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;
background-color: #f9f9f9;
}
.calculator-container {
background: #ffffff;
border: 1px solid #e0e0e0;
border-radius: 8px;
padding: 30px;
max-width: 600px;
margin: 20px auto;
box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}
.calculator-title {
text-align: center;
margin-bottom: 25px;
color: #2c3e50;
font-size: 24px;
font-weight: 700;
border-bottom: 2px solid #3498db;
padding-bottom: 10px;
display: inline-block;
width: 100%;
}
.form-group {
margin-bottom: 20px;
}
.form-group label {
display: block;
margin-bottom: 8px;
font-weight: 600;
color: #555;
}
.input-wrapper {
position: relative;
}
.form-control {
width: 100%;
padding: 12px;
border: 1px solid #ccc;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
transition: border-color 0.3s;
}
.form-control:focus {
border-color: #3498db;
outline: none;
}
.unit-label {
position: absolute;
right: 12px;
top: 50%;
transform: translateY(-50%);
color: #888;
font-size: 14px;
pointer-events: none;
}
.btn-calculate {
background-color: #3498db;
color: white;
border: none;
padding: 15px 30px;
font-size: 18px;
font-weight: bold;
border-radius: 4px;
cursor: pointer;
width: 100%;
transition: background-color 0.3s;
margin-top: 10px;
}
.btn-calculate:hover {
background-color: #2980b9;
}
.results-section {
margin-top: 30px;
background-color: #f0f7fb;
padding: 20px;
border-radius: 4px;
border-left: 5px solid #3498db;
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: #2c3e50;
font-size: 18px;
}
.highlight-result {
color: #e74c3c;
font-size: 22px;
}
.article-content {
background: #fff;
padding: 40px;
margin-top: 40px;
border-radius: 8px;
box-shadow: 0 2px 4px rgba(0,0,0,0.05);
}
h2 {
color: #2c3e50;
margin-top: 30px;
border-bottom: 1px solid #eee;
padding-bottom: 10px;
}
p {
margin-bottom: 15px;
color: #444;
}
ul {
margin-bottom: 20px;
padding-left: 20px;
}
li {
margin-bottom: 8px;
}
.info-table {
width: 100%;
border-collapse: collapse;
margin: 20px 0;
}
.info-table th, .info-table td {
border: 1px solid #ddd;
padding: 12px;
text-align: left;
}
.info-table th {
background-color: #f2f2f2;
font-weight: 600;
}
.error-msg {
color: #e74c3c;
font-size: 14px;
margin-top: 5px;
display: none;
}
@media (max-width: 600px) {
.article-content {
padding: 20px;
}
.calculator-container {
padding: 20px;
}
}
function calculateACH() {
// Get input elements
var lengthInput = document.getElementById('roomLength');
var widthInput = document.getElementById('roomWidth');
var heightInput = document.getElementById('roomHeight');
var cfmInput = document.getElementById('systemCFM');
var errorMsg = document.getElementById('errorMsg');
var resultsDiv = document.getElementById('results');
// Parse values
var length = parseFloat(lengthInput.value);
var width = parseFloat(widthInput.value);
var height = parseFloat(heightInput.value);
var cfm = parseFloat(cfmInput.value);
// Validation
if (isNaN(length) || length <= 0 ||
isNaN(width) || width <= 0 ||
isNaN(height) || height <= 0 ||
isNaN(cfm) || cfm <= 0) {
errorMsg.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
errorMsg.style.display = 'none';
// Calculations
// 1. Calculate Volume in Cubic Feet
var volume = length * width * height;
// 2. Calculate Cubic Feet per Hour (CFH)
var cfh = cfm * 60;
// 3. Calculate Air Changes per Hour (ACH) = (CFM * 60) / Volume
var ach = cfh / volume;
// 4. Time for one complete air change in minutes
var timePerChange = 60 / ach;
// Display Results
document.getElementById('resVolume').textContent = volume.toLocaleString('en-US', {maximumFractionDigits: 0}) + " ft³";
document.getElementById('resAirflowHour').textContent = cfh.toLocaleString('en-US', {maximumFractionDigits: 0}) + " ft³/hr";
// Format time logic
var timeDisplay = "";
if (timePerChange < 1) {
timeDisplay = (timePerChange * 60).toFixed(1) + " seconds";
} else {
timeDisplay = timePerChange.toFixed(1) + " minutes";
}
document.getElementById('resTimePerChange').textContent = timeDisplay;
document.getElementById('resACH').textContent = ach.toFixed(2);
// Show results
resultsDiv.style.display = 'block';
}
How to Calculate Air Exchange Rate per Hour
Calculating the Air Exchange Rate per Hour (often referred to as ACH or ACPH) is a critical step in ensuring proper ventilation for residential homes, commercial buildings, and industrial spaces. The Air Exchange Rate measures how many times the entire volume of air within a specific room or space is replaced with new air every hour.
