Enter room volume in cubic feet and an explicitly chosen ACH. The example 4 ACH is a scenario, not a general residential or clinical ventilation minimum. Volume is length × width × height.
Whether the airflow represents outdoor replacement air, exhaust or effective filtered clean air matters; those quantities are not automatically interchangeable.
The tool converts volume and ACH; it does not select a code minimum or establish infection-control clearance. Short-circuiting, stagnant zones, continuing sources and ineffective filtration change removal. Match delivered airflow to the operating resistance rather than relying on a free-air fan rating.
Makeup-air and pressure effects need separate design.
Required airflow (CFM) from room volume
| Room volume | 1,200 |
|---|---|
| Air changes per hour | 4 |
| Required airflow (CFM) | 80 |
| Minutes per air change | 15 |
Common error: Buying a fan by its free-air rating. Duct length, bends and a hood or grille all cut the airflow, and the delivered figure can be well below the label.
CFM / Air Changes Per Hour: formula
ASHRAE Handbook; Fundamentals · CDC: Airborne-contaminant removal table
CFM / Air Changes Per Hour: changing Room volume
| Room volume | Required airflow (CFM) | Minutes per air change |
|---|---|---|
| 960 | 64 | 15 |
| 1,080 | 72 | 15 |
| 1,200 (example) | 80 | 15 |
| 1,320 | 88 | 15 |
| 1,440 | 96 | 15 |
CFM / Air Changes Per Hour: compare input ranges
| Input | Tested from | To | Required airflow (CFM): low input | Required airflow (CFM): high input | Swing |
|---|---|---|---|---|---|
| Room volume | 1,080 | 1,320 | 72 | 88 | 16 (20%) |
| Air changes per hour | 3.6 | 4.4 | 72 | 88 | 16 (20%) |
CFM / Air Changes Per Hour: individual input comparisons
| Input | Value | Required airflow (CFM) | Minutes per air change |
|---|---|---|---|
| Room volume | 960 | 64 | 15 |
| 1,080 | 72 | 15 | |
| 1,200 | 80 | 15 | |
| 1,320 | 88 | 15 | |
| 1,440 | 96 | 15 | |
| Air changes per hour | 3.2 | 64 | 18.8 |
| 3.6 | 72 | 16.7 | |
| 4 | 80 | 15 | |
| 4.4 | 88 | 13.6 | |
| 4.8 | 96 | 12.5 |
Required airflow (CFM): Room volume against Air changes per hour
| Room volume / Air changes per hour | 3.2 | 4 | 4.8 |
|---|---|---|---|
| 960 | 51.2 | 64 | 76.8 |
| 1,080 | 57.6 | 72 | 86.4 |
| 1,200 | 64 | 80 | 96 |
| 1,320 | 70.4 | 88 | 105.6 |
| 1,440 | 76.8 | 96 | 115.2 |
CFM / Air Changes Per Hour: values and definitions
| Quantity and definition | Value used |
|---|---|
| Room volume — Room volume in cubic feet (length × width × height). | 1,200 |
| Air changes per hour — Target number of complete air changes per hour. | 4 |
| Required airflow (CFM) | 80 |
| Minutes per air change | 15 |
Room volume: half and double
| Room volume | Required airflow (CFM) |
|---|---|
| 600 | 40 |
| 1,200 | 80 |
| 2,400 | 160 |
An air change rate turns a room's volume into a fan size. A 1,200 cubic foot room at 4 air changes per hour needs about 80 cubic feet per minute, which replaces the room's air every 15 minutes. Kitchens, bathrooms and workshops need higher rates than bedrooms, and the right number depends on what is being cleared: moisture, odor, fumes or dust.
The rate for your case comes from the ventilation code or the equipment maker.
Find the recommended air change rate for the room's use and check the fan's rating at the static pressure of the actual duct run, which is lower than the free-air rating.
Size ducts on the duct sizing calculator, pressure on the static pressure page and equipment on the HVAC tonnage calculator. The cubic feet and rectangular prism calculators give room volume, and the SEER upgrade page covers efficiency.
Does 4 ACH remove all old air every fifteen minutes?
No. Fifteen minutes is one volume-equivalent turnover. Ideal mixing gives exponential removal rather than complete one-pass replacement.
How long does ideal 99% removal take at 4 ACH?
About 69 minutes with no continuing source and perfect mixing, consistent with the CDC reference table. Real rooms can take longer.
Does this choose bathroom or laboratory ventilation requirements?
No. Enter the applicable target from the actual design and governing requirements. A generic ACH assumption does not establish those requirements.