Calculating Fire Flow: How to Size an NFPA 22 Suction Tank for a Commercial Sprinkler System

Quick Answer
To size a fire water tank, add your sprinkler demand and hose stream allowance in gallons per minute, then multiply by the required duration in minutes. NFPA 13 sets those three numbers by hazard class. NFPA 22 then decides how much of the tank actually counts, so buy for net capacity, not the nameplate number.

Fire flow is the rate of water, in gallons per minute, that a fire protection system needs to control a fire, held for a set number of minutes. For a sprinklered building fed by a tank, that rate is the sprinkler demand plus any hose streams the tank also supplies. Multiply the rate by the time and you get the volume your tank has to protect.

If your project is racing to file permits before year end, the fire tank is often the line item that slows plan review. Reviewers want the net capacity shown on the drawings, and a wrong guess now can mean a resubmittal in January. Here is how the math works, step by step.

What Is the Difference Between Fire Flow and Tank Capacity?

Fire flow is a rate. Tank capacity is a volume. A fire tank simply stores enough flow to last the required time. If a sprinkler system needs a certain number of gallons per minute for a certain number of minutes, the tank must hold at least that many gallons of usable water.

One warning on words. Fire codes also use "fire flow" for the water needed at hydrants for manual firefighting. That figure is set by your local fire code, not by NFPA 13, and it can be larger than the sprinkler demand. This guide covers the sprinkler side. Ask your fire marshal early whether your tank must cover hydrant flow too.

Who Sets the Numbers: NFPA 13, NFPA 22 or Your Fire Marshal?

Several documents touch a fire water tank, and each one owns a different piece of the answer. Knowing which is which saves a lot of back and forth with your design team.

Check which editions your jurisdiction has adopted. Many cities run a cycle or two behind the newest edition, and the tables can shift between editions. For a full overview of the tank standard itself, see our NFPA 22 guide to fire protection tanks.

Which Hazard Class Is Your Building?

NFPA 13 sorts spaces by how fast a fire is likely to grow and how much heat it is likely to release. There are five classes: Light Hazard, Ordinary Hazard Group 1, Ordinary Hazard Group 2, Extra Hazard Group 1 and Extra Hazard Group 2. The class sets three of the numbers you need: the design density, the hose allowance and the duration. The NFPA 13 annex gives examples, but your designer makes the call. Offices, schools and churches are typical light hazard spaces. Bakeries, laundries and canneries are typical Ordinary Hazard Group 1. Mercantile spaces fall in Ordinary Hazard Group 2, and the 2022 edition moved parking garages there too. Extra hazard covers spaces such as some combustible dust hazards and flammable liquid spraying. If your site handles serious quantities of flammable liquids, our guide to high-hazard industrial fire protection tanks covers the bigger picture.

*Starting tank volume = (density x design area + full hose allowance) x duration. It is a floor estimate for budgeting, not a design. Your engineer's hydraulic calculation will usually come out higher. High-piled and rack storage use separate NFPA 13 storage chapters and are not covered by this table.

Why a range for duration? NFPA 13 lets designers use the lower value when the waterflow alarm and supervisory devices are electrically supervised and monitored at a constantly attended location. Without that monitoring, plan on the higher value.

How Do You Calculate Fire Water Tank Capacity, Step by Step?

Step? The Fire Tank Formula

Net capacity (gallons) = (Sprinkler demand in GPM + Hose stream allowance in GPM) x Duration in minutes

Then add your designer's safety margin, and add the water the tank cannot deliver, to reach the nominal tank size you buy.

1. Get the sprinkler demand. Use the flow at the tank or pump suction from your hydraulic calculation. Before that exists, density x design area gives a floor.
2. Add the hose allowance the tank must supply. The NFPA 13 annex says a tank that serves sprinklers only can skip hose demand. If it feeds hydrants, standpipes or hose stations, include them.
3. Multiply by the duration. Use the minutes from your hazard class, at the higher end unless your alarms are monitored.
4. Add a safety margin if your designer wants one. NFPA 13 does not prescribe a margin. It is a judgment call that covers small changes before the final calculation.
5. Convert net to nominal. Add the water below the anti-vortex plate and the space above the overflow, then pick a real tank whose net capacity clears your number.

