function calculateFlowRate() {
// Get Inputs
var dInput = document.getElementById('pipeDiameter').value;
var dUnit = document.getElementById('diameterUnit').value;
var vInput = document.getElementById('flowVelocity').value;
var vUnit = document.getElementById('velocityUnit').value;
// Validation
if (dInput === "" || vInput === "" || isNaN(dInput) || isNaN(vInput)) {
alert("Please enter valid numerical values for Diameter and Velocity.");
return;
}
var d = parseFloat(dInput);
var v = parseFloat(vInput);
if (d <= 0 || v < 0) {
alert("Diameter must be positive and Velocity cannot be negative.");
return;
}
// 1. Normalize Diameter to Meters (SI base)
var diameterInMeters = 0;
if (dUnit === 'inch') {
diameterInMeters = d * 0.0254;
} else if (dUnit === 'mm') {
diameterInMeters = d / 1000;
} else if (dUnit === 'cm') {
diameterInMeters = d / 100;
} else {
diameterInMeters = d;
}
// 2. Normalize Velocity to Meters per Second (SI base)
var velocityInMPS = 0;
if (vUnit === 'ft_s') {
velocityInMPS = v * 0.3048;
} else if (vUnit === 'mph') {
velocityInMPS = v * 0.44704;
} else { // m_s
velocityInMPS = v;
}
// 3. Calculate Area (A = pi * r^2 = pi * (d/2)^2) in m^2
var radius = diameterInMeters / 2;
var areaM2 = Math.PI * Math.pow(radius, 2);
// 4. Calculate Base Flow Rate (Q = A * v) in m^3/s
var flowRateM3S = areaM2 * velocityInMPS;
// 5. Conversions for Output
// US GPM = m3/s * 15850.3231
var gpm = flowRateM3S * 15850.3231;
// CFM = m3/s * 2118.88
var cfm = flowRateM3S * 2118.880003;
// L/min = m3/s * 60000
var lpm = flowRateM3S * 60000;
// m3/h = m3/s * 3600
var m3h = flowRateM3S * 3600;
// Area in sq inches for display
var areaSqIn = areaM2 * 1550.0031;
// Display Results
document.getElementById('resGPM').innerText = gpm.toFixed(2);
document.getElementById('resCFM').innerText = cfm.toFixed(2) + " CFM";
document.getElementById('resLPM').innerText = lpm.toFixed(2) + " L/min";
document.getElementById('resM3H').innerText = m3h.toFixed(2) + " m³/h";
document.getElementById('resArea').innerText = areaSqIn.toFixed(4) + " in²";
// Show result box
document.getElementById('results-area').style.display = 'block';
}
Understanding Flow Rate in General Engineering
Flow rate is a fundamental concept in fluid dynamics and general engineering, critical for the design and maintenance of piping systems, hydraulics, HVAC systems, and water treatment facilities. Whether you are dealing with GE industrial turbines, household plumbing, or irrigation systems, determining the correct volumetric flow rate ensures system efficiency and safety.
This calculator allows engineers and technicians to determine the volumetric flow rate ($Q$) based on the internal pipe diameter and the velocity of the fluid moving through it.
The Flow Rate Formula
The basic formula used for calculating volumetric flow rate in a pipe is derived from the continuity equation:
Q = A × v
Where:
Q is the Volumetric Flow Rate (e.g., m³/s, GPM, CFM).
A is the Cross-Sectional Area of the pipe (calculated from the diameter).
v is the Velocity of the fluid.
How to Calculate Pipe Area
Since pipes are circular, the cross-sectional area is calculated using the diameter ($D$):
A = π × (D / 2)²
It is crucial that the units for diameter and velocity match before multiplication. Our calculator handles these unit conversions automatically, converting inputs like inches and feet per second into a standard metric base before providing results in Gallons Per Minute (GPM), Liters Per Minute (LPM), and Cubic Feet Per Minute (CFM).
Applications of Flow Rate Calculation
1. HVAC Systems: In heating and cooling, air flow (measured in CFM) determines the efficiency of heat exchange. Duct sizing is based directly on the required flow rate and permissible velocity to reduce noise and pressure drop. 2. Hydraulic Engineering: Calculating the flow of water through pipes helps in sizing pumps and determining the fill time for tanks and reservoirs. 3. Industrial Process Control: Precise flow rates are required for mixing chemical ratios in manufacturing.
Common Units of Measurement
GPM (Gallons Per Minute): The standard unit for liquid flow in the United States.
CFM (Cubic Feet Per Minute): Commonly used for airflow in ventilation systems.
L/min (Liters Per Minute): Standard metric unit for liquid flow.
m³/h (Cubic Meters per Hour): Used for large-scale waterworks and industrial gas flow.
Tips for Accurate Calculation
When using this tool, ensure you are using the internal diameter of the pipe, not the outer diameter. The wall thickness of the pipe reduces the available area for fluid flow. For example, a standard 2-inch Schedule 40 pipe has an internal diameter of approximately 2.067 inches, not exactly 2 inches.