Air Changes per Hour (ACH) Calculator
Enter your room dimensions and airflow rate to find how many times per hour the air in the space is completely replaced. Switch to reverse mode to find the airflow (CFM or CMH) needed to hit a target ACH. The calculator also shows how long it takes to remove 99% and 99.9% of airborne contaminants at your current ventilation rate, and compares your result against recommended ranges for common room types.
What is air changes per hour (ACH)?
Air changes per hour (ACH, also written ACPH) measures how many times the entire volume of air in a room is replaced or recirculated in one hour. A room with 6 ACH has its full volume of air exchanged six times every sixty minutes. Higher ACH means faster dilution of pollutants, pathogens, odors, and excess moisture, which is why healthcare facilities, laboratories, and commercial kitchens require far more ventilation than a typical bedroom. ACH is the standard metric used by ASHRAE (Standard 62.1 and 170), the CDC, and most building codes to specify ventilation requirements.
How ACH is calculated - the formula
The core formula is: ACH = (CFM x 60) / Room Volume (ft3). In metric units: ACH = CMH / Room Volume (m3). CFM stands for cubic feet per minute, the airflow rating of your HVAC system or air purifier. CMH is cubic metres per hour. The factor of 60 converts minutes to hours in the imperial version. For example, a 20 ft x 15 ft x 9 ft room has a volume of 2,700 ft3. If your HVAC delivers 200 CFM, the ACH is (200 x 60) / 2,700 = 4.44 ACH. To find the airflow needed to achieve a target ACH, rearrange the formula: CFM = (Target ACH x Room Volume) / 60. A target of 6 ACH in the same room needs (6 x 2,700) / 60 = 270 CFM.
ACH and contaminant removal: the first-order decay model
If you inject a burst of contamination into a perfectly mixed room, the concentration falls exponentially over time. The fraction remaining after T minutes is e^(-ACH x T/60). At 6 ACH it takes about 46 minutes to remove 99% of airborne particles, and 69 minutes for 99.9%. Doubling ACH from 6 to 12 halves those times to 23 and 35 minutes. This is why healthcare guidelines are so specific: every additional air change meaningfully reduces exposure risk. Real-world rooms are never perfectly mixed, so actual removal is slower than the model predicts, making the calculation a best-case estimate. The CDC recommends at least 5 ACH for occupied shared spaces; the Lancet Commission rates 4 ACH as "Good," 6 as "Better," and above 6 as "Best."
Measuring and improving your ACH
To use this calculator you need your room dimensions (measured from wall to wall at the floor and from floor to ceiling) and the airflow rate of your ventilation equipment. HVAC airflow in CFM can be read from the equipment label, measured with an anemometer or balometer at the supply register, or estimated from the unit capacity. Air purifier airflow is listed as CADR (Clean Air Delivery Rate) in the manufacturer specifications. If you have multiple devices, add their individual CFM values and enter the total. To increase ACH without upgrading ductwork: add a portable air purifier with a high CADR, use an energy recovery ventilator (ERV) or heat recovery ventilator (HRV) to bring in fresh outside air, or simply open windows when outdoor air quality and weather allow.
Recommended ACH by room type
| Room type | Recommended ACH | Notes |
|---|---|---|
| Residential bedroom | 2-4 | Minimum comfortable ventilation |
| Living room / office | 4-6 | Standard occupied space |
| Commercial office | 4-10 | ASHRAE 62.1 minimum; higher for dense occupancy |
| Classroom | 6-20 | Higher end for crowded or poorly-ventilated rooms |
| Conference room | 6-10 | Higher occupancy density than private offices |
| Restaurant / dining | 8-12 | Cooking odors and higher occupancy require more |
| Bathroom / restroom | 8-10 | Moisture and odor control |
| Commercial kitchen | 15-30 | Heat, steam, and grease require aggressive extraction |
| Hospital patient room | 6-12 | ASHRAE 170 minimum 6 ACH total |
| Operating room | 20-25 | ASHRAE 170 and CDC guidelines |
| Isolation room | 12+ | Negative-pressure rooms per CDC guidance |
| Laboratory | 6-12 | Chemical fume hoods add to requirement |
| Pharmacy / clean room | 10-15 | Contamination prevention |
| Gymnasium | 4-10 | High activity and occupancy |
These ranges reflect ASHRAE, CDC, and industry guidelines. Actual requirements depend on occupancy, local codes, and specific activities.
Frequently asked questions
What ACH do I need for a healthy home?
The CDC recommends at least 5 ACH for occupied shared spaces to reduce airborne pathogen exposure. The Lancet Commission on COVID-19 rates 4 ACH as "Good," 6 as "Better," and above 6 as "Best." For residential bedrooms and living areas a range of 2-6 ACH is typical, with 4-6 being a sensible target for spaces where multiple people gather.
What is the difference between CFM and ACH?
CFM (cubic feet per minute) is an absolute measure of airflow volume delivered by a fan, HVAC unit, or air purifier. ACH is a relative measure that puts that airflow in the context of the room size. The same 200 CFM fan provides 8.9 ACH in a small 400 ft3 room but only 3.3 ACH in a 1,080 ft3 room. Use CFM when comparing equipment; use ACH when evaluating whether a room is adequately ventilated.
Does ACH account for air quality or just air quantity?
ACH counts how often the air volume is exchanged, not whether the replacement air is clean. A high ACH using recirculated air without filtration dilutes contaminants less effectively than a lower ACH using fresh filtered air. For infection control and indoor air quality, the type of ventilation matters: HEPA filtration, UV-C treatment, and outdoor air supply all improve effective air quality beyond what raw ACH alone captures.
What ACH is required for hospital rooms?
ASHRAE Standard 170 requires a minimum of 6 total ACH for general patient rooms, with at least 2 of those being outdoor air. Operating rooms require 20-25 ACH. Negative-pressure airborne infection isolation rooms must maintain at least 12 ACH with 2 outdoor air changes, plus continuous negative pressure relative to adjacent spaces, per CDC and ASHRAE guidelines.
How many air changes per hour does an air purifier provide?
It depends on the purifier CADR and the room size. Use the forward mode of this calculator: enter your room dimensions and the purifier CADR (in CFM) as the airflow. A purifier rated 200 CFM CADR in a 400 ft3 room delivers (200 x 60) / 400 = 30 ACH, but in a 1,000 ft3 room the same purifier only delivers 12 ACH. Most manufacturers rate their purifiers for a specific room size at a target of 4 or 5 ACH.
What is the 99% contaminant removal time shown in the results?
This is a theoretical estimate based on the first-order exponential decay model: concentration drops by a factor of e each time the full air volume is exchanged. The formula is time = -ln(1-0.99) / (ACH/60) = ln(100) x 60 / ACH. It assumes instantaneous perfect mixing throughout the room, which overstates real-world performance. In practice, rooms have dead zones and concentration gradients that make actual removal slower. Treat it as a best-case benchmark for comparing ventilation scenarios.