Understanding the NFPA Fire Flow Formula
The required fire flow rate ($F$) is the minimum water supply in **Gallons Per Minute (GPM)** needed to fight a fire in a given structure. The calculation is essential for water utility planning, fire department sizing, and building code compliance.
The Base Fire Flow Formula
This calculator uses the general formula derived from **NFPA 1, Fire Code** and **NFPA 1141, Fire Protection in Planned Urban, Suburban, and Rural Developments**, which is based on the building's size and construction type: $$\mathbf{F = 18 \cdot C \cdot \sqrt{A}}$$ The final result is multiplied by the **Exposure Factor ($\mathbf{E}$)** to account for adjacent hazards.
- **$\mathbf{F}$:** Required Fire Flow in GPM (Gallons Per Minute).
- **$\mathbf{18}$:** An empirical constant coefficient.
- **$\mathbf{C}$:** The **Construction Type Factor**, a decimal value from 0.6 (most non-combustible) to 1.0 (most combustible/wood frame).
- **$\mathbf{A}$:** The **Area in Sq. Ft.**, typically the total floor area or the area of the largest story.
- **$\mathbf{E}$:** The **Exposure Factor** (1.0 to 1.5) applied to the base flow rate.
Factor Definitions (NFPA 1, Table 50.3.2.2.1)
Construction Type Factor (C)
- **Type I & II (C=0.6):** Fire-resistive and non-combustible construction.
- **Type III (C=0.8):** Exterior-protected/ordinary construction (e.g., masonry walls, combustible interior).
- **Type IV (C=0.8):** Heavy Timber construction.
- **Type V (C=1.0):** Wood frame construction (most combustible type).
Exposure Factor (E)
- **E=1.0:** No or minimal fire exposure from adjacent properties.
- **E=1.25:** Moderate exposure due to building proximity.
- **E=1.5:** Severe exposure, or if the structure is contiguous (connected) to other buildings.
Important Disclaimer:
This calculation provides a theoretical minimum based on the general NFPA formula. It does **NOT** account for building contents, hazardous materials, specific occupancy classifications (e.g., storage, high-hazard), or local code amendments, which often increase the final required flow. **Always consult with the local Authority Having Jurisdiction (AHJ) and a certified fire protection engineer for official requirements.**