Air changes per hour (ACH) measures how many times the total volume of air in a room is replaced in one hour. To calculate ACH, multiply your air purifier's CADR (in CFM) by 60, then divide by your room's volume in cubic feet. A 250 CFM purifier in a 2,400 cu ft room delivers 6.25 ACH—right in the sweet spot for allergy management.
ACH is the definitive metric for evaluating whether your air purifier, ventilation system, or HVAC setup is actually cleaning your air fast enough. CADR tells you how much clean air a purifier produces; ACH tells you whether that's enough for your specific room.
ACH Calculator
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The ACH Formula
The formula for calculating ACH is straightforward:
ACH = (Airflow in CFM × 60) ÷ Room Volume in cubic feet
Where:
- Airflow (CFM) = Your purifier's CADR rating or your HVAC system's supply airflow
- × 60 = Converts CFM (per minute) to cubic feet per hour
- Room Volume = Length × Width × Ceiling Height (all in feet)
Worked Example
Room: 20 ft long × 15 ft wide × 8 ft ceiling = 2,400 cu ft Purifier: Coway AP-1512HH with 233 CFM CADR (smoke)
ACH = (233 × 60) ÷ 2,400 = 13,980 ÷ 2,400 = 5.83 ACH
This means all the air in the room passes through the purifier's HEPA filter 5.83 times every hour. At 5+ ACH, you'll see significant reductions in airborne particles within the first 20–30 minutes.
Reverse Calculation: Finding Required CADR
If you know your target ACH and room volume, solve for CADR:
Required CADR (CFM) = (Room Volume × Target ACH) ÷ 60
Room: 2,400 cu ft, Target: 6 ACH Required CADR = (2,400 × 6) ÷ 60 = 240 CFM
Recommended ACH Rates by Application
Healthcare vs. residential ACH: Hospital rooms require 6–12+ ACH by code (ASHRAE Standard 170). Operating rooms need 20–25 ACH. Residential spaces have no code requirement for ACH from air purifiers, but the recommendations above are based on published health research showing measurable improvements in air quality and symptoms.
What ACH Actually Means for Air Quality
ACH doesn't mean that after one air change, 100% of pollutants are removed. Filtration follows an exponential decay curve. Each air change removes a percentage of remaining particles.
After just 3 air changes, 95% of airborne particles are removed. At 5 ACH, you reach 95% removal in 36 minutes and 99%+ removal within an hour. This is why 5 ACH is considered the minimum for allergy relief—it gets you to 95% clean within roughly half an hour.
Why Higher ACH Matters
Higher ACH doesn't just filter faster—it maintains cleaner air against continuous contamination. Your room isn't sealed. You're constantly introducing new particles by:
- Opening doors and windows
- Walking, cooking, cleaning
- Pet movement and grooming
- Off-gassing from furniture and materials
- HVAC system cycling outdoor air in
At 3 ACH, there's a lag between new contamination and removal. At 8+ ACH, the purifier removes new particles almost as fast as they're introduced, maintaining consistently low levels.
ACH for HVAC Systems vs. Air Purifiers
ACH can apply to both air purifiers and HVAC systems, but the calculations differ slightly.
Air purifier ACH uses the CADR rating—the volume of actually filtered air. This accounts for filter efficiency and air bypass.
HVAC ACH uses the supply air volume at the register. However, most HVAC systems use MERV 8–13 filters, not HEPA. The effective particle removal per pass is lower. A MERV 13 filter captures about 85% of PM2.5 particles per pass, compared to 99.97% for HEPA.
MERV 8 filters (standard HVAC) provide very low effective ACH for fine particles. Your home HVAC running at 4 nominal ACH with a MERV 8 filter only delivers roughly 1.2 effective ACH for PM2.5. Upgrading to MERV 13 (if your system supports it) or adding a portable HEPA purifier dramatically improves effective air cleaning.
