How Do You Calculate Available Fire Flow at 20 psi?

Available fire flow at 20 psi is calculated by taking the flow measured during a hydrant flow test and scaling it by the ratio of two pressure drops raised to the 0.54 power: the drop from static pressure to 20 psi, divided by the drop from static pressure to the residual pressure read during the test. The result is the flow, in gallons per minute (gpm), that the main could deliver at the gauge hydrant while holding 20 psi, and it is the single number that water supply reports, sprinkler calculations and fire pump selections are built on.

The method comes from National Fire Protection Association (NFPA) 291, Recommended Practice for Water Flow Testing and Marking of Hydrants (2022), Chapter 4, which sets out both the pitot discharge formula that turns a gauge reading into flow and the projection to 20 psi. The American Water Works Association (AWWA) Manual M17 is the parallel reference for hydrant practice on the purveyor’s side. This article covers the inputs, the two formulas, a worked example computed step by step, the errors that produce a wrong number and the cases where the simple projection is not enough. How the test itself is run in the street is a separate question, covered in DGA’s article on how a hydrant flow test is run and read under NFPA 291.

What Inputs Does the Available Fire Flow Calculation Need?

Seven numbers go into the calculation, and every one of them comes from the field test except the outlet coefficient, which comes from looking at the shape of the hydrant outlet. The table lists each input, its units and where it comes from.

The static and residual readings have to come from the same gauge on the same hydrant, and that hydrant must not be the one flowing. A pitot reading taken at the gauge hydrant, or a residual read at the flow hydrant, produces a flow and a projection that do not describe the main. The gauge hydrant also has to sit between the supply and the flow hydrant, so the residual reading carries the friction loss the building’s service will see.

What Is the Method for Projecting the Test to 20 psi?

The method has two formulas and five steps. The first formula converts each pitot reading into a flow; the second scales the total measured flow to the flow that would exist at 20 psi residual. Both are standard hydraulics, and NFPA 291 (2022) Chapter 4 is the source of the method as applied to hydrant tests.

Figure 1: The Two Formulas Behind Available Fire Flow

Flow from a pitot reading: Q = 29.84 × c × d² × √p, where Q is the flow from one outlet in gpm, c is the outlet coefficient (0.9, 0.8 or 0.7), d is the outlet diameter in inches and p is the pitot pressure in psi. The total test flow Q_F is the sum of Q for every flowing outlet.

Projection to 20 psi: Q_R = Q_F × (h_r^0.54 ÷ h_f^0.54), where h_r = P_static − 20 psi (the drop the main is allowed to take) and h_f = P_static − P_residual (the drop the test produced). Q_R is the available flow at 20 psi residual.

Elevation correction: where the system served sits above or below the gauge hydrant, project to 20 psi from the hydrant readings first, then shift the whole supply curve down or up by 0.433 psi for every foot of difference, so the curve is expressed at the base of riser while the 20 psi floor stays a pressure in the main.

Figure 2: From Pitot Reading to Available Flow in Five Steps

1. Convert each pitot reading to flowApply Q = 29.84 × c × d² × √p to each flowing outlet with its own coefficient and diameter.

2. Sum the outletsAdd the flows from every outlet that was open when the residual was read. That sum is the test flow Q_F.

3. Compute the 0.54 power ratioTake h_r = static − 20 and h_f = static − residual, divide, and raise the quotient to the 0.54 power.

4. Project to 20 psiMultiply Q_F by the ratio from step 3. The product is the available flow at 20 psi, Q_R.

5. Correct for elevation and plotShift all three points, static, test point and 20 psi point, by 0.433 psi per ft to the base of riser, then plot them on N^1.85 graph paper or tabulate them.

The 20 psi residual is the reporting point because it is the lowest pressure most water purveyors will allow a main to be drawn down to while fire flow is taken. A positive pressure in the main keeps groundwater and cross-connections from being pulled into the drinking water supply and keeps the pipe from collapsing under external load. Many purveyors write 20 psi into their rules as the minimum a fire pump may draw the main down to, and some set a higher floor, which is why the projection is reported at 20 psi and then checked against the purveyor’s number on each project.

The fifth step matters more than it looks. A sprinkler system calculated at the base of riser 12 ft above the gauge hydrant sees 5.2 psi less static and residual than the hydrant gauge read, and the whole supply curve shifts down by 5.2 psi, while the flow available at 20 psi in the main does not change. The same correction runs the other way when the building sits below the street.

