Ventilation Rate Calculator
Room volume to air changes per hour from CFM or L/s.
What it does
Air changes per hour, or ACH, says how fast room air is replaced. A bedroom at 4 ACH gets its full volume swapped about four times each hour. That one number lets you compare a den with a hall without guessing from fan noise.
The tool above takes room size and airflow and returns volume, ACH, and a plain rating. Imperial mode uses feet and CFM; metric mode uses meters and liters per second. The math matches ASHRAE wording: airflow divided by room volume, scaled to an hour.
ASHRAE terminology defines air changes as ventilation airflow divided by room volume. EPA ventilation worksheets and California public health guidance use the same relation: ACH = CFM x 60 / volume in cubic feet. Health pages point to about 5 ACH or 30 CFM per person as a boost target during outbreaks.
Use the result to size action. Under 3 ACH, a room relies on luck and leaks. From 3 to 6, most living spaces feel fresh. Above 6, fans and drafts start to dominate, which suits gyms and workshops but can annoy in a quiet study.
How to use
- Pick imperial (ft, CFM) or metric (m, L/s).
- Enter room length, width, and ceiling height.
- Enter supply or outdoor airflow for the room.
- Read volume, ACH, and the Low, Good, or High rating.
- Compare with the table for the room type.
- Adjust fan speed or openings and rerun the numbers.
Formula
- Q
- Airflow: cubic feet per minute (imperial) or liters per second (metric).
- V
- Room volume: cubic feet or cubic meters.
- ACH
- Air changes per hour.
Volume uses cubic feet with CFM and cubic meters with liters per second. To go the other way, required airflow = ACH target x volume / 60 (CFM) or x volume / 3.6 (L/s). The rating bands are planning guides, not code.
Worked example
Example: bedroom at 125 CFM
A 16 ft by 13 ft by 9 ft room with 125 CFM of outdoor air.
- Volume: 16 x 13 x 9 = 1,872.0 ft3.
- Airflow is 125 CFM.
- ACH: 125 x 60 / 1872 = 4.0 ACH.
- Rating: 4.0 sits in the Good band.
- Read: suits a bedroom or office; add airflow for gyms or workshops.
Reference table
| Space | Common target | Note |
|---|---|---|
| Bedroom or office | 3 to 6 ACH | Fresh without strong drafts |
| Living room | 3 to 5 ACH | Comfort band |
| Classroom | 4 to 6 ACH | Boost when full |
| Gym or workshop | 6 or more ACH | High turnover wanted |
| Bath or kitchen (boost) | 8 or more ACH | Short bursts while in use |
Limits
- One zone only. Leaky doors and open plans share air with neighbors.
- Assumes mixed air. Short circuiting supply to exhaust reads better than it feels.
- Rating bands are guides, not building code.
- Filter grade and humidity sit outside this math.
Common mistakes
- Entering floor area instead of volume.
- Using total system CFM for one small room.
- Mixing liters per second with cubic feet.
- Counting a recirculating purifier as outdoor air.
- Sealing a room tight, then wondering why CO2 climbs.
Try it yourself
Quick checks. Answers sit tucked under each question.
ACH for 6 m by 5 m by 3 m at 100 L/s?
Volume is 90.0 m3. ACH is 100 x 3.6 / 90 = 4.0 ACH, rated Good.
ACH for 20 ft by 16 ft by 10 ft at 200 CFM?
Volume is 3,200.0 ft3. ACH is 200 x 60 / 3200 = 3.8 ACH, rated Good.
Questions
What is the difference between ACH and CFM?
ACH counts how many times the room air volume is replaced in an hour. CFM is the flow that does it. Same CFM gives a high ACH in a small room and a low ACH in a hall.
What ACH should a bedroom have?
Bedrooms and offices often target 3 to 6. Kitchens, baths, and gyms want more when in use. Health guidance has pointed to about 5 during outbreaks, so treat that as a boost mode rather than a daily must.
How do I get room volume with an open plan?
Multiply length by width by height in matching units. For open plan areas, use the connected volume the same system serves, not just the seating corner.
Does a portable air purifier raise ACH?
No. Filters clean recirculated air but do not replace it. The tool counts outdoor or equivalent clean airflow. A purifier helps, but fresh air or filtered supply still sets ACH.
What are signs of weak ventilation?
Dust, cooking smoke, and stuffiness that clears slowly after airing out. CO2 meters also help: levels that stay high with people present point to weak air turnover.
How do I raise a low reading?
Raise fan speed, open a window on the far side for cross flow, clear supply grilles, and check filters. Recompute after changes; small rooms respond fast.