Real-World ACH Scenarios
Example 1: New Parent Setting Up a Nursery
Room: 12 ft × 12 ft, 8 ft ceiling = 1,152 cu ft Target: 6 ACH (recommended for infant rooms) Required CADR: (1,152 × 6) ÷ 60 = 115 CFM
A Levoit Core 300S (141 CFM CADR) achieves 7.3 ACH in this room—exceeding the target. Running on medium (estimated 85 CFM) still delivers 4.4 ACH, adequate for general protection.
Example 2: School Classroom Post-COVID
Room: 30 ft × 28 ft, 9 ft ceiling = 7,560 cu ft Target: 6 ACH (CDC classroom recommendation) Required CADR: (7,560 × 6) ÷ 60 = 756 CFM
No single portable unit achieves this. Solution: Two Coway Airmega 400 units (400 CFM each = 800 combined CFM) achieve 6.3 ACH. Alternatively, the school's HVAC system at MERV 13 can contribute 2–3 effective ACH, requiring the portables to supplement only 3–4 ACH.
Example 3: Home Gym
Room: 20 ft × 16 ft, 8 ft ceiling = 2,560 cu ft Target: 8 ACH (exercise increases particle generation and breathing rate) Required CADR: (2,560 × 8) ÷ 60 = 341 CFM
A Blueair Blue Pure 211i Max (350 CFM) achieves 8.2 ACH—just over target. During intense workouts, running on high maximizes particle removal during peak generation.
Example 4: Dental Office Operatory
Room: 10 ft × 10 ft, 9 ft ceiling = 900 cu ft Target: 12 ACH (CDC dental guidance for aerosol-generating procedures) Required CADR: (900 × 12) ÷ 60 = 180 CFM
A medical-grade unit like the IQAir HealthPro Plus (300 CFM) achieves 20 ACH in this room—well above the requirement. The extra capacity provides a safety margin during high-aerosol procedures.
ACH and Indoor Air Quality Standards
Several organizations set ACH recommendations:
ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) — Sets ventilation standards for commercial and healthcare facilities. Standard 62.1 covers commercial buildings; Standard 62.2 covers residential. Standard 170 covers healthcare.
CDC (Centers for Disease Control) — Published ACH guidance for infection control in healthcare settings and schools, especially post-COVID. Recommends 5+ ACH equivalent for classrooms and 6+ for healthcare settings.
WELL Building Standard — A certification program for commercial buildings that includes ACH requirements for occupant health. Requires specific ACH thresholds based on room type and occupancy.
EPA — Provides residential guidance on air cleaner effectiveness. Does not set specific ACH requirements but references the AHAM two-thirds rule (approximately 4.8 ACH) as a baseline.
Factors That Affect Your Actual ACH
Your calculated ACH assumes ideal conditions. Several real-world factors can reduce effective ACH:
Filter loading — As your HEPA filter accumulates particles, airflow resistance increases and actual CADR decreases. A filter at 80% capacity may deliver 10–20% less CADR than a fresh filter.
Room leakage — Gaps under doors, open windows, and air exchange with adjacent rooms introduce new contaminants that reset the filtration baseline. Higher ACH compensates for this.
Purifier placement — A purifier in a corner or behind furniture has restricted intake, reducing effective CADR. Place units with clear airflow paths (see our placement guide).
Running speed — Manufacturers report CADR at maximum speed. On low or medium, CADR drops 40–70%. Your actual ACH on a comfortable speed is much lower than the rated maximum.
Temperature stratification — In rooms with high ceilings, warm air rises and creates layers. Ground-level purifiers may not effectively cycle air at ceiling height. Ceiling fans help mix the air.
Key Takeaways
- ACH = (CADR × 60) ÷ Room Volume — this is the formula that matters
- 5 ACH is the minimum for allergy/asthma relief; 8+ ACH for smoke; 12+ ACH for medical use
- At 5 ACH with HEPA, 95% of particles are removed in 36 minutes
- Manufacturer room ratings often assume 2 ACH—far below what's needed for health benefits
- MERV 8 HVAC filters provide only ~30% particle capture per pass, drastically lowering effective ACH
- Higher ACH compensates for continuous contamination from doors, windows, cooking, and movement
- Real-world ACH is lower than calculated due to filter loading, placement, and running speed