body {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
line-height: 1.6;
color: #333;
max-width: 1200px;
margin: 0 auto;
padding: 20px;
}
.calculator-wrapper {
background-color: #f8f9fa;
border: 1px solid #e9ecef;
border-radius: 8px;
padding: 30px;
margin-bottom: 40px;
box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}
.calc-header {
text-align: center;
margin-bottom: 25px;
}
.calc-header h2 {
margin: 0;
color: #2c3e50;
}
.input-grid {
display: grid;
grid-template-columns: 1fr 1fr;
gap: 20px;
}
.input-group {
margin-bottom: 15px;
}
.input-group label {
display: block;
font-weight: 600;
margin-bottom: 5px;
color: #495057;
}
.input-group input, .input-group select {
width: 100%;
padding: 10px;
border: 1px solid #ced4da;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
}
.input-group select {
background-color: white;
}
.full-width {
grid-column: span 2;
}
.btn-calculate {
background-color: #007bff;
color: white;
border: none;
padding: 12px 20px;
font-size: 18px;
font-weight: bold;
border-radius: 4px;
cursor: pointer;
width: 100%;
margin-top: 10px;
transition: background-color 0.2s;
}
.btn-calculate:hover {
background-color: #0056b3;
}
.results-area {
margin-top: 30px;
padding: 20px;
background-color: #fff;
border: 1px solid #dee2e6;
border-radius: 4px;
display: none;
}
.results-area h3 {
margin-top: 0;
border-bottom: 2px solid #007bff;
padding-bottom: 10px;
color: #2c3e50;
}
.result-row {
display: flex;
justify-content: space-between;
margin-bottom: 10px;
font-size: 18px;
}
.result-label {
color: #6c757d;
}
.result-value {
font-weight: bold;
color: #28a745;
}
.error-msg {
color: #dc3545;
display: none;
margin-top: 10px;
text-align: center;
font-weight: bold;
}
/* Article Styling */
.content-article {
background: #fff;
padding: 20px;
}
.content-article h2 {
color: #2c3e50;
border-bottom: 1px solid #eee;
padding-bottom: 10px;
margin-top: 30px;
}
.content-article p {
margin-bottom: 15px;
}
.content-article ul {
margin-bottom: 20px;
}
.content-article table {
width: 100%;
border-collapse: collapse;
margin: 20px 0;
}
.content-article th, .content-article td {
border: 1px solid #ddd;
padding: 12px;
text-align: left;
}
.content-article th {
background-color: #f2f2f2;
font-weight: bold;
}
@media (max-width: 600px) {
.input-grid {
grid-template-columns: 1fr;
}
.full-width {
grid-column: span 1;
}
}
Understanding Exhaust Air Flow Rate
Calculating the correct exhaust air flow rate is critical for maintaining indoor air quality, controlling temperature, and removing contaminants like moisture, odors, smoke, or fumes. Whether you are designing a residential bathroom, a commercial kitchen, or an industrial workshop, the airflow rate determines the capacity of the fan required to effectively ventilate the space.
The Calculation Method: Air Changes Per Hour (ACH)
The most common method for determining required exhaust flow is the Air Changes Per Hour (ACH) method. This metric indicates how many times the total volume of air in a room is replaced within one hour.
