Room Air Changes Calculator
ACH and minutes per change from volume and flow.
What it does
Air changes per hour turn fan flow into room rhythm. A 1,152 ft3 bedroom with 100 CFM sees 5.21 changes an hour, or one full pass every 11.5 minutes. Bathrooms, kitchens, and shops each have their own targets because moisture and fumes differ.
The tool above divides hourly flow by volume. Defaults give 5.21 ACH imperial and 5.17 ACH metric on the mirrored room, with minutes per change beside each. Hourly airflow sits third so fans compare head to head regardless of room.
Flow honesty matters more than volume precision. Nameplate CFM ignores ducts, grilles, and dirty filters, while metered flow at working pressure tells what the room feels. Small rooms swing fast with tiny flow changes, so measure after the install settles.
This page times ideal mixing only. It does not model short circuits, closed doors, filters, or pollutant sources. Pair the rate with source control and filtration for real air quality.
How to use
- Pick imperial (ft3, CFM) or metric (m3, m3 per h).
- Enter the room volume.
- Enter the metered airflow.
- Read ACH first, then minutes per change.
- Read hourly airflow for fan comparison.
- Compare against the room target before sizing.
Formula
- volume
- Room volume in ft3 or m3.
- flow
- Metered fan flow in CFM or m3 per h.
- ACH
- Full room volumes replaced per hour.
Ventilation rate with one steady airflow and one mixed room. At zero flow there are no changes. Short circuiting and closed doors sit outside this ideal mixing math.
Worked example
Example: 1,152 ft3 at 100 CFM
Defaults: imperial 1,152 ft3 (12x12x8) with 100 CFM. Metric mirror is 32.9 m3 at 170 m3 per h.
- Inputs (imperial default): 1,152 ft3 room with 100 CFM.
- Hourly: 100 times 60 is 6,000 ft3 per h.
- ACH: 6,000 / 1,152 is 5.21 ACH.
- Minutes: 60 / 5.21 is 11.5 min per change.
- Metric mirror (32.9 m3 at 170 m3 per h) gives 5.17 ACH and 11.6 min. Tool shows the same.
Reference table
| Room and flow | ACH | Minutes |
|---|---|---|
| 1152 ft3 at 100 CFM | 5.21 ACH | 11.5 min |
| 2000 ft3 at 150 CFM | 4.5 ACH | 13.3 min |
| 32.9 m3 at 170 m3/h | 5.17 ACH | 11.6 min |
| 50 m3 at 200 m3/h | 4 ACH | 15 min |
| 800 ft3 at 80 CFM | 6 ACH | 10 min |
Limits
- Ideal mixing assumed. Short circuits excluded.
- Steady flow only. Cycling fans need runtime math.
- No filtration or source model.
- Closed doors and zones excluded.
Common mistakes
- Entering nameplate CFM instead of metered flow.
- Using floor area as volume without height.
- Chasing ACH while ignoring the source.
- Comparing rooms with different targets as if equal.
- Forgetting ducts that halve the flow.
Try it yourself
Quick checks. Answers sit tucked under each question.
2,000 ft3 at 150 CFM?
ACH is 150 times 60 / 2000 is 4.5 ACH. Minutes are 60 / 4.5 is 13.3 min per change.
50 m3 at 200 m3 per h?
ACH is 200 / 50 is 4 ACH. Minutes are 15 min per change.
Questions
How are air changes figured?
ACH = CFM times 60 / volume. Here 6,000 / 1,152 is 5.21 ACH.
How is minutes per change figured?
Minutes = 60 / ACH. Here 11.5 min per change.
What ACH suits a bathroom?
Bathrooms often target about 8 ACH, or 1 CFM per ft2 with 8 ft ceilings. Kitchens and workshops run higher.
Which airflow number do I enter?
Use metered flow at working pressure. Long ducts and dirty filters cut nameplate flow a lot.
Does metric work the same?
Yes with the same math using m3 per h over m3. Here 170 / 32.9 is 5.17 ACH.
Does high ACH mean clean air?
No. This times air movement only. Source control and filtration decide actual air quality.