Worked problem

Fire-Water Storage: Demand, Duration and Usable Volume

Size a fire-water tank for a 750 gpm demand over 90 minutes, add unusable volume, and find the water depth in a 30 ft diameter tank.

Fire Protection · Last reviewed October 17, 2026

Given

A fire-water tank must supply a combined demand of 750 gpm (for example, sprinklers plus hose streams) for 90 minutes. Treat the demand and duration as exercise inputs: the required values come from the applicable standard, the hazard classification and the authority having jurisdiction. The tank has 4,500 gal of unusable water below the suction outlet and 3,000 gal of unusable volume above the working water level (air gap and freeboard). The tank is a vertical cylinder of 30 ft diameter. Assume no refill during the event.

Find

The required water volume, the total tank volume to the overflow level, and the water depth.

Solution

Demand volume = 750 gpm × 90 min = 67,500 gal.
Add the unusable volume: 67,500 + 4,500 + 3,000 = 75,000 gal.
Tank area = π ÷ 4 × 30² = 706.9 ft². Each foot of depth holds 706.9 × 7.481 = 5,288 gal.
Depth for the demand = 67,500 ÷ 5,288 = 12.77 ft.
Depth of unusable water below the outlet = 4,500 ÷ 5,288 = 0.85 ft, and above the working level = 3,000 ÷ 5,288 = 0.57 ft.
Total depth from the tank floor to the top of the freeboard = 12.77 + 0.85 + 0.57 = 14.2 ft.

Answer

usable 67,500 gal  |  tank volume 75,000 gal  |  about 14.2 ft of water depth in a 30 ft diameter tank

Component	Volume (gal)	Depth (ft)
Unusable, below the outlet	4,500	0.85
Usable demand, 750 gpm × 90 min	67,500	12.77
Unusable, air gap and freeboard	3,000	0.57
Total	75,000	14.2

What changes the answer

Duration: at 120 minutes the demand is 90,000 gal and the tank volume 97,500 gal.
Demand: every 100 gpm adds 9,000 gal at 90 minutes.
Refill: a reliable refill source during the event may reduce the storage that the standard requires. This exercise does not count it.
Where the standard allows, other supplies may reduce the storage needed. Check the applicable edition.

Common mistakes

Using the gross tank volume as the available water. The water below the outlet and above the working level cannot be used.
Mixing gpm with hours instead of minutes, which is a factor of 60 error.
Treating one hazard type or duration as a universal rule. Required demand and duration depend on the occupancy and the standard.
Forgetting that the combined demand includes every item the water must supply at the same time.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

Questions

Is 90 minutes a standard duration?

It is an exercise value. Durations in standards vary with the hazard, and some occupancies need less or much more.

Why is there unusable water?

The suction outlet sits above the floor to avoid sediment and vortices, and a tank cannot be filled completely up to the overflow because of the air gap and the level control.

How does tank size relate to pump size?

The pump sets the flow rate (here 750 gpm). The tank sets how long that flow can be maintained.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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