Fire Protection Water Tank Sizing
Fire Protection Water Tank Sizing Guide
Align Storage Capacity with Fire Flow, Duration & NFPA 22 Requirements
When a municipal water supply can’t provide enough flow or reliability, a dedicated fire protection water tank keeps your sprinklers, standpipes and hydrants supplied when it matters most. This guide walks through the key concepts behind fire water tank sizing—fire flow, duration, refill, and NFPA 22 requirements—so you can have a more informed conversation with your fire protection engineer and authority having jurisdiction.
Why Fire Protection Water Tank Sizing Matters
When public water isn’t available or can’t provide enough fire flow, a dedicated fire protection water tank becomes the primary water supply for sprinklers, standpipes and private hydrants. If the tank is too small, the system may not deliver the flow and duration required by code or the authority having jurisdiction (AHJ). If it is oversized, you may be paying for capacity you will never use.
The goal of fire water tank sizing is simple: provide enough usable water at the right pressure to meet the required fire flow for the full design duration. The details are governed by NFPA standards such as NFPA 22 (water tanks), NFPA 13 (sprinklers), NFPA 14 (standpipes) and NFPA 20 (fire pumps), along with local fire codes and insurance requirements.
Important: This guide explains the concepts behind fire water tank sizing. Actual sizing should always be performed and stamped by a qualified fire protection engineer and approved by your AHJ.
Step 1 – Identify the Codes, Standards & Stakeholders
Before anyone starts calculating tank volume, the design team should confirm which codes and standards apply to the project. Typical references include:
- NFPA 22 – Standard for Water Tanks for Private Fire Protection (tank design and sizing).
- NFPA 13 – Sprinkler system design and water demand.
- NFPA 14 – Standpipe system design and demand (if present).
- NFPA 20 – Fire pumps and suction requirements.
- Local fire code, building code and any insurance carrier guidelines.
NFPA 22 states that the size and elevation of tanks are determined by the required fire flow, duration and pressure for the attached fire protection system(s). Local code officials and insurance representatives may add their own requirements on top of these standards.
Step 2 – Define the Fire Protection Systems Your Tank Must Support
The next step is to identify which systems will draw from the fire water storage tank. Common combinations include:
Sprinkler-Only Supply
The tank supplies one or more automatic sprinkler systems. In this case, the sprinkler demand used for hydraulic calculations typically governs the tank size.
- Light, ordinary or high-hazard sprinkler demand.
- ESFR or storage sprinkler designs with higher flows.
Sprinklers + Standpipes
In many mid- and high-rise buildings, the water tank and fire pump supply both sprinklers and standpipes. The more demanding of the two systems controls the required fire flow and duration.
- Combined sprinkler/standpipe risers.
- Separate standpipe zones for stairwells or large floor plates.
Private Hydrants & Site Fire Flow
At remote sites without public hydrants, the tank may also be required to provide water for hose streams and private hydrants to meet site fire flow requirements.
- Industrial plants, warehouses and tank farms.
- Rural sites, campuses and facilities without reliable public water.
Knowing which systems are tied to the tank allows the engineer to determine the governing water demand and design duration.
Step 3 – Determine Required Fire Flow & Duration
Fire flow is the rate at which water must be delivered to control or suppress a design fire. For sprinkler and standpipe systems, this is typically calculated using the applicable NFPA standard (NFPA 13 or NFPA 14) and verified through hydraulic calculations. For site fire flow and hydrants, the fire department or local fire code usually defines the required rate and duration.
In simplified terms, the base storage volume for a dedicated fire protection tank is often calculated as:
Required Fire Flow (gpm) × Required Duration (minutes) = Base Gallons Needed
For example, a system that requires 1,500 gpm for 90 minutes would have a base water demand of 135,000 gallons. From there, the engineer considers refill, usable volume, freeboard and other factors.
Step 4 – Account for Reliable Refill & Net Usable Volume
Most modern codes and standards allow the use of reliable, automatic refill to reduce the required storage volume. NFPA 22 states that the stored water plus reliable refill over the design duration must meet the system demand. In other words, the tank does not always need to hold the entire demand on its own if it can be refilled fast enough during a fire event.
