CFM stands for Cubic Feet per Minute. It is a measurement of the volume of air flow. In HVAC (Heating, Ventilation, and Air Conditioning) design, calculating the correct CFM is vital to ensure that a space receives enough conditioned air to maintain comfort levels and proper air exchange.
The CFM Calculation Formula
Airflow is calculated by multiplying the cross-sectional area of the duct by the velocity of the air moving through it:
CFM = Area (sq. ft.) × Velocity (FPM)
Area: The physical opening of the duct in square feet.
Velocity: How fast the air is moving in Feet Per Minute.
Recommended Air Velocities
High velocity causes noise (whistling or rushing sounds), while low velocity can result in poor air distribution. Use these standard guidelines:
Application
Recommended Velocity (FPM)
Residential Supply Branches
500 – 700
Residential Main Trunk
700 – 900
Commercial Supply
1,000 – 1,200
Return Air Ducts
400 – 600
Examples of CFM Calculation
Example 1 (Round Duct): An 8-inch round duct has an area of approximately 0.349 sq. ft. If the air velocity is 600 FPM, the airflow is 0.349 × 600 = 209.4 CFM.
Example 2 (Rectangular Duct): A 12×8 inch rectangular duct has an area of 96 sq. inches. To convert to square feet: 96 / 144 = 0.667 sq. ft. At 600 FPM, the airflow is 0.667 × 600 = 400 CFM.
Why Airflow Matters
If your CFM is too low, your furnace or air conditioner may freeze up or overheat because there isn't enough air moving across the heat exchanger or coil. If it's too high, you'll experience excessive noise and potentially high static pressure, which shortens the lifespan of the blower motor.
function calculateCFM() {
var shape = document.getElementById("ductShape").value;
var velocity = parseFloat(document.getElementById("velocity").value);
var areaSqFt = 0;
if (isNaN(velocity) || velocity <= 0) {
alert("Please enter a valid air velocity.");
return;
}
if (shape === "round") {
var diameter = parseFloat(document.getElementById("diameter").value);
if (isNaN(diameter) || diameter <= 0) {
alert("Please enter a valid diameter.");
return;
}
// Area in sq inches = PI * r^2
var radius = diameter / 2;
var areaSqIn = Math.PI * Math.pow(radius, 2);
areaSqFt = areaSqIn / 144;
} else {
var width = parseFloat(document.getElementById("width").value);
var height = parseFloat(document.getElementById("height").value);
if (isNaN(width) || width <= 0 || isNaN(height) || height <= 0) {
alert("Please enter valid width and height.");
return;
}
// Area in sq inches = W * H
var areaSqIn = width * height;
areaSqFt = areaSqIn / 144;
}
var cfm = areaSqFt * velocity;
document.getElementById("cfmResult").style.display = "block";
document.getElementById("cfmOutput").innerText = Math.round(cfm) + " CFM";
document.getElementById("areaOutput").innerText = "Calculated Duct Area: " + areaSqFt.toFixed(3) + " sq. ft.";
}