The formula depends on your unit of measurement:
Imperial Units (Cubic Feet per Minute – CFM)
Formula: CFM = (Room Volume in ft³ × ACH) / 60
- Room Volume: Length × Width × Height (in feet)
- ACH: Desired Air Changes Per Hour
- 60: Converts hours to minutes
Metric Units (Cubic Meters per Hour – CMH)
Formula: CMH = Room Volume in m³ × ACH
- Room Volume: Length × Width × Height (in meters)
- CMH: Cubic Meters per Hour
Recommended Air Changes Per Hour (ACH) Standards
Different spaces require different ventilation intensities based on the pollutants generated. Below is a reference table for common applications:
| Space Type |
Recommended ACH Range |
Typical Usage |
| Residential Bedroom |
2 – 4 |
General air freshness, CO2 removal |
| Bathroom / Toilet |
6 – 10 |
Moisture and odor removal |
| Residential Kitchen |
15 – 20 |
Heat, steam, and cooking odor removal |
| Office Space |
4 – 6 |
General occupancy ventilation |
| Garage / Workshop |
6 – 12 |
Exhaust fumes and dust control |
| Commercial Kitchen |
20 – 30+ |
Heavy grease, smoke, and heat extraction |
| Server Room |
4 – 6 (or Heat Load based) |
Temperature control |
Why Proper Sizing Matters
Undersizing: If your exhaust fan moves less air than calculated, pollutants will remain in the room. In bathrooms, this leads to mold and mildew growth. In kitchens, grease settles on surfaces, and odors linger.
Oversizing: Installing a fan that is too powerful can create negative pressure, sucking conditioned air (heating or cooling) out of the building faster than necessary, leading to increased energy bills. It may also generate excessive noise.
Static Pressure Considerations
This calculator determines the airflow requirement. However, when selecting a fan, you must also consider Static Pressure. Ductwork, elbows, and filters create resistance to airflow. A fan rated for 100 CFM at 0 static pressure might only deliver 50 CFM if pushed through long, winding ductwork. Always consult the fan's performance curve to ensure it can deliver the calculated CFM/CMH against the system's resistance.
function updateLabels() {
var unit = document.getElementById('unitType').value;
var lblLength = document.getElementById('lblLength');
var lblWidth = document.getElementById('lblWidth');
var lblHeight = document.getElementById('lblHeight');
if (unit === 'imperial') {
lblLength.innerText = 'Room Length (ft)';
lblWidth.innerText = 'Room Width (ft)';
lblHeight.innerText = 'Ceiling Height (ft)';
} else {
lblLength.innerText = 'Room Length (m)';
lblWidth.innerText = 'Room Width (m)';
lblHeight.innerText = 'Ceiling Height (m)';
}
}
function calculateAirFlow() {
// Inputs
var length = parseFloat(document.getElementById('roomLength').value);
var width = parseFloat(document.getElementById('roomWidth').value);
var height = parseFloat(document.getElementById('roomHeight').value);
var ach = parseFloat(document.getElementById('targetACH').value);
var unit = document.getElementById('unitType').value;
var errorDiv = document.getElementById('errorMsg');
var resultsDiv = document.getElementById('results');
// Validation
if (isNaN(length) || isNaN(width) || isNaN(height) || isNaN(ach) || length <= 0 || width <= 0 || height <= 0 || ach < 0) {
errorDiv.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
errorDiv.style.display = 'none';
resultsDiv.style.display = 'block';
// Calculation Logic
var volume = length * width * height;
var cfm = 0;
var cmh = 0;
var lps = 0;
var volDisplay = '';
if (unit === 'imperial') {
// Volume is in Cubic Feet
// CFM formula: (Volume * ACH) / 60
cfm = (volume * ach) / 60;
// Convert CFM to CMH (1 CFM = 1.69901 CMH)
cmh = cfm * 1.69901;
volDisplay = volume.toFixed(2) + ' ft³';
} else {
// Volume is in Cubic Meters
// CMH formula: Volume * ACH
cmh = volume * ach;
// Convert CMH to CFM (1 CMH = 0.588578 CFM)
cfm = cmh * 0.588578;
volDisplay = volume.toFixed(2) + ' m³';
}
// Calculate Liters per Second (L/s)
// 1 CMH = 0.277778 L/s
lps = cmh * 0.277778;
// Display Results
document.getElementById('resVolume').innerText = volDisplay;
document.getElementById('resCFM').innerText = cfm.toFixed(2) + " CFM";
document.getElementById('resCMH').innerText = cmh.toFixed(2) + " m³/h";
document.getElementById('resLPS').innerText = lps.toFixed(2) + " L/s";
}