How to Calculate the Ventilation Rate

Ventilation Rate Calculator (ACH & CFM)

Understanding Ventilation Rate Calculation

The ventilation rate is a measure of how much fresh outdoor air is introduced into an indoor space. Proper ventilation is critical for diluting indoor pollutants, controlling moisture, and ensuring the health and comfort of occupants. Most building codes and health organizations use either Air Changes per Hour (ACH) or Cubic Feet per Minute (CFM) as the primary metrics.

The ACH Method Formula

To calculate the ventilation rate using the Air Changes per Hour (ACH) method, you need to know the physical dimensions of the room and the recommended ACH for that specific environment. The formula is:

CFM = (Volume × ACH) / 60

• Volume: Length × Width × Height (in cubic feet).
• ACH: The number of times the entire volume of air in the room is replaced in one hour.
• 60: Conversion factor to move from hours to minutes.

Common Recommended ACH Values

Room Type	Recommended ACH
Residential Living Rooms	0.35 – 1.0
Classrooms	3.0 – 6.0
Kitchens (Domestic)	5.0 – 10.0
Offices	4.0 – 8.0
Hospitals (Patient Rooms)	6.0+

Step-by-Step Example

Scenario: You have an office space that is 20 feet long, 20 feet wide, and has a 10-foot ceiling. You want to achieve 6 Air Changes per Hour (ACH).

1. Calculate Volume: 20 × 20 × 10 = 4,000 cubic feet.
2. Calculate Hourly Flow: 4,000 × 6 = 24,000 cubic feet per hour.
3. Convert to CFM: 24,000 / 60 = 400 CFM.

In this case, your HVAC system or ventilation fan needs to be capable of moving 400 cubic feet of air every minute.

Why Ventilation Rates Matter

Inadequate ventilation leads to the buildup of CO2, Volatile Organic Compounds (VOCs), and allergens. This can cause "Sick Building Syndrome," characterized by headaches, fatigue, and respiratory issues. Conversely, excessive ventilation can lead to high energy costs due to the need to heat or cool the incoming outdoor air. Finding the "Goldilocks" zone—the specific rate required for your room type—is essential for efficiency and health.

Factors Influencing Ventilation Requirements

• Occupancy: More people require more fresh air (usually 15-20 CFM per person).
• Activity Level: Gyms require higher rates than libraries.
• Pollutant Sources: Labs or workshops with chemicals need dedicated exhaust systems.

function calculateVentilation() { 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('desiredACH').value); var resultDiv = document.getElementById('ventResult'); if (isNaN(length) || isNaN(width) || isNaN(height) || isNaN(ach) || length <= 0 || width <= 0 || height <= 0 || ach <= 0) { alert("Please enter valid positive numbers for all fields."); resultDiv.style.display = "none"; return; } // Calculation logic var volume = length * width * height; var cfh = volume * ach; var cfm = cfh / 60; // Displaying results document.getElementById('resVolume').innerText = volume.toLocaleString(undefined, {maximumFractionDigits: 2}); document.getElementById('resCFM').innerText = cfm.toLocaleString(undefined, {maximumFractionDigits: 2}); document.getElementById('resCFH').innerText = cfh.toLocaleString(undefined, {maximumFractionDigits: 2}); resultDiv.style.display = "block"; }