CFM stands for Cubic Feet per Minute. It is a measurement of the velocity at which air flows into or out of a space. In HVAC (Heating, Ventilation, and Air Conditioning) design, calculating the correct CFM is vital for maintaining air quality, controlling humidity, and ensuring consistent temperatures.
The CFM Formula for Ventilation
To calculate the required CFM for a room based on ventilation needs (Air Changes Per Hour), we use the following formula:
CFM = (Volume of Room × ACH) / 60
Volume: Length × Width × Height (in cubic feet).
ACH: The number of times the total volume of air in a room is replaced in one hour.
60: Converts hours to minutes.
Typical Air Changes Per Hour (ACH) Reference
Room Type
Recommended ACH
Bedrooms / Residential Halls
2 – 4
Kitchens
6 – 10
Bathrooms (High Humidity)
8 – 12
Workshops / Smoking Rooms
12 – 20
Example Calculation
If you have a workshop that is 20 feet long, 15 feet wide, and has a 10-foot ceiling, your total volume is 3,000 cubic feet (20 × 15 × 10). If the workshop requires 12 air changes per hour (ACH):
(3,000 cu ft × 12 ACH) / 60 = 600 CFM.
You would need a fan or ventilation system rated for at least 600 CFM to properly exhaust the air.
function calculateCFM() {
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('achValue').value);
var resultDiv = document.getElementById('cfm-result-box');
var resultValue = document.getElementById('cfm-value');
var resultSummary = document.getElementById('cfm-summary');
if (isNaN(length) || isNaN(width) || isNaN(height) || length <= 0 || width <= 0 || height <= 0) {
alert("Please enter valid positive numbers for dimensions.");
return;
}
var volume = length * width * height;
var cfm = (volume * ach) / 60;
// Formatting the result
var finalCFM = Math.ceil(cfm);
resultValue.innerText = finalCFM + " CFM";
var summaryText = "For a room volume of " + volume.toLocaleString() + " cubic feet, " +
"providing " + ach + " air changes per hour requires a system moving " +
finalCFM + " cubic feet of air per minute.";
resultSummary.innerText = summaryText;
resultDiv.style.display = 'block';
// Smooth scroll to result
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}