Hydrant flow test calculator
To calculate fire hydrant flow, use Q = 29.83 × c × d² × √p, where Q is flow in gallons per minute, c is the outlet discharge coefficient (0.90, 0.80, or 0.70), d is the outlet diameter in inches, and p is the pitot pressure in psi. Enter those three values below to get GPM instantly.
Single-outlet discharge. When you flow more than one outlet, calculate each and add the results for total flow.
The formula
- Q — flow in gallons per minute (gpm)
- 29.83 — the unit conversion constant
- c — outlet discharge coefficient (0.90 / 0.80 / 0.70)
- d — outlet diameter in inches
- p — pitot (velocity) pressure in psi
Discharge coefficient (c)
| Outlet | c |
|---|---|
Smooth / rounded outlet Outlet flush and rounded where it meets the barrel | 0.90 |
Square / sharp outlet Outlet flush with the barrel, square edges | 0.80 |
Projecting into barrel Outlet tube projects into the barrel | 0.70 |
Worked example
A 2½-inch smooth outlet (c = 0.90) reads a pitot pressure of 20 psi.
- d² = 2.5 × 2.5 = 6.25
- √p = √20 ≈ 4.472
- Q = 29.83 × 0.90 × 6.25 × 4.472
- Q ≈ 750 gpm
The hydrant is flowing roughly 750 gpm from that outlet at 20 psi pitot.
How to run the reading in the field
- 1Cap the flow hydrant and open it fully
Choose the flow hydrant, remove one outlet cap, and open the hydrant fully so it discharges through that single outlet. Flow to clear debris before reading.
- 2Read the pitot pressure
Hold the pitot gauge blade in the center of the stream, about one-half the outlet diameter away from the opening, and read the velocity pressure (p) in psi.
- 3Note the outlet diameter and coefficient
Record the outlet diameter (d) in inches and select the discharge coefficient (c) that matches the outlet type: 0.90 smooth/rounded, 0.80 square, or 0.70 projecting.
- 4Compute the flow
Apply Q = 29.83 × c × d² × √p to get the discharge in gpm. Repeat for each flowed outlet and add the results for total flow.
- 5Pair with the residual reading
Record the static and residual pressure at a separate test hydrant during the flow. The flow (Q) plus the pressures are what let an engineer compute available fire flow at any pressure.
The calculator does the arithmetic once you have field readings. A valid hydrant flow test still means flowing the hydrant, reading pitot and residual correctly, and documenting it to NFPA 291 / AWWA M-17 so an engineer or AHJ can rely on it.
Fire hydrant flow testingHydrant flow calculation, answered
How do you calculate fire hydrant flow?
Fire hydrant flow is calculated with the pitot formula Q = 29.83 × c × d² × √p: Q is flow in gpm, c is the outlet discharge coefficient (0.90 smooth, 0.80 square, or 0.70 projecting), d is the outlet diameter in inches, and p is the pitot (velocity) pressure in psi measured at the flowing outlet. Multiply it out to get gpm, and add outlets if you flow more than one.
What is the hydrant flow test formula?
The standard hydrant flow formula is Q = 29.83 × c × d² × √p. The constant 29.83 converts the units, c accounts for how the outlet is shaped, d² is the outlet diameter squared (inches), and √p is the square root of the pitot pressure (psi). It gives the discharge in gallons per minute from a single flowing outlet.
What is the discharge coefficient (c) in a hydrant flow test?
The discharge coefficient adjusts the formula for how water leaves the outlet. Use 0.90 for a smooth, rounded outlet where it meets the barrel; 0.80 for a square outlet flush with the barrel; and 0.70 for an outlet that projects into the barrel. Choosing the right c matters — it changes the calculated flow by up to about 20%.
What is pitot pressure?
Pitot pressure is the velocity pressure of water leaving a flowing hydrant outlet, read with a pitot gauge held in the center of the stream about half an outlet-diameter from the opening. It is the "p" in Q = 29.83 × c × d² × √p, and it is what converts a flowing outlet into a measured gpm.
Does this calculator replace a real hydrant flow test?
No. The calculator does the math once you have field readings, but a valid fire hydrant flow test still requires flowing the hydrant, measuring pitot and residual pressures correctly, and documenting the result to NFPA 291 / AWWA M-17 so an engineer or AHJ can rely on it. FLOW performs the physical test and provides the report — the calculator is just the arithmetic.