Calculate the required Airflow (CFM) based on room volume and air changes.
Imperial (Feet)
Metric (Meters)
Standard Office: 4-6, Kitchen: 15-20
Please enter valid positive numbers for all fields.
Calculation Results
Room Volume:0
Required CFM (Cubic Feet/Min):0 CFM
Required CMH (Cubic Meters/Hour):0 m³/h
Required L/s (Liters/Second):0 L/s
* Recommended fan size should be at least this rating.
Understanding Fan Flow Rate Calculation
Proper ventilation is crucial for maintaining indoor air quality, controlling temperature, and removing contaminants. The Fan Flow Rate Calculator helps HVAC professionals, engineers, and homeowners determine the necessary size of a fan for a specific room based on the volume of the space and the required air turnover frequency.
How to Calculate Required Airflow
The standard formula for calculating the required airflow relies on the concept of Air Changes Per Hour (ACH). The basic logic is to calculate the total volume of air in the room and determine how many times that air needs to be replaced within 60 minutes.
The Formula (Imperial):
CFM = (Room Volume in ft³ × ACH) ÷ 60
The Formula (Metric):
CMH = (Room Volume in m³ × ACH)
Where:
CFM: Cubic Feet per Minute.
CMH: Cubic Meters per Hour.
Volume: Length × Width × Height.
ACH: Air Changes Per Hour.
What are Air Changes Per Hour (ACH)?
ACH is a measure of how many times the air within a defined space is replaced or filtered per hour. A higher ACH indicates better ventilation. Different environments require different ventilation rates depending on the activity level, presence of fumes, or density of occupants.
Recommended ACH Values
When using the calculator above, refer to this table for standard industry recommendations for Air Changes Per Hour:
Room Type
Recommended ACH
Bedroom / Living Room
2 – 4
Office Space
4 – 6
Classrooms
6 – 8
Bathrooms (Private)
6 – 8
Commercial Kitchens
15 – 30
Machine Shops / Workshops
6 – 10
Laboratories
8 – 12
Warehouses
2 – 4
Example Calculation
Let's say you have a workshop that is 20 feet long, 15 feet wide, and has a 10-foot ceiling. You want to clear out sawdust and fumes, so you choose an ACH of 8.
You would need a fan rated for at least 400 CFM to ventilate this workshop effectively.
Why Static Pressure Matters
While this calculator provides the required flow rate, keep in mind that ductwork, filters, and vents create resistance known as static pressure. When purchasing a fan, ensure it can deliver the calculated CFM at the static pressure level of your system (often measured in inches of water gauge or Pascals).
function toggleUnitLabels() {
var unit = document.getElementById('measureUnit').value;
var labelLength = document.getElementById('labelLength');
var labelWidth = document.getElementById('labelWidth');
var labelHeight = document.getElementById('labelHeight');
if (unit === 'feet') {
labelLength.innerHTML = 'Room Length (ft)';
labelWidth.innerHTML = 'Room Width (ft)';
labelHeight.innerHTML = 'Ceiling Height (ft)';
} else {
labelLength.innerHTML = 'Room Length (m)';
labelWidth.innerHTML = 'Room Width (m)';
labelHeight.innerHTML = 'Ceiling Height (m)';
}
}
function calculateFlowRate() {
// Get Inputs
var unit = document.getElementById('measureUnit').value;
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('ach').value);
var errorDisplay = document.getElementById('errorDisplay');
var resultsDiv = document.getElementById('results');
// Validation
if (isNaN(length) || isNaN(width) || isNaN(height) || isNaN(ach) || length <= 0 || width <= 0 || height <= 0 || ach <= 0) {
errorDisplay.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
errorDisplay.style.display = 'none';
// Calculation Logic
var volume = length * width * height;
var cfm = 0;
var cmh = 0;
var lps = 0;
var volumeDisplay = '';
if (unit === 'feet') {
// Imperial Calculation
// Volume is in Cubic Feet
volumeDisplay = volume.toFixed(2) + " ft³";
// Formula: CFM = (Volume x ACH) / 60
cfm = (volume * ach) / 60;
// Conversions
// 1 CFM = 1.699011 m³/h (CMH)
cmh = cfm * 1.699011;
// 1 CFM = 0.4719474 L/s
lps = cfm * 0.4719474;
} else {
// Metric Calculation
// Volume is in Cubic Meters
volumeDisplay = volume.toFixed(2) + " m³";
// Formula: CMH = Volume x ACH
cmh = volume * ach;
// Conversions
// 1 CMH = 0.5885778 CFM
cfm = cmh * 0.5885778;
// 1 CMH = 0.277778 L/s
lps = cmh * 0.277778;
}
// Display Results
resultsDiv.style.display = 'block';
document.getElementById('resVolume').innerText = volumeDisplay;
document.getElementById('resCFM').innerText = Math.ceil(cfm).toLocaleString() + " CFM";
document.getElementById('resCMH').innerText = Math.ceil(cmh).toLocaleString() + " m³/h";
document.getElementById('resLPS').innerText = Math.ceil(lps).toLocaleString() + " L/s";
}