Air Changes Per Hour
5
ACH
Meets the 5 ACH target
Room Volume
960
ft³
Full Air Change
12 min
per change
How we calculated this
= 960 ft³
= 5 ACH
Assumes the rated CFM is actually delivered to the room. Duct losses, dirty filters, and door undercuts can cut real-world airflow well below the label.
Reference
Recommended ACH by Room Type
Air circulation targets for residential rooms. These describe total air movement, not the much smaller fresh-air requirement.
| Room Type | Target ACH | Why |
|---|---|---|
| Bedroom | 5–6 | Stationary occupants, modest demands |
| Living Room | 6–8 | More activity and occupancy |
| Kitchen | 7–8 | Cooking fumes and heat |
| Bathroom | 8–10 | Moisture and odors, code minimum 50 CFM |
| Office | 6–8 | Electronics heat, long occupancy |
| Garage | 6–8 | Exhaust and fumes when in use |
| Workshop | 10–12 | Dust, vapors, combustion byproducts |
Learn
Understanding Air Changes Per Hour
What ACH measures
ACH (air changes per hour) is how many times a room's entire air volume gets replaced in an hour. It's the great equalizer of ventilation: a fan's CFM rating means nothing until you know the size of the room it serves. Five ACH in a bedroom feels fresh; the same fan in a room four times the size delivers barely one change an hour and the air goes stale.
The formula
ACH = CFM × 60 / room volume (ft³). Airflow in cubic feet per minute, times 60 minutes, divided by the room's volume. It rearranges usefully in both directions: the CFM you need for a target is volume × ACH / 60, and the time for one full air change is 60 / ACH minutes.
Circulation vs. fresh air
The room-by-room targets here (5–12 ACH) describe air circulation, air moved through the room by fans or HVAC, most of it recirculated. Fresh outdoor air is a separate, much smaller number: ASHRAE 62.2 calls for about 0.35 ACH of outdoor air for a whole house. A room can have excellent circulation and still need more fresh air, which is what ERVs and HRVs are for. Don't compare a bathroom fan spec against the 0.35 figure; they answer different questions.
Rated CFM is not delivered CFM
The number on the fan box assumes free air with no resistance. Real installations push through ducts, elbows, dampers, and grilles, and delivered airflow commonly lands 20–40% below the rating. A bathroom fan on a long flex-duct run can deliver half its label. If the calculated ACH is borderline, measure or assume the pessimistic end before deciding the fan is adequate.
When higher ACH matters
Push to the top of the target range, or past it, for rooms that generate moisture, odors, or particulates: bathrooms without windows, kitchens with serious cooking, workshops with dust or fumes, and any room used for isolation during illness. The CDC's guidance for infection control uses 6–12 ACH as the reference band for clinical spaces, which is a useful calibration for "a lot of ventilation."
FAQ
Common Questions About ACH
How do you calculate air changes per hour?
Multiply the airflow in CFM by 60 to get cubic feet per hour, then divide by the room's volume: ACH = CFM × 60 / volume. An 80 CFM fan in a 12 × 10 × 8 room (960 ft³) gives 80 × 60 / 960 = 5 ACH. This calculator also runs it backwards and tells you the CFM you'd need to hit your target.
What is a good ACH for a house?
Two different questions hide in there. For fresh outdoor air, ASHRAE 62.2 targets a modest 0.35 ACH for the whole house. For air circulation through the HVAC system or fans, individual rooms target much higher numbers: bedrooms 5–6, living areas 6–8, kitchens 7–8, bathrooms 8–10, and workshops 10–12. This calculator measures the circulation number.
What ACH do I need for a bathroom?
8–10 ACH. Codes typically require at least a 50 CFM exhaust fan for a full bath, and in a typical 5 × 8 × 8 bathroom (320 ft³) that works out to about 9 ACH, right in the target band. Bigger bathrooms need proportionally more: a 10 × 10 × 9 master bath needs 120–150 CFM to hit the same turnover.
What's the difference between ACH and CFM?
CFM is the raw flow rate; ACH is that flow normalized by room size. The same 80 CFM fan turns over a small powder room 15 times an hour but a large great room barely twice. That's why targets are written in ACH (they describe the outcome) while equipment is rated in CFM (it describes the machine). Convert with ACH = CFM × 60 / volume.
How can I increase the ACH in a room?
Two levers: add airflow or stop losing it. Adding is straightforward: a higher-CFM fan, a second exhaust point, or another supply register. Doubling delivered CFM doubles ACH. The losses are sneakier: dirty filters, crushed flex duct, and a closed door with no return path can cut delivered airflow far below the fan's label. Fix the cheap stuff first, then size up. The CFM Calculator gives the airflow to aim for.
Related Tools
CFM Calculator
The inverse problem: the airflow a room needs for a target ACH.
↳ 12×10 bedroom · 5 ACH → 80 CFM
Duct Size Calculator
Size the duct that has to deliver the airflow.
↳ 400 CFM @ 800 fpm → 10" round
BTU Calculator
Pair ventilation with the room's heating and cooling needs.
↳ 250 sq ft · hot climate → ~9,000 BTU