About this calculator This calculator determines the required airflow (CFM) based on room dimensions and a target number of air changes per hour (ACH). Use it when designing ventilation for spaces with prescriptive ACH requirements such as kitchens, labs, or mechanical rooms. The result represents the minimum supply or exhaust airflow needed to achieve the specified ventilation rate.
Use when You need to determine airflow requirements based on room size and desired air changes.
Formula CFM = (Length × Width × Height × ACH) ÷ 60
Variables
Length Room length in feetWidth Room width in feetHeight Room height in feetACH Air changes per hour
Length (ft)
Room length in feet
Width (ft)
Room width in feet
Height (ft)
Room height (floor to ceiling) in feet
Air Changes per Hour (ACH)
Required ventilation rate in air changes per hour
CFM —

What are air changes per hour (ACH)?

Air changes per hour (ACH) is the number of times the total volume of air in a room is replaced in one hour. It is used to specify ventilation rates for spaces with prescriptive code requirements, including healthcare facilities, laboratories, and commercial kitchens. The required airflow to achieve a target ACH is: CFM = (Length × Width × Height × ACH) / 60, where room dimensions are in feet and the division by 60 converts cubic feet per hour to cubic feet per minute. ACH requirements vary by space type and governing code. ASHRAE Standard 170 specifies 20 total ACH for operating rooms and 6 total ACH for patient rooms. Laboratory ventilation typically requires 6–12 ACH depending on hazard classification. For most commercial spaces, ASHRAE Standard 62.1 uses the Ventilation Rate Procedure based on occupancy and floor area rather than ACH.

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