tanks, or cisterns) can sustain firefighting operations for the expected duration of a significant structural fire. Water systems must be tested periodically to verify they can deliver the required flow at adequate pressure (minimum 20 psi residual pressure at the hydrant).
Worked Example
A 3,000 sq ft, 2-story, joisted masonry (Class 2) residential building with no sprinklers:
Ci = 6.0 (Class 2)
Oi = 0.85 (residential)
NFF = 6.0 × √6,000 × 0.85 = 6.0 × 77.46 × 0.85 = 395 GPM
Rounded to nearest 250 = 500 GPM (minimum)
Duration = 2 hours | Total = 60,000 gallons
Frequently Asked Questions
What is a typical fire flow for a house?
Most single-family homes require a fire flow of 500-1,000 GPM for 2 hours. The exact requirement depends on the home's size, construction type, and distance from neighboring structures. The minimum NFF for any building is 500 GPM. Larger or older wood-frame homes may require 1,000-1,500 GPM due to the higher fire load of combustible construction.
How does sprinkler protection reduce fire flow?
A fully compliant NFPA 13 sprinkler system can reduce the needed fire flow by up to 50%, with a minimum NFF of 500 GPM after the reduction. Partial sprinkler systems or NFPA 13R residential systems typically qualify for a 25% reduction. Sprinklers control fires early, limiting the area of involvement and reducing the total water demand for suppression.
How many hydrants do I need?
The number of hydrants depends on the NFF and individual hydrant capacity (typically 500-1,500 GPM each depending on main size and pressure). A general guideline is one hydrant per 500-1,000 GPM of required fire flow. The first hydrant should be within 300 feet of the building, with additional hydrants within 600 feet. Local fire codes specify exact spacing requirements.
What is the difference between fire flow and available flow?
Fire flow (NFF) is the calculated water demand needed to fight a fire in a specific building. Available flow is the actual water delivery capacity of the municipal water system at a given hydrant location, measured during a flow test. The available flow must meet or exceed the NFF. If it does not, the building may require on-site fire water storage, a fire pump, or design modifications to reduce the NFF.