Determine the volumetric flow rate based on pipe diameter and air velocity.
Metric (mm, m/s)
Imperial (inches, ft/min)
Flow Rate (CFM):0
Flow Rate (m³/h):0
Flow Rate (L/s):0
How to Calculate Air Flow Rate in a Pipe
Calculating the air flow rate is critical for designing HVAC systems, pneumatic transport lines, and industrial exhaust systems. The flow rate represents the volume of air passing through a specific cross-section of a pipe over a set period of time.
The Fundamental Formula
The calculation relies on the continuity equation for incompressible flow:
Q = A × v
Q: Volumetric flow rate
A: Cross-sectional area of the pipe (π × r²)
v: Velocity of the air
Step-by-Step Calculation Guide
Follow these steps to calculate the flow manually:
Find the Internal Diameter: Measure the inside diameter (D) of your pipe. For a 4-inch pipe, D = 4 inches.
Calculate the Area: Convert the diameter to radius (r = D/2). Area = π × r². If using inches, convert to square feet (divide by 144) for CFM calculations.
Measure Velocity: Use an anemometer to find the air speed (v) in feet per minute (fpm) or meters per second (m/s).
Multiply: Multiply the Area by the Velocity to get the total flow rate.
Example Calculation (Imperial)
Suppose you have a 6-inch diameter pipe and an air velocity of 1,000 feet per minute (fpm):
Radius = 3 inches = 0.25 feet
Area = π × (0.25)² = 0.1963 square feet
Flow Rate = 0.1963 sq ft × 1,000 fpm = 196.3 CFM
Common Air Velocity Guidelines
In ductwork design, different velocities are recommended to balance noise and efficiency:
Residential: 600 – 900 fpm
Commercial: 1,000 – 1,500 fpm
Industrial: 2,000 – 4,000 fpm
function updateUnits() {
var unit = document.getElementById("unitSystem").value;
var labelD = document.getElementById("labelDiameter");
var labelV = document.getElementById("labelVelocity");
if (unit === "metric") {
labelD.innerText = "Pipe Internal Diameter (mm)";
labelV.innerText = "Air Velocity (m/s)";
} else {
labelD.innerText = "Pipe Internal Diameter (inches)";
labelV.innerText = "Air Velocity (ft/min)";
}
}
function calculateFlow() {
var unit = document.getElementById("unitSystem").value;
var diameter = parseFloat(document.getElementById("pipeDiameter").value);
var velocity = parseFloat(document.getElementById("airVelocity").value);
if (isNaN(diameter) || isNaN(velocity) || diameter <= 0 || velocity <= 0) {
alert("Please enter valid positive numbers for diameter and velocity.");
return;
}
var area_m2, velocity_ms, cfm, m3h, ls;
if (unit === "metric") {
// Diameter in mm to m
var d_m = diameter / 1000;
area_m2 = Math.PI * Math.pow((d_m / 2), 2);
velocity_ms = velocity;
var m3s = area_m2 * velocity_ms;
m3h = m3s * 3600;
ls = m3s * 1000;
cfm = m3s * 2118.88;
} else {
// Diameter in inches to feet
var d_ft = diameter / 12;
var area_ft2 = Math.PI * Math.pow((d_ft / 2), 2);
// Velocity in ft/min
cfm = area_ft2 * velocity;
// Convert CFM to others
m3h = cfm * 1.69901;
ls = cfm * 0.471947;
}
document.getElementById("resCFM").innerText = cfm.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resM3H").innerText = m3h.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resLS").innerText = ls.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resultArea").style.display = "block";
}