Understanding the water flow capacity of fire hydrants is critical for municipal planning, fire department operations, and insurance rating purposes. This calculator determines the discharge flow rate in Gallons Per Minute (GPM) based on the physical properties of the hydrant outlet and the pressure measured during a flow test.
The Hydraulic Formula
This calculator uses the standard theoretical discharge formula utilized by the National Fire Protection Association (NFPA) and water authorities worldwide:
Q = 29.83 × c × d² × √p
Where:
Q = Flow rate in Gallons Per Minute (GPM).
c = Coefficient of discharge (efficiency of the outlet shape).
d = Diameter of the outlet in inches (typically 2.5″ or 4.5″).
p = Pitot pressure (velocity head) in psi.
29.83 = A constant conversion factor.
Discharge Coefficients
The internal shape of the hydrant outlet affects how smoothly water exits. This is represented by the coefficient of discharge (c):
0.90 (Rounded): The most efficient shape, offering a smooth transition for water flow.
0.80 (Square): A sharp edge where the outlet meets the barrel, creating some turbulence.
0.70 (Protruding): The pipe protrudes into the barrel, causing significant turbulence and reducing flow efficiency.
NFPA 291 Color Coding
According to NFPA 291 "Recommended Practice for Fire Flow Testing and Marking of Hydrants," hydrants are color-coded based on their flow capacity at 20 psi residual pressure. This helps firefighters quickly identify the strongest water sources.
Class
Color
Flow Capacity (GPM)
Description
Class AA
Light Blue
1,500 +
Very Good Flow
Class A
Green
1,000 – 1,499
Good Flow for Residential
Class B
Orange
500 – 999
Adequate for Single Line
Class C
Red
< 500
Inadequate / Limited Supply
How to Perform the Test
To use this calculator accurately, you need to measure the pitot pressure. This is done using a hand-held Pitot gauge with a blade inserted into the stream of water coming out of the nozzle. The blade should be held in the center of the stream, at a distance away from the opening equal to half the nozzle diameter.
Note: While this calculator provides the theoretical discharge of a single orifice, comprehensive fire flow testing often involves calculating residual pressure drops across the water main system to determine total available fire flow.
function calculateFlowRate() {
// Get input values
var diameterInput = document.getElementById('diameter');
var pressureInput = document.getElementById('pressure');
var coefficientInput = document.getElementById('coefficient');
var resultBox = document.getElementById('result');
var flowResult = document.getElementById('flowResult');
var classResult = document.getElementById('classResult');
var d = parseFloat(diameterInput.value);
var p = parseFloat(pressureInput.value);
var c = parseFloat(coefficientInput.value);
// Validation logic
if (isNaN(d) || d <= 0) {
alert("Please enter a valid outlet diameter greater than 0.");
return;
}
if (isNaN(p) || p = 1500) {
className = "Class AA (Light Blue)";
badgeColor = "#3498db"; // Light Blue
} else if (roundedFlow >= 1000) {
className = "Class A (Green)";
badgeColor = "#2ecc71"; // Green
} else if (roundedFlow >= 500) {
className = "Class B (Orange)";
badgeColor = "#f39c12"; // Orange
} else {
className = "Class C (Red)";
badgeColor = "#e74c3c"; // Red
}
// Display Results
flowResult.innerHTML = roundedFlow.toLocaleString();
classResult.innerHTML = '' + className + '';
resultBox.style.display = "block";
// Auto scroll to result for better UX on mobile
resultBox.scrollIntoView({behavior: "smooth", block: "nearest"});
}