Whether you are installing a new HVAC system, setting up an air purifier, or ensuring safety compliance in a workshop, understanding your ACH is vital for maintaining healthy Indoor Air Quality (IAQ).
The Air Exchange Rate Formula
To calculate the Air Changes per Hour manually, you need two primary pieces of data: the volume of the room and the airflow capacity of your ventilation equipment (usually measured in Cubic Feet per Minute, or CFM).
ACH Formula:
ACH = (CFM × 60) / Volume of Room
Step-by-Step Calculation Guide:
- Measure Room Dimensions: Measure the Length, Width, and Height of the room in feet.
- Calculate Room Volume: Multiply Length × Width × Height to get the volume in cubic feet (ft³).
- Determine Airflow (CFM): Check the specifications of your fan, air purifier, or HVAC system to find its CFM rating.
- Convert CFM to CFH: Multiply the CFM by 60 (since there are 60 minutes in an hour) to get Cubic Feet per Hour.
- Divide: Divide the total hourly airflow by the room volume.
Calculation Example
Let's say you have a living room that is 20 feet long, 15 feet wide, and has an 8-foot ceiling. You are using an air purifier rated at 300 CFM.
- Step 1 (Volume): 20 × 15 × 8 = 2,400 cubic feet.
- Step 2 (Hourly Airflow): 300 CFM × 60 minutes = 18,000 cubic feet per hour.
- Step 3 (ACH): 18,000 / 2,400 = 7.5 ACH.
This means the air in your living room is completely replaced 7.5 times every hour, or roughly once every 8 minutes.
Recommended Air Exchange Rates
Different spaces require different levels of ventilation depending on their usage and the presence of pollutants. Below are general guidelines for recommended ACH levels:
| Room Type |
Recommended ACH |
| Residential Bedroom |
2 – 4 |
| Kitchens |
15 – 20 |
| Bathrooms |
6 – 10 |
| Offices |
4 – 6 |
| Laboratories/Workshops |
6 – 12 |
| Warehouses |
1 – 2 |
Why is Air Exchange Important?
1. Pollutant Removal: High ACH helps dilute and remove airborne contaminants like dust, allergens, VOCs (Volatile Organic Compounds), and viruses.
2. Humidity Control: Proper air exchange prevents moisture buildup, reducing the risk of mold growth, particularly in bathrooms and basements.
3. CO2 Reduction: In crowded spaces, CO2 levels can rise quickly, causing drowsiness and reduced cognitive function. Adequate fresh air exchange keeps CO2 levels low.
Factors That Affect ACH
While the calculation provides a theoretical number, real-world performance can be influenced by:
- Obstructions: Furniture and layout can impede airflow circulation.
- Leakage: Drafty windows or poor insulation can introduce uncontrolled air exchange (infiltration).
- System Efficiency: Clogged filters or dirty ducts reduce the actual CFM of your HVAC system.