Air Changes Per Hour Calculator

How Often the Air in a Room Gets Replaced

Air changes per hour (ACH) measures how many times the entire volume of air in a space is replaced in an hour — a number that matters far beyond comfort. Kitchens, bathrooms, labs, and mechanical rooms all have minimum ACH targets set by code or by the equipment inside them, and ventilation design starts with knowing what a given airflow actually achieves in a specific room.

The Formula

Room Volume = Length × Width × Height
ACH = (CFM × 60) / Room Volume

CFM is the airflow rate in cubic feet per minute; multiplying by 60 converts it to cubic feet per hour, which is then divided by the room's total volume to get the number of complete air changes per hour.

A Worked Example

150 CFM exhaust fan in a 20 × 15 × 8 ft room
Step	Value
Room Volume (20 × 15 × 8)	2,400 cu ft
Airflow (150 × 60)	9,000 cu ft/hr
ACH	3.75

Common ACH Targets

Typical minimum ACH targets by space type
Space Type	Typical ACH Target
Residential living space (general ventilation)	0.35–1
Residential bathroom (intermittent exhaust)	8
Residential kitchen (exhaust)	15+
Office space	4–6
Laboratory (general)	6–12

Targets vary by local code and specific use; always check applicable building or mechanical code for a required minimum.

Where This Calculation Matters

Exhaust fan selection — sizing a bathroom or kitchen fan to hit a required ACH for a specific room volume.
Ventilation code compliance — many building and mechanical codes specify minimum ACH for particular room types.
Indoor air quality — higher ACH dilutes odors, moisture, and airborne contaminants faster.
Grow rooms and workshops — spaces with elevated humidity or fume concerns often target a specific ACH well above residential norms.

How to Use This Calculator

Enter the Airflow in CFM.
Enter the Room Length, Width, and Height in feet.
Select Calculate to see the resulting air changes per hour.

Related Calculations

Work out the airflow a target temperature rise or drop requires with the Airflow CFM Calculator, or size the duct that will carry it with the HVAC Duct Sizer Calculator.