.air-flow-calculator-wrapper {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
max-width: 800px;
margin: 0 auto;
padding: 20px;
background: #f9f9f9;
border: 1px solid #e0e0e0;
border-radius: 8px;
}
.afc-header {
text-align: center;
margin-bottom: 25px;
}
.afc-header h2 {
margin: 0;
color: #2c3e50;
font-size: 24px;
}
.afc-grid {
display: grid;
grid-template-columns: 1fr 1fr;
gap: 20px;
}
@media (max-width: 600px) {
.afc-grid {
grid-template-columns: 1fr;
}
}
.afc-input-group {
margin-bottom: 15px;
}
.afc-input-group label {
display: block;
margin-bottom: 5px;
font-weight: 600;
color: #444;
font-size: 14px;
}
.afc-input-group input, .afc-input-group select {
width: 100%;
padding: 10px;
border: 1px solid #ccc;
border-radius: 4px;
box-sizing: border-box;
font-size: 16px;
}
.afc-input-group input:focus {
border-color: #3498db;
outline: none;
}
.afc-button {
grid-column: 1 / -1;
background-color: #3498db;
color: white;
padding: 12px;
border: none;
border-radius: 4px;
cursor: pointer;
font-size: 18px;
font-weight: bold;
width: 100%;
margin-top: 10px;
transition: background 0.3s;
}
.afc-button:hover {
background-color: #2980b9;
}
.afc-results {
grid-column: 1 / -1;
background: #ffffff;
padding: 20px;
border-radius: 6px;
border-left: 5px solid #2ecc71;
margin-top: 20px;
display: none;
box-shadow: 0 2px 5px rgba(0,0,0,0.05);
}
.afc-result-row {
display: flex;
justify-content: space-between;
align-items: center;
padding: 10px 0;
border-bottom: 1px solid #eee;
}
.afc-result-row:last-child {
border-bottom: none;
}
.afc-result-label {
color: #555;
font-weight: 500;
}
.afc-result-value {
font-weight: 800;
color: #2c3e50;
font-size: 18px;
}
.afc-big-result {
text-align: center;
font-size: 28px;
color: #2ecc71;
margin: 10px 0;
font-weight: bold;
}
.afc-content-section {
margin-top: 40px;
padding-top: 20px;
border-top: 2px solid #eee;
color: #333;
line-height: 1.6;
}
.afc-content-section h3 {
color: #2c3e50;
margin-top: 20px;
}
.afc-content-section ul {
padding-left: 20px;
}
.hidden {
display: none;
}
.input-suffix {
color: #888;
font-size: 0.85em;
margin-left: 5px;
}
function toggleDuctInputs() {
var shape = document.getElementById('ductShape').value;
var rectDiv = document.getElementById('rectInputs');
var roundDiv = document.getElementById('roundInputs');
if (shape === 'rectangular') {
rectDiv.style.display = 'grid';
roundDiv.style.display = 'none';
roundDiv.classList.add('hidden');
rectDiv.classList.remove('hidden');
} else {
rectDiv.style.display = 'none';
roundDiv.style.display = 'block';
rectDiv.classList.add('hidden');
roundDiv.classList.remove('hidden');
}
}
function calculateAirFlow() {
// Get inputs
var shape = document.getElementById('ductShape').value;
var velocity = parseFloat(document.getElementById('airVelocity').value);
var areaSqFt = 0;
var isValid = true;
// Validate Velocity
if (isNaN(velocity) || velocity <= 0) {
alert("Please enter a valid Air Velocity greater than 0.");
return;
}
// Calculate Area based on shape
if (shape === 'rectangular') {
var width = parseFloat(document.getElementById('ductWidth').value);
var height = parseFloat(document.getElementById('ductHeight').value);
if (isNaN(width) || width <= 0 || isNaN(height) || height <= 0) {
alert("Please enter valid width and height dimensions.");
isValid = false;
} else {
// Convert inches to feet for area calculation: (W * H) / 144
areaSqFt = (width * height) / 144;
}
} else {
var diameter = parseFloat(document.getElementById('ductDiameter').value);
if (isNaN(diameter) || diameter <= 0) {
alert("Please enter a valid duct diameter.");
isValid = false;
} else {
// Area of circle = pi * r^2. Diameter in inches needs converting to radius in feet.
// Radius in inches = D/2. Radius in feet = (D/2)/12.
var radiusFt = (diameter / 2) / 12;
areaSqFt = Math.PI * Math.pow(radiusFt, 2);
}
}
if (isValid) {
// Formula: Q (CFM) = V (FPM) * A (sq ft)
var cfm = velocity * areaSqFt;
// Update UI
document.getElementById('results').style.display = 'block';
document.getElementById('resCFM').innerText = Math.round(cfm).toLocaleString() + " CFM";
document.getElementById('resArea').innerText = areaSqFt.toFixed(3) + " sq ft";
document.getElementById('resVelocity').innerText = velocity.toLocaleString() + " FPM";
}
}
Air Flow Rate Calculator (CFM)
Calculate Cubic Feet per Minute based on Duct Size and Velocity
Rectangular Duct
Round Duct
Calculated Air Flow Rate
0 CFM
Duct Area (Square Feet):
0 sq ft
Air Velocity:
0 FPM
How to Calculate Air Flow Rate (CFM)
Understanding air flow rate is critical for HVAC technicians, engineers, and facility managers to ensure proper ventilation, heating, and cooling within a space. The standard unit of measurement for air flow in the United States is CFM (Cubic Feet per Minute).
The Air Flow Formula
The fundamental equation used to calculate air flow rate is known as the continuity equation:
Q = V × A
Where:
- Q = Air Flow Rate (CFM – Cubic Feet per Minute)
- V = Air Velocity (FPM – Feet per Minute)
- A = Cross-Sectional Area of the duct (Square Feet)
Step-by-Step Calculation Guide
To use this calculator manually, follow these steps:
- Measure Velocity: Use an anemometer to measure the speed of the air moving through the duct or register. Take multiple readings and average them for accuracy.
- Measure Dimensions: Measure the interior dimensions of the duct in inches.
- Calculate Area in Square Feet:
- For Rectangular ducts: (Width inches × Height inches) ÷ 144
- For Round ducts: π × (Diameter inches ÷ 24)²
- Multiply: Multiply your measured Velocity (FPM) by the Area (Sq Ft) to get the CFM.
Why Air Flow Rate Matters
Correct air flow ensures that HVAC systems operate efficiently. If the CFM is too low, the system may freeze up (AC) or overheat (furnace). If the CFM is too high, it can cause noise issues, uncomfortable drafts, and inefficient humidity control. A standard rule of thumb for air conditioning is approximately 400 CFM per ton of cooling capacity.