Air Changes Per Hour Chart
Typical air changes per hour (ACH) by space type — how many times per hour the air in a room is replaced. For most spaces these are design rules of thumb; the healthcare rows are code minimums from ASHRAE Standard 170. Convert any ACH to airflow with CFM = ACH × room volume ÷ 60.
ACH by space type
| Space type | Typical ACH | Basis |
|---|---|---|
| Office | 4–8 | Rule of thumb |
| Classroom | 4–8 | Rule of thumb |
| Restaurant dining | 8–12 | Rule of thumb |
| Commercial kitchen | 15–30 | Rule of thumb |
| Restroom / toilet | 6–15 | Rule of thumb |
| Retail | 6–10 | Rule of thumb |
| Gym | 4–8 | Rule of thumb |
| Warehouse | 4–6 | Rule of thumb |
| Laboratory (general) | 6–12 | Rule of thumb |
| Hospital patient room | 6 (2 outdoor) | Code (ASHRAE 170) |
| Hospital operating room | 20 (4 outdoor) | Code (ASHRAE 170) |
| Airborne infection isolation room | 12 | Code (ASHRAE 170) |
Air changes vs. outdoor air
Air changes per hour and outdoor-air ventilation are two different measures, and it is easy to conflate them. ACH counts how many times the total supply air replaces the room volume per hour — most of that is recirculated air moved for heating, cooling, and mixing. The outdoor-air rate from ASHRAE 62.1 is the fraction that must be fresh. A space can have a high ACH but a modest outdoor-air requirement, or the reverse. For ordinary occupancies the enforceable minimum is the 62.1 outdoor-air cfm; ACH is a convenient design target on top of that.
Where ACH is the requirement is healthcare and certain exhaust-driven spaces. ASHRAE 170 sets operating rooms at 20 air changes per hour, patient rooms at 6, and isolation rooms at 12 — these protect against infection and are mandatory, not guidelines. Kitchens and restrooms are driven by exhaust rates in the mechanical code.
Common questions
How do you calculate CFM from air changes per hour?
Multiply the ACH by the room volume in cubic feet and divide by 60: CFM = ACH × volume ÷ 60. For example, a 1,000 sqft room with a 10 ft ceiling is 10,000 cubic feet, so 5 air changes per hour is 5 × 10,000 ÷ 60 = about 833 CFM.
How many air changes per hour does a commercial kitchen need?
A commercial kitchen typically runs 15–30 air changes per hour — far more than most spaces — because it is exhaust-driven, pulling out heat, grease, and odors through the hoods with makeup air replacing it. An office, by contrast, is usually 4–8 ACH.
Are ACH values a code requirement?
Mostly no. For ordinary spaces, ACH is a design rule of thumb — the actual code minimum is the outdoor-air ventilation rate from ASHRAE 62.1 (in cfm), not an air-change count. The exception is healthcare: hospital operating rooms, patient rooms, and isolation rooms have air-change minimums mandated by ASHRAE Standard 170. Some restroom and kitchen exhaust rates are also set by the mechanical code.
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