Worked Example: A Rural Bakery

A single-story commercial bakery outside city limits is classed Ordinary Hazard Group 1. The tank feeds the sprinklers and the site hydrant. Waterflow alarms are not remotely monitored, so the 90-minute duration applies.

• Sprinkler demand: 0.15 gpm/ft² x 1,500 ft² = 225 gpm (852 L/min)
• Hose allowance: 250 gpm (946 L/min)
• Total flow: 225 + 250 = 475 gpm (1,798 L/min)
• Net water: 475 gpm x 90 min = 42,750 gallons (161,826 L)
• With a 10 percent design margin: 47,025 gallons (178,009 L)

Picking the tank: a corrugated steel tank 24 ft (7.3 m) across with an 18 ft 4 in (5.6 m) eave height holds 61,388 gallons (232,379 L) nominal. At that diameter each foot of height holds about 3,350 gallons (12,681 L). Set aside about a foot at the bottom and a foot at the top, and roughly 54,700 gallons (207,062 L) of net water remains. That clears the 47,025 gallon target.

What changes the answer: if the alarms were monitored, 60 minutes would apply and the net water drops to 28,500 gallons (107,884 L) before margin. If the hydrant ran on the city main and the tank fed sprinklers only, the net water would be 225 x 90 = 20,250 gallons (76,655 L). Same building, three different tanks.

That last line is the point. Monitoring, hydrant supply and shared domestic use can each swing the tank size by tens of thousands of gallons. Settle those questions with your engineer before you price a tank.

Why Does Net Capacity Matter More Than the Number on the Tank?

NFPA 22 counts only the water a suction tank can actually deliver to the fire pump. For a suction tank, net capacity is the water between the inlet of the overflow and the level of the anti-vortex plate. Everything below the plate stays in the tank when the pump runs. Everything above the overflow drains away before it can be stored.

The anti-vortex plate sits over the suction outlet to stop the pump from pulling in air. Plan reviewers in Los Angeles, for example, check that the plate is at least twice the outlet diameter and at least 6 inches (152 mm) above the tank bottom, that the overflow inlet sits at least 2 inches (51 mm) below the top, and that the net capacity in gallons appears on the plans.

The practical rule: always ask for the net capacity on the tank drawing, and compare that number, not the nominal one, to your calculation.

Which Tank Materials Work for a Fire Pump Suction Tank?

NFPA 22 covers steel tanks (welded and bolted), concrete, wood, fiberglass and embankment-supported coated fabric tanks. For most commercial sprinkler projects the choice comes down to steel or fiberglass. Our comparison of the best water storage tanks for fire protection goes deeper on each material.

Planning a tank through the winter? In cold climates NFPA 22 calls for tank heating that keeps the water at or above 42°F (5.6°C). Your engineer will check whether your site's climate triggers that rule, and the heater shows up on the tank drawings.

When Should You Bring In a Fire Protection Engineer?

Always for the final design. This guide helps you check numbers and set a budget, but the stamped calculation and drawings come from a licensed professional. Bring one in on day one if any of these apply:

• The building has high-piled or rack storage, or uses storage-specific sprinklers.
• The site mixes hazard classes, such as offices beside a production floor.
• The tank will also feed hydrants, standpipes or domestic and process water.
• A fire pump is needed, which brings NFPA 20 into play.
• The site sits in a seismic or high-wind zone, or your AHJ asks for stamped structural drawings.

Tank Depot can supply stamped drawings and engineering documents with our steel fire protection tanks, which keeps the tank side of the submittal moving while your engineer finishes the system design.

The Bottom Line on Fire Tank Sizing

Fire tank sizing is three numbers and one rule. NFPA 13 gives you the sprinkler demand, the hose allowance and the duration. You add the flows, multiply by the minutes, and NFPA 22 tells you that only the water between the overflow and the anti-vortex plate counts. Get those right before year-end permit filings, and the tank stops being the thing that holds up your project.

Key Takeaways

• Tank size starts with one line of math: (sprinkler demand + hose allowance) x duration. NFPA 13 supplies all three numbers.
• Only the water between the overflow and the anti-vortex plate counts under NFPA 22, so the tank you buy must be bigger than the number you calculate.
• Your fire protection engineer runs the final hydraulic calculation and your fire marshal approves it. Use this guide to check the math and budget early.

Frequently Asked Questions (FAQ)