Worked Example: 74 psi Static, 61 psi Residual, One 2.5 in. Outlet

A flow test on a 10-inch municipal main in front of a 2-story commercial building reads 74 psi static at the gauge hydrant. With one 2.5-inch outlet open on the flow hydrant downstream, the pitot reads 38 psi and the residual at the gauge hydrant settles at 61 psi. The flow hydrant outlet is smooth and rounded where it meets the barrel, so the coefficient is 0.9. The gauge hydrant and the base of riser are at the same elevation, so no correction is applied.

The main can deliver about 2,230 gpm at the gauge hydrant while holding 20 psi. The three points that define the supply curve are 74 psi at 0 gpm, 61 psi at 1,034 gpm and 20 psi at 2,232 gpm. Those are the numbers a sprinkler designer plots the demand point against and a fire pump designer reads suction pressure from; DGA’s article on how to plot sprinkler demand against the supply curve takes the curve from here.

The arithmetic can be checked in a few seconds with the hydrant flow test calculator, which accepts the same seven inputs, sums multiple outlets and returns the projected flow and the plotted curve. On a test with two outlets open, each outlet gets its own pitot reading and its own line in step 3, and the two flows are added before step 6.

Why Does a Small Residual Drop Make the Projection Unreliable?

Suppose the same test had been run on a day when the flow hydrant was partly obstructed and the residual only fell to 71 psi, a drop of 3 psi, or 4.1 percent of static. The test drop h_f is now 3 psi instead of 13 psi, and the ratio (54 ÷ 3)^0.54 = 18^0.54 = 4.763. The projected flow becomes 1,034.7 × 4.763 = 4,928 gpm, more than double the first result from a test on the same main.

The second number is not evidence that the main got stronger; it is evidence that the projection has become hypersensitive to gauge error. A 1 psi error in the residual reading moves the first projection from 2,232 gpm to between 2,145 gpm and 2,331 gpm, a swing of about 190 gpm. The same 1 psi error on the 3 psi drop moves the second projection to between 4,219 gpm and 6,134 gpm, a swing of about 1,900 gpm, which is larger than the entire first projection.

That is why NFPA 291 (2022) Chapter 4 expects the residual to drop by a meaningful percentage of the static, at least 10 percent in the 2022 edition (the 2019 edition said 25 percent), and why AWWA Manual M17 practice looks for a drop of at least 10 psi. A test that does not reach the drop is not wrong, but its projection is not usable for design: the fix is to open a second outlet or a second hydrant and run the test again. The table and the figure below show both cases side by side.

Figure 3: Supply Curve from the Worked Example on N^1.85 Paper

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What Are the Common Errors in the Available Fire Flow Calculation?

Most wrong projections trace to one of eight errors, and the first three account for the majority DGA’s engineers find in review. The table lists each error, how it shows up in the numbers and the fix.

The coefficient error is the one that survives longest because it is invisible in the arithmetic. A spreadsheet that takes 0.9 as the default will produce a plausible number for every test, and only a photograph of the outlet or a field note about its shape reveals that 0.8 was the right value. Whether the test itself was set up correctly, from hydrant selection to gauge calibration, is covered in the hydrant flow test article linked above.

Where Does the Simple Projection Stop and a Professional Engineer’s Calculation Begin?

The 0.54 power projection assumes that the main behaves like a single pipe with a fixed friction characteristic between the source and the gauge hydrant, and that the flow test sampled that behavior. Several situations break that assumption, and in each the result needs a licensed Professional Engineer (PE) to build the supply curve from more than one test point or from a hydraulic model of the distribution system.

In each case the formula still does the arithmetic; what changes is which pressures and flows go into it and how many test points are needed to trust the slope. Whether the projected supply is enough on its own or the building needs a pump is the question DGA’s article on how engineers decide whether a fire pump is needed answers from the same curve.

Does the Calculation Change by Jurisdiction?

No. The pitot formula, the 0.54 power projection and the 20 psi reporting point are the same in New York State, New York City, New Jersey, Pennsylvania, Maryland and Delaware, because every jurisdiction adopts an edition of NFPA 13 whose hydraulic calculation chapter (Chapter 27 in 2019, Chapter 28 in 2022 and 2025) reads the supply from a flow test run and projected the same way. What varies is the water purveyor’s rule on how far a main may be drawn down and whether a fire pump may take suction from it: the New York City Department of Environmental Protection (DEP), the investor-owned and municipal purveyors in New Jersey, and the Washington Suburban Sanitary Commission (WSSC) in Maryland each publish their own conditions. DGA checks the purveyor’s minimum residual on each project and reports the available flow at 20 psi and, where the purveyor sets a higher floor, at that floor as well.

Frequently Asked Questions

This article is general information, not a water supply analysis for any project. Purveyor rules, code editions and local amendments change; confirm the adopted edition of NFPA 13 and NFPA 291 and the purveyor’s minimum residual pressure with the authority having jurisdiction before a system or pump is designed.