Two important concepts are:
- Refill rate: the proven flow rate from a well, municipal connection or other source that can replenish the tank during a fire.
- Net usable capacity: the volume of water above the outlet or suction level and below the overflow, excluding dead volume and freeboard.
Fire protection engineers often size the tank for the deficiency between required demand and reliable refill over the design duration, while ensuring the net usable volume meets code requirements.
Step 5 – Special Case: Break Tanks
In some projects, a break tank is used between a public water main and the fire pump. A break tank acts as a buffer when the municipal supply cannot directly meet the pump’s suction requirements but can refill a smaller tank over time.
In this case, NFPA 22 requires a minimum tank volume based on the fire pump rated capacity and the time the tank needs to support the pump at 150% of its rated flow. Your fire protection engineer will determine whether a full fire protection storage tank or a smaller break tank is appropriate for your project.
Step 6 – Consider Elevation, Pressure & Site Conditions
Sizing a fire protection water tank is not just about volume. The elevation and location of the tank also affect fire pump suction conditions, available pressure and exposure to nearby structures.
- Tank elevation: gravity head and suction conditions at the fire pump.
- Seismic, wind and snow loads: structural demands on the tank and foundation.
- Access and clearances: fire department access, inspection and maintenance.
- Exposure protection: separation from buildings and other hazards, as required by NFPA 22 and local codes.
These site-specific factors are incorporated into the tank design along with volume.
Example – Conceptual Fire Water Tank Sizing
Every project is unique, but the example below illustrates how the concepts in this guide come together. Actual projects may require more detailed analysis and multiple demand scenarios.
Sample Facility
- High-piled storage warehouse with ESFR sprinklers.
- Remote site with no reliable municipal water supply.
- Dedicated fire protection water tank and fire pump.
Design Assumptions
- Sprinkler system demand: 2,000 gpm for 90 minutes (from hydraulic calculations).
- Hose stream allowance: 250 gpm for 90 minutes.
- Required fire flow: 2,250 gpm total.
- No reliable refill during a major fire event.
Conceptual Tank Volume
Required tank volume (ignoring refill) would be:
2,250 gpm × 90 minutes = 202,500 gallons
The engineer then adds allowances for unusable volume at the bottom of the tank, freeboard at the top, and any additional water the AHJ requires for site fire flow or multiple events. This often results in a nominal tank size somewhat larger than the simple flow × duration calculation.
Tip: When you request pricing on a fire protection water tank, share your fire flow, duration and refill assumptions. This helps ensure the tank proposal aligns with your engineer’s design.
Fire Protection Water Tank Sizing – FAQs
Who is responsible for sizing the fire water tank?
The fire protection engineer of record is typically responsible for calculating the required fire water storage volume and documenting it on the design drawings. The AHJ and insurance carrier review and approve these calculations as part of the permitting process.
Can I reduce tank size by using municipal water as refill?
In many cases, yes—if the municipal supply can reliably refill the tank fast enough during a fire. The refill rate must be documented and accepted by the AHJ. NFPA standards generally allow the stored water plus proven refill over the design duration to meet the demand, as long as the net usable volume of the tank is adequate.
Does the tank need to include water for both sprinklers and hydrants?
It depends on how your system is arranged and what your local fire code requires. On some projects the fire protection tank is sized only for the sprinkler and standpipe demand; on others, additional volume is included for site fire flow and hose streams. Your fire protection engineer and AHJ will determine which scenarios must be covered.
How do I know if my existing tank is large enough?
If you are changing occupancies, storage heights or system types, the original water supply calculations may no longer be valid. A fire protection engineer can review your existing system, re-run the hydraulics and compare the required volume to your tank’s net usable capacity.
Need help selecting a fire protection water tank for your project? Our tanks are engineered to support NFPA-based designs and real-world site conditions for warehouses, plants, campuses and remote facilities.