body {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
line-height: 1.6;
color: #333;
max-width: 800px;
margin: 0 auto;
padding: 20px;
background-color: #f9f9f9;
}
.calculator-container {
background: #ffffff;
padding: 30px;
border-radius: 12px;
box-shadow: 0 4px 15px rgba(0,0,0,0.1);
margin-bottom: 40px;
border: 1px solid #e0e0e0;
}
.calculator-header {
text-align: center;
margin-bottom: 25px;
border-bottom: 2px solid #0073e6;
padding-bottom: 15px;
}
.calculator-header h2 {
margin: 0;
color: #0073e6;
font-size: 24px;
}
.input-group {
margin-bottom: 20px;
}
.input-group label {
display: block;
margin-bottom: 8px;
font-weight: 600;
color: #444;
}
.input-row {
display: flex;
gap: 15px;
align-items: center;
}
.input-wrapper {
flex: 1;
}
input[type="number"], select {
width: 100%;
padding: 12px;
border: 1px solid #ddd;
border-radius: 6px;
font-size: 16px;
box-sizing: border-box;
transition: border-color 0.3s;
}
input[type="number"]:focus, select:focus {
border-color: #0073e6;
outline: none;
}
.radio-group {
display: flex;
gap: 20px;
margin-bottom: 15px;
}
.radio-label {
display: flex;
align-items: center;
cursor: pointer;
font-weight: normal;
}
.radio-label input {
margin-right: 8px;
}
.calc-btn {
width: 100%;
background-color: #0073e6;
color: white;
padding: 14px;
border: none;
border-radius: 6px;
font-size: 18px;
font-weight: bold;
cursor: pointer;
transition: background-color 0.2s;
margin-top: 10px;
}
.calc-btn:hover {
background-color: #005bb5;
}
.results-area {
margin-top: 25px;
background-color: #f0f7ff;
padding: 20px;
border-radius: 8px;
border-left: 5px solid #0073e6;
display: none;
}
.result-row {
display: flex;
justify-content: space-between;
margin-bottom: 10px;
padding-bottom: 10px;
border-bottom: 1px solid #dae1e7;
}
.result-row:last-child {
border-bottom: none;
margin-bottom: 0;
padding-bottom: 0;
}
.result-label {
font-weight: 600;
color: #555;
}
.result-value {
font-weight: 700;
color: #0073e6;
font-size: 18px;
}
.article-section {
background: white;
padding: 30px;
border-radius: 12px;
box-shadow: 0 2px 10px rgba(0,0,0,0.05);
}
h1 { font-size: 32px; color: #222; margin-bottom: 20px; }
h2 { font-size: 24px; color: #333; margin-top: 30px; margin-bottom: 15px; }
h3 { font-size: 20px; color: #444; margin-top: 20px; margin-bottom: 10px; }
p { margin-bottom: 15px; color: #555; }
ul { margin-bottom: 20px; padding-left: 20px; }
li { margin-bottom: 10px; color: #555; }
.formula-box {
background: #eee;
padding: 15px;
border-left: 4px solid #666;
font-family: monospace;
margin: 20px 0;
}
.hidden { display: none; }
How to Calculate Air Flow Rate of a Fan
Calculating the air flow rate of a fan or through a duct system is a fundamental skill in HVAC engineering, industrial ventilation, and even computer cooling management. Understanding how to determine Cubic Feet per Minute (CFM) ensures that your system is providing adequate ventilation, cooling, or exhaust capabilities.
The calculation is primarily based on the relationship between the speed of the air (velocity) and the size of the opening it flows through (area). This guide explains the mathematics behind the calculator above.
The Air Flow Formula
The most common method to calculate volumetric flow rate (Q) is using the continuity equation:
Q = V × A
- Q = Air Flow Rate (typically CFM or CMM)
- V = Air Velocity (Feet Per Minute or Meters Per Second)
- A = Cross-Sectional Area of the duct or fan (Square Feet or Square Meters)
Step-by-Step Calculation Guide
1. Measure Air Velocity
First, you must determine how fast the air is moving. This is usually done using an anemometer.
If you measure in m/s (meters per second), you should convert it to FPM (feet per minute) to calculate CFM directly.
