Air Changes per Hour Calculator
Calculate ACH for any room or find the required CFM to meet ventilation standards. Includes ASHRAE recommendations and CDC guidelines for indoor air quality.
ACH Formula
CDC recommends 5+ ACH to reduce airborne virus transmission in occupied spaces.
📏 Room & Airflow Details
Check your HVAC system or air purifier rating for this value
🌬️ Air Changes per Hour
Your Air Changes per Hour
7.5 ACH
Air replaced 7.5 times every hour
✅ Excellent - Exceeds recommended ACH
Comparison: Office
Calculation Details
Room Volume: 2,400 ft³ (68.0 m³)
Airflow: 300 CFM (141.6 L/s)
Air/Hour: 18,000 ft³/hr
📊 Recommended ACH by Room Type (ASHRAE & CDC)
| Room Type | Minimum ACH | Recommended ACH | Source |
|---|---|---|---|
| Residential - General | 0.35 | 5 | ASHRAE 62.2 |
| Bedroom | 2 | 5 | ASHRAE |
| Living Room | 3 | 5 | ASHRAE |
| Kitchen | 7 | 12 | ASHRAE |
| Bathroom | 6 | 10 | ASHRAE |
| Office | 4 | 6 | ASHRAE 62.1 |
| Classroom | 4 | 6 | ASHRAE |
| Conference Room | 6 | 10 | ASHRAE |
| Restaurant / Dining | 6 | 10 | ASHRAE |
| Retail Store | 4 | 8 | ASHRAE |
| Gym / Fitness Center | 8 | 12 | ASHRAE |
| Hospital - General | 6 | 10 | ASHRAE 170 |
| Hospital - Operating Room | 15 | 20 | ASHRAE 170 |
| Hospital - Isolation Room | 12 | 12 | CDC |
| Laboratory | 6 | 10 | ASHRAE |
| Clean Room (ISO 7) | 15 | 50 | ISO 14644 |
| Clean Room (ISO 5) | 250 | 400 | ISO 14644 |
* These are general guidelines. Specific requirements may vary based on local codes, occupancy levels, and specific activities within the space.
🌬️ Understanding Air Changes per Hour
What is ACH?
Air Changes per Hour (ACH) measures how many times the total air volume in a room is completely replaced with fresh or filtered air in one hour. An ACH of 6 means the entire room's air is replaced 6 times per hour, or once every 10 minutes. This metric is crucial for maintaining indoor air quality, controlling pollutants, and reducing airborne disease transmission.
Why ACH Matters for Health
The CDC recommends at least 5 ACH in occupied spaces to reduce airborne virus transmission. Higher ACH rates dilute and remove airborne contaminants including viruses, bacteria, allergens, and volatile organic compounds (VOCs). In healthcare settings, operating rooms require 20+ ACH to prevent surgical site infections, while isolation rooms need 12+ ACH with negative pressure.
ACH vs. CFM: What's the Difference?
CFM (Cubic Feet per Minute) is an absolute measure of airflow - how much air moves per minute regardless of room size. ACH is relative to room volume - the same 300 CFM fan provides excellent ventilation (7.5 ACH) in a small office but inadequate ventilation (3 ACH) in a large classroom. Always consider ACH when evaluating ventilation adequacy, not just CFM.
📐 Quick Formulas
Calculate ACH:
ACH = (CFM × 60) ÷ Volume
Find Required CFM:
CFM = (ACH × Volume) ÷ 60
Room Volume:
V = L × W × H
✅ CDC Guidelines
- Aim for 5+ ACH minimum
- Higher is better for virus protection
- Combine HVAC + air purifiers
- Open windows when possible
- Use HEPA filters (MERV 13+)
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Frequently Asked Questions
Air Changes per Hour (ACH) is calculated using the formula: ACH = (CFM × 60) ÷ Room Volume. First, find your room volume by multiplying Length × Width × Height in feet. Then multiply your airflow rate (CFM) by 60 to convert to cubic feet per hour. Finally, divide by the room volume. For example, a 300 CFM fan in a 2,400 cubic foot room: (300 × 60) ÷ 2,400 = 7.5 ACH.
For a 2000 sq ft space with 8-foot ceilings (16,000 cubic feet volume), the CFM needed depends on your target ACH. For residential (5 ACH): 16,000 × 5 ÷ 60 = 1,333 CFM. For office (6 ACH): 1,600 CFM. For kitchen (12 ACH): 3,200 CFM. Always check local codes and ASHRAE standards for specific requirements.
The CDC recommends at least 5 ACH for reducing airborne virus transmission in occupied spaces. ASHRAE standards vary by room type: Residential 4-6 ACH, Offices 6-8 ACH, Kitchens 12-15 ACH, Bathrooms 8-10 ACH, Hospital operating rooms 20-25 ACH. Higher ACH means better air quality but increased energy costs.
For residential homes, ASHRAE recommends a minimum of 0.35 ACH of outdoor air exchange. However, for good indoor air quality and virus protection, aim for 4-6 ACH total. Modern tight homes may need mechanical ventilation to achieve this. Kitchens and bathrooms should have higher rates (8-15 ACH) due to moisture and odor concerns.
ISO 14644 is the international standard for cleanrooms. It specifies ACH based on cleanliness class: ISO 5 (Class 100) requires 250-600 ACH, ISO 6 (Class 1000) requires 90-180 ACH, ISO 7 (Class 10,000) requires 30-60 ACH, and ISO 8 (Class 100,000) requires 10-25 ACH. These high rates ensure particle contamination is minimized for pharmaceutical and semiconductor manufacturing.
CFM (Cubic Feet per Minute) measures the volume of air moved per minute - it's a rate. ACH (Air Changes per Hour) measures how many times the entire room volume is replaced in one hour - it's relative to room size. A 300 CFM fan provides different ACH in different sized rooms: 7.5 ACH in a small 2,400 ft³ room, but only 3 ACH in a 6,000 ft³ room. ACH is more useful for comparing ventilation adequacy.
🌬️ Disclaimer: This calculator provides estimates based on standard formulas. Actual ventilation requirements depend on many factors including occupancy, activities, contaminant sources, and local building codes. Consult HVAC professionals and local codes for specific applications. ACH values assume well-mixed air distribution.