Common values: Office (4-10), Kitchen (15-20), Workshop (6-12)
Room Volume:0 ft³
Base Airflow Required:0 CFM
Recommended Exhaust Fan Rating
0 CFM
Includes 20% efficiency buffer
function calculateExhaust() {
// Get input values
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);
// Validation
if (isNaN(length) || isNaN(width) || isNaN(height) || isNaN(ach)) {
alert("Please enter valid numeric values for all fields.");
return;
}
if (length <= 0 || width <= 0 || height <= 0 || ach <= 0) {
alert("Dimensions and ACH must be greater than zero.");
return;
}
// Calculations
var volume = length * width * height;
// Formula: CFM = (Volume * ACH) / 60 minutes
var baseCFM = (volume * ach) / 60;
// Add 20% safety margin for static pressure and efficiency loss
var recommendedCFM = baseCFM * 1.2;
// Display results
document.getElementById('resVolume').innerHTML = volume.toLocaleString() + " ft³";
document.getElementById('resBaseCFM').innerHTML = Math.round(baseCFM).toLocaleString() + " CFM";
document.getElementById('resFinalCFM').innerHTML = Math.round(recommendedCFM).toLocaleString() + " CFM";
// Show results container
document.getElementById('results').style.display = "block";
}
Understanding Exhaust Rate Calculations
Proper ventilation is critical for maintaining air quality, controlling temperature, and ensuring safety in both residential and industrial environments. The Exhaust Rate is the measurement of air volume that must be removed from a space within a specific timeframe to ensure adequate air turnover.
This calculator determines the required Cubic Feet per Minute (CFM) rating for exhaust fans or ventilation systems based on the volume of the room and the desired frequency of air replacement, known as Air Changes Per Hour (ACH).
The Core Formula
The standard industry formula for calculating the required exhaust rate for general ventilation is:
ACH = Air Changes Per Hour (how many times the total air volume is replaced in one hour).
60 = The number of minutes in an hour (converts the hourly rate to a per-minute rate).
Recommended Air Changes Per Hour (ACH)
Different environments require different rates of air exchange depending on the pollutants, heat, or moisture generated within the space. Use the table below to determine the appropriate ACH for your calculation:
Room Type
Recommended ACH Range
General Office / Bedroom
4 – 6
Kitchens (Residential)
15 – 20
Bathrooms
8 – 15
Garages / Workshops
6 – 12
Laboratories
6 – 15
Grow Rooms / Greenhouses
20 – 30
Warehouses
2 – 4
Why Include a Safety Buffer?
Our calculator automatically provides a "Recommended Exhaust Fan Rating" that includes a 20% buffer above the mathematically required base CFM. This is crucial for real-world applications because:
Static Pressure: Ductwork, bends, and filters create resistance (static pressure) that reduces the actual airflow performance of a fan.
System Efficiency: Fans rarely operate at 100% efficiency over their lifespan as dust accumulates.
Leakage: Not all rooms are perfectly sealed, leading to loss of pressure.
How to Select the Right Fan
Once you have your required CFM from the calculator above, look for an inline duct fan, wall exhaust fan, or hood fan that meets or slightly exceeds this number. If your setup involves long runs of ducting or carbon filters (common in hydroponics or workshops), you may need to increase your requirement by up to 30-50% to overcome the increased static pressure.