Calculated using standard air density. Ensure measurements are internal dimensions.
function toggleFfrInputs() {
var shape = document.getElementById('ffr_shape').value;
var dim1Label = document.getElementById('ffr_lbl_dim1');
var dim2Group = document.getElementById('ffr_grp_dim2′);
if (shape === 'round') {
dim1Label.innerText = "Duct Diameter (Inches)";
dim2Group.style.display = 'none';
} else {
dim1Label.innerText = "Duct Width (Inches)";
dim2Group.style.display = 'block';
}
}
function calculateFanFlow() {
var shape = document.getElementById('ffr_shape').value;
var dim1 = parseFloat(document.getElementById('ffr_dim1').value);
var velocity = parseFloat(document.getElementById('ffr_velocity').value);
var areaSqFt = 0;
var flowCFM = 0;
// Validation
if (isNaN(dim1) || isNaN(velocity) || dim1 <= 0 || velocity < 0) {
alert("Please enter valid positive numbers for dimensions and velocity.");
return;
}
if (shape === 'rectangular') {
var dim2 = parseFloat(document.getElementById('ffr_dim2').value);
if (isNaN(dim2) || dim2 <= 0) {
alert("Please enter a valid height.");
return;
}
// Area = (W * H) / 144 to convert sq inches to sq feet
areaSqFt = (dim1 * dim2) / 144;
} else {
// Area = PI * (r^2) / 144
// Radius = Diameter / 2
var radius = dim1 / 2;
areaSqFt = (Math.PI * (radius * radius)) / 144;
}
// Q = V * A
flowCFM = velocity * areaSqFt;
// Display Results
document.getElementById('ffr_area_res').innerText = areaSqFt.toFixed(3) + " sq. ft.";
document.getElementById('ffr_cfm_res').innerText = Math.round(flowCFM).toLocaleString() + " CFM";
document.getElementById('ffr_results').style.display = 'block';
}
Understanding Fan Volume Flow Rate Calculation
Calculating the fan volume flow rate is a fundamental task in HVAC design, industrial ventilation, and aerodynamic engineering. The flow rate determines how much air a fan moves over a specific period, typically measured in Cubic Feet per Minute (CFM) in imperial units or Cubic Meters per Second (m³/s) in metric systems. Correct calculation ensures that air circulation systems meet the required air changes per hour (ACH) for safety, cooling, or air quality standards.
The Core Formula: Q = V × A
The most direct method to calculate the volumetric flow rate is by measuring the air velocity moving through a duct of a known size. The continuity equation used is:
Q = V × A
Q (Flow Rate): The volume of air moving per unit of time (e.g., CFM).
V (Velocity): The speed of the air (e.g., Feet Per Minute, FPM).
A (Area): The cross-sectional area of the duct (e.g., Square Feet).
How to Use This Calculator
Select Duct Shape: Choose between rectangular (square) or round (circular) ducts.
Input Dimensions: Enter the internal width/height or diameter in inches. The calculator automatically converts these to square feet.
Input Velocity: Enter the average air velocity measured in Feet Per Minute (FPM). This is often obtained using an anemometer.
Calculate: The tool computes the cross-sectional area and multiplies it by the velocity to provide the flow rate in CFM.
Why is CFM Calculation Critical?
1. Proper Ventilation (ACH):
Rooms require a specific number of Air Changes Per Hour (ACH). For example, a server room might need 30 ACH, while a standard office might only need 4-6 ACH. To verify if a fan is sufficient, you calculate the required CFM using the formula:
Required CFM = (Room Volume × ACH) / 60.
2. System Balancing:
In HVAC systems, balancing ensures that every room receives the designed amount of air. By measuring velocity at the register or in the duct and calculating CFM, technicians can adjust dampers to balance the system.
3. Fan Efficiency:
Comparing the calculated airflow against the manufacturer's fan curve helps determine if the fan is operating efficiently or if high static pressure is restricting performance.
Example Calculation
Suppose you have a 12-inch round duct and you measure an average air velocity of 800 FPM.
Step 1: Calculate Area.
Radius = 6 inches. Area = π × 6² = 113.1 square inches.
Convert to square feet: 113.1 / 144 ≈ 0.785 sq. ft.