A fire cistern doesn’t have one standard capacity. The initial water requirement is generally calculated by multiplying the required fire-flow rate by the length of time that flow must be sustained.
Required fire flow in GPM × required duration in minutes = base fire-water volume in gallons
The base fire-water volume represents the water required based solely on fire flow and duration.
This formula is an educational starting point, not a final cistern specification. States and local jurisdictions may adopt different codes and NFPA editions, amend provisions, or impose additional fire-water requirements. Before selecting a tank, confirm the required flow, duration, usable capacity, and system configuration with the Authority Having Jurisdiction (AHJ) and qualified project professionals.
Fire Flow and Duration Establish the Base Fire-Water Volume
Fire flow is the required rate of water delivery for fire protection, typically expressed in gallons per minute (GPM). The required duration identifies how long that rate must be sustained. Together, these values establish the base fire-water volume.
Duration Determines How Long Water Must Remain Available
Duration is the period during which the required fire flow must be sustained. Storage volume increases in direct proportion to duration. Doubling the duration at the same flow rate doubles the base fire-water volume.
Example Fire-Water Calculations
The base fire-water volume is calculated by multiplying the required flow rate by the required duration.
| Required Fire Flow | Required Duration | Calculated Fire-Water Volume |
|---|---|---|
| 250 GPM | 40 minutes | 10,000 gallons |
| 500 GPM | 60 minutes | 30,000 gallons |
| 1,000 GPM | 60 minutes | 60,000 gallons |
These examples demonstrate the calculation only. The flow rates and durations are hypothetical and do not represent standard capacity recommendations or project-specific design guidance.
Account for Usable Water When Selecting the Tank
The calculated fire-water volume must be available as usable water. Nominal capacity describes a tank’s stated total volume, while usable capacity reflects the water available based on the complete system design. Tank size, layout, fittings, piping, and drafting configuration should be planned together to meet the approved usable-water requirement.
Coordinating these components helps the system support the required flow rate during drafting. Evaluating the tank and hydraulic design as a complete system helps ensure the stored water can be accessed at the required flow rate.
Why Fire Cistern Requirements Differ by Project
The required fire-water supply depends on the characteristics of the property, its fire-protection systems, available water sources, and the requirements adopted by the local jurisdiction.
Structure and Use: Building size, construction, occupancy, and specific fire hazards can affect the required fire flow and duration.
Exposure Conditions: The distance between buildings may influence the water needed to control a fire and protect nearby structures from exposure.
Existing Fire Protection: Approved sprinklers and other fire-protection systems may affect the calculation. Any reduction requires approval from the Authority Having Jurisdiction (AHJ).
Available Water Sources: Municipal systems, wells, and other sources may be considered only when their capacity and reliability have been verified.
Adopted and Local Requirements: The applicable codes and NFPA editions vary based on what each state or local jurisdiction has adopted. Counties, municipalities, and fire districts may also amend provisions or establish additional requirements.
These factors help determine when fire cisterns are required for new development and how much water a project must provide.
Fire Cistern Capacities Across Different Projects
Darco Tanks has supplied fiberglass fire cisterns for projects across the United States. These installations demonstrate how property types, storage requirements, tank dimensions, and site conditions can influence the selected configuration.
| PROJECT | LOCATION | FIRE CISTERN SIZE |
|---|---|---|
| Project | Location | Fire Cistern |
| Subdivision | Mountain Home, Idaho | 10,000 gallons; 8-foot diameter |
| School | Sanford, Colorado | 30,000 gallons; 10-foot diameter |
| Fire department | Santa Fe, New Mexico | 35,000 gallons; 10-foot diameter |
| Hotel | Canyon Lake, Texas | 40,000 gallons; 12-foot diameter |
| School | Pueblo, Colorado | 4 × 65,000 gallons; 12-foot diameter; 260,000 gallons total |
These completed installations show how project requirements can translate into different tank capacities, dimensions, and configurations. They are examples of completed work, not standard sizing recommendations for similar properties or locations. Darco Tanks supplied each system according to the project specifications and helped identify a suitable tank configuration.
Can a Refill Source Reduce the Required Storage Capacity?
A refill source may reduce the amount of water that must already be stored if it can reliably supply water during the required fire-flow period. Incoming water may be counted only if its performance has been verified and the AHJ approves it as part of the fire-water supply.
The evaluation should address:
- Verified flow rate and ability to sustain that rate during the required fire-flow period
- Year-round availability
- Dependence on electrical power
- Effects of drought, freezing, or seasonal conditions
- Recovery time after the cistern is used
If approved, only water available during the required fire-flow period may be credited toward the fire-water supply. Water delivered afterward affects system recovery and does not reduce the volume required for the fire event.
When Fire-Water Capacity Requires Multiple Tanks
A multiple-tank system may be appropriate when the required storage volume exceeds the practical capacity of a single tank. It can also provide greater flexibility when site dimensions, underground utilities, delivery routes, or installation access limit the size of tank the property can accommodate.
The required volume may be divided among multiple fiberglass tanks with available capacities and dimensions suited to the project. For example, Darco Tanks supplied four 65,000-gallon tanks for a school in Pueblo, Colorado, creating a system with 260,000 gallons of total nominal capacity.
Using multiple tanks does not reduce the approved fire-water requirement. The complete system must collectively provide the required usable water supply and satisfy the project’s design requirements.
Confirm Requirements Before Selecting a Fire Cistern
Determining the required water supply and selecting a tank are separate steps involving different parties.
1. Requirements and Approval
The Authority Having Jurisdiction (AHJ) establishes or approves the applicable fire-water requirements.
2. Calculation and System Design
Qualified project professionals perform the necessary calculations and coordinate the complete fire-protection design.
3. Tank Capacity and Configuration
Darco Tanks helps match the approved storage requirement with an appropriate fiberglass tank capacity and configuration.
Before selecting a tank, consult the local fire department, fire marshal, building or code officials, and qualified project professionals. Applicable codes, adopted editions, amendments, and approval procedures can vary by state and among jurisdictions within the same state.
Plan Around the Approved Fire-Water Requirement
The right fire cistern is not selected by a rule of thumb. It must provide the approved usable water supply through a system suited to the project.
Once your requirements are established, Darco Tanks can help match them with an appropriate fiberglass fire cistern capacity and configuration.
Planning a fire cistern project? Contact Darco Tanks or call 800-232-8660 to discuss your storage requirements, site conditions, and tank options.
