function toggleDuctInputs() {
var shape = document.getElementById('dfrShape').value;
var rectDiv = document.getElementById('dfrRectInputs');
var roundDiv = document.getElementById('dfrRoundInputs');
if (shape === 'rectangular') {
rectDiv.style.display = 'block';
roundDiv.style.display = 'none';
} else {
rectDiv.style.display = 'none';
roundDiv.style.display = 'block';
}
}
function calculateAirFlow() {
var shape = document.getElementById('dfrShape').value;
var velocity = parseFloat(document.getElementById('dfrVelocity').value);
var areaSqFt = 0;
var dimString = "";
// Validation
if (isNaN(velocity) || velocity <= 0) {
alert("Please enter a valid Air Velocity in FPM.");
return;
}
if (shape === 'rectangular') {
var width = parseFloat(document.getElementById('dfrWidth').value);
var height = parseFloat(document.getElementById('dfrHeight').value);
if (isNaN(width) || width <= 0 || isNaN(height) || height <= 0) {
alert("Please enter valid width and height dimensions.");
return;
}
// Area in square inches = w * h
// Area in square feet = (w * h) / 144
areaSqFt = (width * height) / 144;
dimString = width + '" x ' + height + '" Rectangular';
} else {
var diameter = parseFloat(document.getElementById('dfrDiameter').value);
if (isNaN(diameter) || diameter <= 0) {
alert("Please enter a valid duct diameter.");
return;
}
// Radius = diameter / 2
// Area = pi * r^2
// Area in sq ft = (pi * (d/2)^2) / 144
var radius = diameter / 2;
areaSqFt = (Math.PI * Math.pow(radius, 2)) / 144;
dimString = diameter + '" Round';
}
// Calculate CFM: Q = A * V
// CFM = Area (sq ft) * Velocity (FPM)
var cfm = areaSqFt * velocity;
// Display Results
document.getElementById('resAreaSqFt').innerHTML = areaSqFt.toFixed(3) + " sq. ft.";
document.getElementById('resCFM').innerHTML = Math.round(cfm) + " CFM";
document.getElementById('resDims').innerHTML = dimString;
document.getElementById('dfrResult').style.display = 'block';
}
Understanding Duct Flow Rate Calculations
Proper airflow is critical for the efficiency and longevity of HVAC systems. The Duct Flow Rate Calculator helps HVAC technicians, engineers, and homeowners determine the volume of air moving through a specific duct size at a given velocity. This is essential for balancing systems, ensuring rooms receive adequate heating or cooling, and maintaining optimal static pressure.
The Formula: How to Calculate CFM
The calculation of airflow relies on the fundamental relationship between Volume, Area, and Velocity. The standard formula used in the HVAC industry is:
Q = A × V
Q (Quantity): Air Flow Rate measured in Cubic Feet per Minute (CFM).
A (Area): The internal cross-sectional area of the duct in Square Feet (ft²).
V (Velocity): The speed of the air moving through the duct in Feet per Minute (FPM).
Step-by-Step Calculation Logic
Determine Duct Area: Most duct dimensions are measured in inches, but the flow rate formula requires square feet.
For Rectangular Ducts: Area (sq ft) = (Width inches × Height inches) ÷ 144.
For Round Ducts: Area (sq ft) = (π × (Diameter inches ÷ 2)²) ÷ 144.
Measure Velocity: Velocity is usually measured using an anemometer or determined by system design standards (e.g., residential supply trunks often run between 700 and 900 FPM).
Multiply: Multiply the Area (in sq ft) by the Velocity (in FPM) to get the CFM.
Why Duct Size Matters
If a duct is too small for the required CFM, the air velocity will increase, leading to excessive noise (turbulence) and high static pressure, which can damage the blower motor. Conversely, if the duct is too large, velocity drops, causing poor air mixing in the room and inefficient temperature control.
Standard Air Velocity Guidelines
When designing a system, the following velocity ranges are typically recommended to balance noise and efficiency:
Residential Supply Trunks: 700 – 900 FPM
Residential Branch Ducts: 500 – 700 FPM
Commercial Main Ducts: 1000 – 1300 FPM
Return Air Grilles: 400 – 600 FPM (to minimize noise)
Use the calculator above to ensure your duct sizing matches your target airflow requirements.