Airflow Rate Calculator

An airflow rate calculator finds air changes per hour (ACH): the number of times a room's air is fully replaced in one hour. It also works in reverse, finding the airflow a fan or vent must supply to reach a target ACH.

What is air changes per hour?

A fan or vent moves air into a room and pushes stale air out. If that airflow equals one roomful of air per hour, the room gets 1 air change per hour. If it moves two roomfuls in an hour, it gets 2 ACH. A higher ACH usually means fresher air, less trapped moisture, and fewer airborne particles. Very high ACH also costs more, since a fan running harder uses more energy and pulls in more air that needs heating or cooling.

Airflow rate formula

ACH equals airflow divided by room volume. Volume is length times width times height, all in the same unit.

$\text{Volume} = L \times W \times H$

In metric units, airflow is measured in cubic metres per hour (m³/h), which already matches the "per hour" in ACH:

$\text{ACH} = \frac{\text{Airflow (m³/h)}}{\text{Volume (m³)}}$

In imperial units, airflow is usually given in CFM, cubic feet per minute. Multiplying by 60 converts it to cubic feet per hour before dividing by the room volume:

$\text{ACH} = \frac{60 \times \text{Airflow (CFM)}}{\text{Volume (ft}^3\text{)}}$

Airflow needed for a target ACH

To size a fan or air purifier, the formula is rearranged to solve for airflow instead of ACH.

$\text{Required airflow (m}^3\text{/h)} = \text{ACH} \times \text{Volume (m}^3\text{)}$

$\text{Required airflow (CFM)} = \frac{\text{ACH} \times \text{Volume (ft}^3\text{)}}{60}$

Worked example

A meeting room measures 5 m by 4 m by 3 m. Its volume is:

$5 \times 4 \times 3 = 60 \text{ m}^3$

A vent supplies 360 m³/h of air. Its ACH is:

$\text{ACH} = \frac{360}{60} = 6$

The room gets 6 air changes per hour.

The same numbers work in reverse. A 6,000 ft³ room that needs 6 ACH requires:

$\frac{6 \times 6{,}000}{60} = 600 \text{ CFM}$

Recommended air changes by room type

The right ACH depends on how a space is used. A kitchen or bathroom needs more air changes than a bedroom, because cooking and bathing add moisture and odor. These are common engineering guideline ranges. Local building codes and standards such as ASHRAE 62.1 take precedence on regulated projects.

Room type	Typical ACH
Warehouse	1–2
Bedroom	5–6
Classroom	5–6
Living room	6–8
Bathroom	6–8
Office	6–8
Hospital room	6–12
Retail store	6–10
Kitchen (residential)	7–8
Conference room	8–12
Restaurant / bar	8–12
Laboratory	8–12
Commercial kitchen	15–30

For reducing the risk of airborne illness, the US Centers for Disease Control and Prevention (CDC) suggests aiming for 5 or more air changes per hour of clean air in occupied indoor spaces.

Frequently asked questions

What is the difference between airflow rate and ACH? Airflow rate is a volume of air per unit of time, measured in m³/h or CFM. ACH compares that airflow to the size of a specific room. The same fan gives a high ACH in a small closet and a low ACH in a large hall.

What does CFM mean? CFM stands for cubic feet per minute, the standard airflow unit for fans, air purifiers, and HVAC equipment. Multiplying CFM by 60 converts it to cubic feet per hour, which is what the ACH formula needs.

Is a higher ACH always better? Not always. Air changes beyond what a room needs waste fan energy and the energy used to heat or cool the incoming air. Matching the recommended range for the room type is usually the goal.

Does this calculator size a whole HVAC system? No. It sizes the ventilation rate for one room. A full HVAC design also accounts for heating and cooling loads, duct losses, and outdoor-air minimums based on occupancy.

Why does the imperial formula multiply by 60? CFM measures airflow per minute, but ACH measures air changes per hour. Multiplying CFM by 60 converts minutes to hours before dividing by room volume.

Airflow Rate Calculator — Air Changes Per Hour (ACH) & CFM