Fire Water Demand Calculation is used to determine the required flow rate and water storage capacity for a building’s firefighting system according to NFPA standards.
The calculation helps engineers size:
- Fire pumps
- Fire water tanks
- Underground piping
- Sprinkler systems
- Hydrant systems
Main NFPA Standards Used
Components of Fire Water Demand
The total fire water demand is generally:
Qtotal = Qsprinkler + Qhose stream
Where:
- Qtotal = Total fire water demand
- Qsprinkler = Sprinkler system flow
- Qhose stream = Hose reel / hydrant allowance
Step-by-Step Fire Water Demand Calculation
Step 1 — Determine Hazard Classification
According to NFPA 13, buildings are classified as:
Step 2 — Select Density and Design Area
NFPA 13 provides required sprinkler density.
Example for Ordinary Hazard Group 1:
Step 3 — Calculate Sprinkler Flow
Formula:
Q = D × A
Where:
- Q = Sprinkler demand (gpm)
- D = Density (gpm/ft²)
- A = Design area (ft²)
Example:
Q = 0.15 × 1500
Result:
Q = 225 gpm
Step 4 — Add Hose Stream Allowance
NFPA 13 requires additional hose demand.
Typical values:
For Ordinary Hazard:
Qhose = 250 gpm
Step 5 — Calculate Total Fire Water Demand
Formula:
Qtotal = 225 + 250
Result:
Qtotal = 475 gpm
Therefore:
- Required fire pump flow = 475 gpm minimum
- Fire tank must support this duration
Step 6 — Calculate Fire Water Storage Tank Capacity
NFPA requires water duration depending on hazard type.
Typical duration:
Tank volume formula:
V = Q × t
Where:
- V = Tank volume
- Q = Total demand
- t = Duration
Example:
V = 475 × 60
V = 28,500 gallons
Important Design Considerations
Fire Pump Selection
According to NFPA 20:
- Pump should meet flow and pressure requirements
- Include jockey pump
- Diesel or electric driver selection
Pressure Requirements
Hydraulically most remote sprinkler must receive:
- Minimum residual pressure
- Required sprinkler operating pressure
Hydrant Systems
Additional outdoor hydrant demand may be required by local civil defense authority.
Safety Factor
Engineers often provide:
- 5–10% additional tank capacity
- Future expansion allowance
Typical Fire Water Demand Values
Units Commonly Used
Simplified Example Summary
Key Engineering Objective
The purpose of fire water demand calculation is to ensure:
- Adequate water supply during fire emergency
- Proper fire pump selection
- Compliance with NFPA and local authority requirements
- Reliable operation of sprinkler and hydrant systems
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