Air Changes Per Hour: What ACH Means and How to Calculate It
Understand air changes per hour (ACH): what it measures, ASHRAE targets for different room types, how to calculate the airflow needed, and why it matters for indoor air quality.
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What Are Air Changes Per Hour?
Air changes per hour (ACH or ACPH) measures how many times per hour the total volume of air in a space is replaced by fresh or conditioned air. An ACH of 6 means the room's full air volume is exchanged six times every hour.
ACH is used for two distinct purposes: 1. HVAC design — determines how much airflow (CFM) a supply duct must deliver 2. Indoor air quality (IAQ) — measures ventilation adequacy for occupant health and contaminant dilution
Recommended ACH by Room Type
ASHRAE Standard 62.1 governs ventilation for non-residential buildings. ASHRAE 62.2 governs residential ventilation.
How to Calculate ACH
From known CFM:
ACH = CFM × 60 ÷ Room Volume
Where Room Volume = Length × Width × Ceiling Height
Example: A 20 ft × 15 ft conference room with 10-ft ceilings receiving 800 CFM: - Volume: 20 × 15 × 10 = 3,000 cubic feet - ACH = 800 × 60 ÷ 3,000 = 16 ACH ✓ (within 8–12 target range for this space)
From target ACH to required CFM:
CFM = Room Volume × ACH ÷ 60
Example: A 12 ft × 10 ft bedroom (9-ft ceilings) targeting 6 ACH: - Volume: 12 × 10 × 9 = 1,080 cubic feet - CFM = 1,080 × 6 ÷ 60 = 108 CFM
ACH and Indoor Air Quality
Low ACH leads to accumulation of indoor air pollutants:
- CO₂ from occupant respiration accumulates above 1,000 ppm in poorly ventilated spaces. Above 2,500 ppm, cognitive performance measurably decreases. ASHRAE targets outdoor air supply to keep CO₂ below 1,000 ppm.
- VOCs (volatile organic compounds) from furniture, adhesives, cleaning products, and building materials require dilution ventilation.
- Particulates (PM2.5) from cooking, combustion, and outdoor pollution penetrate without adequate filtration and air exchange.
- Biological contaminants (mold spores, bacteria) concentrate in low-ACH spaces, particularly in humid rooms like bathrooms and kitchens.
COVID-19 guidance drove significant attention to ACH: the CDC and ASHRAE recommend at least 5 ACH of clean air (filtered and/or outdoor air) in occupied indoor spaces during community transmission periods.
Measured ACH (Blower Door Test)
In building science, ACH50 is the air changes per hour at 50 Pascals of pressure differential — measured by a blower door test. This is the standard for measuring building envelope airtightness:
Natural ACH under normal wind conditions is roughly ACH50 ÷ 20 for most climates. A home at ACH50 = 8 has approximately 0.4 natural air changes per hour — well below the minimum recommended for good IAQ.
How the Calculator Works
The calculator takes room dimensions (length, width, ceiling height) and target ACH, and outputs: - Required airflow in CFM - Room volume in cubic feet
Alternatively, enter existing CFM and room volume to calculate current ACH for comparison against targets.
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Frequently Asked Questions
- What is a good ACH for a bedroom?
- ASHRAE recommends 5–6 air changes per hour for residential bedrooms. This balances adequate ventilation for occupant health with energy efficiency. Lower ACH (3–4) may be acceptable in well-sealed, filtered homes with mechanical ventilation systems.
- How do I calculate air changes per hour from CFM?
- ACH = (CFM × 60) ÷ Room Volume. Room Volume = Length × Width × Ceiling Height. Example: A room 15 × 12 × 9 ft receiving 180 CFM: Volume = 1,620 cu ft. ACH = (180 × 60) ÷ 1,620 = 6.7 ACH.
- What ACH is required for an operating room?
- ASHRAE 170 requires 15–25 air changes per hour for hospital operating rooms, with a minimum of 20% outdoor air. These requirements ensure surgical site infection control and anesthetic gas dilution. They are dramatically higher than residential requirements.
- What is ACH50 in a blower door test?
- ACH50 measures how many times per hour a building's full air volume leaks through the envelope when a 50 Pascal pressure difference is applied (simulating a moderate wind). It measures airtightness, not ventilation. A tight modern home scores 1–3 ACH50; typical existing homes score 7–15.
- How does ACH relate to indoor air quality?
- Higher ACH dilutes indoor air pollutants including CO₂, VOCs, particulates, and biological contaminants. Below 4 ACH, CO₂ accumulates above 1,000 ppm in occupied rooms, which impairs cognitive performance. Bathrooms need 6–8 ACH to control humidity and odors; commercial kitchens need 20–60 ACH for smoke and grease management.
Last updated 7/28/2026