Hydrant Flow Calculator for Engineering | Discharge, Velocity and Available Flow

Calculator inputs

Static pressure (psi)

Pressure before any hydrant outlet flows.

Residual pressure (psi)

Pressure measured while the test outlet is flowing.

Pitot pressure (psi)

Pitot reading at the flowing outlet stream center.

Outlet diameter (inches)

Use the actual outlet or nozzle diameter.

Outlet type

Choose a preset or enter a custom coefficient.

Outlet coefficient (C)

Typical values: 0.90 smooth, 0.80 rounded, 0.70 square.

Flowing outlets

Use identical outlets for the total test flow estimate.

Target residual pressure (psi)

Common planning checks often use 20 psi.

Example data table

Static Pressure (psi)	Residual Pressure (psi)	Pitot Pressure (psi)	Outlet Diameter (in)	Coefficient	Outlets	Target Residual (psi)	Total Test Flow (GPM)	Available Flow @ 20 psi (GPM)
72	58	24	2.5	0.90	2	20	1,644.59	3,340.36
68	50	18	2.5	0.90	1	20	712.19	1,289.47
80	60	28	2.5	0.80	2	20	1,577.23	2,858.63

Formula used

1) Single outlet flow
Q = 29.84 × C × d² × √P

2) Total test flow
Q_total = Q × N

3) Available flow at target residual pressure
Q_available = Q_total × ((P_s − P_t) / (P_s − P_r))^0.54

4) Outlet velocity
V = 0.4085 × Q / d²

Where:

Q = single outlet flow in GPM
C = outlet coefficient
d = outlet diameter in inches
P = pitot pressure in psi
N = number of identical flowing outlets
P_s = static pressure in psi
P_r = residual pressure in psi
P_t = target residual pressure in psi
V = estimated outlet velocity in feet per second

How to use this calculator

Enter the static pressure before flowing any outlet.
Enter the residual pressure recorded during the hydrant test.
Enter the pitot pressure from the flowing stream measurement.
Provide the outlet diameter and select the outlet type.
Adjust the coefficient if your field condition needs a custom value.
Enter how many identical outlets are flowing.
Set the target residual pressure for the available flow estimate.
Press the calculate button to display results above the form.
Review the table, graph, and exports for reporting.

Frequently asked questions

1) What does this hydrant flow calculator estimate?

It estimates single outlet discharge, total test flow, available flow at a selected residual pressure, pressure drop, outlet velocity, and supporting reporting values.

2) Why are static and residual pressures both required?

Static pressure shows the system before flow starts. Residual pressure shows the system while water is flowing. Their difference helps estimate network performance under demand.

3) What is pitot pressure used for?

Pitot pressure measures stream impact pressure at the outlet. The calculator uses it with outlet diameter and coefficient to estimate actual discharge through the flowing opening.

4) Why does the calculator use an outlet coefficient?

The coefficient adjusts the discharge formula for outlet geometry and edge shape. Smooth outlets usually pass water more efficiently than rougher or square edged openings.

5) What does available flow at target residual mean?

It is an engineering estimate of how much flow the system could provide while maintaining the selected residual pressure, often used for planning and fire protection checks.

6) Can I use this for multiple flowing outlets?

Yes. Enter the number of identical flowing outlets. The calculator multiplies the single outlet flow to estimate the total test flow across those openings.

7) Why is the result block shown above the form?

That layout makes reports easier to read. After submission, the main answers appear immediately below the header, so users can review outcomes without scrolling first.

8) Are the CSV and PDF buttons useful for reporting?

Yes. CSV works well for spreadsheets and project records. PDF is useful for sharing a quick formatted summary with inspectors, engineers, or field teams.