- Conversion: 1 m/s ≈ 196.85 FPM.
2. Calculate the Cross-Sectional Area
The area calculation depends on the shape of your duct or fan housing. Since most tape measures use inches, but the CFM formula requires Square Feet (ft²), we must convert the units carefully.
For Round Ducts (Circular):
The formula for the area of a circle is A = π × r². However, since we usually measure diameter in inches:
Area (ft²) = π × (Diameter in inches / 24)²
Note: We divide by 24 because the radius is half the diameter, and we divide by 12 to convert inches to feet.
For Rectangular Ducts:
The formula for a rectangle is Width × Height. To get square feet from inches:
Area (ft²) = (Width inches × Height inches) / 144
3. Calculate CFM (Cubic Feet per Minute)
Once you have the velocity in FPM and the Area in ft², simply multiply them:
CFM = Velocity (FPM) × Area (ft²)
Why is Calculating Air Flow Important?
HVAC Balancing: Ensures that every room in a building receives the correct amount of conditioned air for temperature control.
Industrial Safety: In exhaust systems, maintaining a specific air flow rate is critical to remove dust, fumes, or hazardous gases effectively.
Equipment Efficiency: Fans operating against high static pressure may not deliver their rated CFM. Calculating the actual flow helps in diagnosing system inefficiencies.
Common Unit Conversions
- CFM: Cubic Feet per Minute (Imperial Standard)
- CMM: Cubic Meters per Minute (Metric Standard)
- Conversion: 1 CFM ≈ 0.0283 CMM
- Conversion: 1 CMM ≈ 35.31 CFM
function toggleShapeInputs() {
var shape = document.querySelector('input[name="ductShape"]:checked').value;
var roundDiv = document.getElementById('roundInputs');
var rectDiv = document.getElementById('rectInputs');
if (shape === 'round') {
roundDiv.classList.remove('hidden');
rectDiv.classList.add('hidden');
} else {
roundDiv.classList.add('hidden');
rectDiv.classList.remove('hidden');
}
}
function calculateAirFlow() {
// 1. Get Velocity
var velocityInput = parseFloat(document.getElementById('airVelocity').value);
var velUnit = document.getElementById('velocityUnit').value;
// Validation for velocity
if (isNaN(velocityInput) || velocityInput <= 0) {
alert("Please enter a valid air velocity greater than 0.");
return;
}
// Convert Velocity to FPM (Feet Per Minute) for standard calculation
var velocityFPM = velocityInput;
if (velUnit === 'MPS') {
// 1 m/s = 196.850394 fpm
velocityFPM = velocityInput * 196.8504;
}
// 2. Get Shape and Calculate Area in Square Feet
var shape = document.querySelector('input[name="ductShape"]:checked').value;
var areaSqFt = 0;
if (shape === 'round') {
var diameterInches = parseFloat(document.getElementById('diameter').value);
if (isNaN(diameterInches) || diameterInches <= 0) {
alert("Please enter a valid diameter.");
return;
}
// Radius in feet = (Diameter in inches / 2) / 12 = Diameter / 24
var radiusFt = diameterInches / 24;
areaSqFt = Math.PI * Math.pow(radiusFt, 2);
} else {
var widthInches = parseFloat(document.getElementById('ductWidth').value);
var heightInches = parseFloat(document.getElementById('ductHeight').value);
if (isNaN(widthInches) || widthInches <= 0 || isNaN(heightInches) || heightInches <= 0) {
alert("Please enter valid width and height dimensions.");
return;
}
// Area in sq ft = (w * h) / 144
areaSqFt = (widthInches * heightInches) / 144;
}
// 3. Calculate CFM (Q = V * A)
var cfm = velocityFPM * areaSqFt;
// 4. Calculate CMM (1 CFM = 0.0283168 CMM)
var cmm = cfm * 0.0283168;
// 5. Display Results
document.getElementById('resArea').innerHTML = areaSqFt.toFixed(3) + " ft²";
document.getElementById('resCFM').innerHTML = cfm.toFixed(1).replace(/\B(?=(\d{3})+(?!\d))/g, ",") + " CFM";
document.getElementById('resCMM').innerHTML = cmm.toFixed(2) + " CMM";
document.getElementById('results').style.display = 'block';
}