How do I calculate air changes per hour (ACH)?

ACH = (CFM x 60) / Volume. Multiply the airflow rate in cubic feet per minute (CFM) by 60 to get cubic feet per hour, then divide by the room volume in cubic feet. For example, 200 CFM in a 1,440 cu ft room (15 x 12 x 8) gives 8.33 ACH.

What is a good ACH50 for a house?

ACH50 below 3 is very tight (passive house level), 3-5 is tight (modern energy-efficient), 5-10 is average (typical existing construction), 10-20 is leaky (older home), and above 20 is very leaky. Energy Star requires ACH50 of 4-7 depending on climate zone.

How do I convert CFM50 to natural ACH?

Use the LBL (Lawrence Berkeley National Laboratory) correlation: Natural ACH = CFM50 / (N x Volume). The N-factor depends on the number of stories, wind exposure, and climate severity. For a typical 1-story home in a cold climate, N is approximately 16.7.

What is the ASHRAE 62.2 ventilation requirement?

ASHRAE 62.2-2019 requires minimum mechanical ventilation of 0.01 CFM per square foot of floor area plus 7.5 CFM per person (where persons = bedrooms + 1). For a 2,000 sq ft, 3-bedroom home: 0.01 x 2000 + 7.5 x 4 = 50 CFM. An infiltration credit from a blower door test may reduce this requirement.

Air Change Rate Calculator

Calculate air changes per hour (ACH) from airflow and room volume, convert blower door test results to natural infiltration, and find ASHRAE 62.2 ventilation requirements. Includes a searchable room ACH reference.

Calculation Direction

Airflow Rate

Airflow(CFM)

Room Dimensions

Length(ft)

Width(ft)

Ceiling Height(ft)

Volume: 1,440 ft³

Understanding Air Change Rates

What Is ACH?

Air changes per hour (ACH) measures how many times the entire volume of air in a room is replaced in one hour. An ACH of 6 means the room's air volume is exchanged six times per hour. Higher ACH removes contaminants faster but increases energy costs because conditioned air is exhausted.

Blower Door Tests

A blower door test measures building airtightness by depressurizing the building to 50 Pascals and measuring the airflow (CFM50) needed to maintain that pressure. The LBL correlation converts CFM50 to natural infiltration ACH using an N-factor that accounts for building height, wind exposure, and climate.

ASHRAE 62.2 Ventilation

ASHRAE Standard 62.2-2019 sets minimum ventilation rates for residential buildings. The formula combines a floor-area component (0.01 CFM/sq ft) and an occupant component (7.5 CFM per person, where occupants = bedrooms + 1). Buildings with demonstrated natural infiltration from blower door testing can take a credit against the mechanical ventilation requirement.

Tightness vs. Ventilation

A tight building envelope is always desirable — it reduces energy waste and improves comfort. But tight buildings need mechanical ventilation to maintain indoor air quality. The principle is “build tight, ventilate right” — seal the envelope, then add controlled mechanical ventilation sized to ASHRAE 62.2 requirements.

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