CFM from room volume, by air changes per hour.

Short answer

Airflow in CFM = room volume in cubic feet x air changes per hour / 60. A 12 x 10 ft room with an 8 ft ceiling holds 960 ft3, so 6 air changes per hour needs 96 CFM.

Worked example

A 12 x 10 ft room with an 8 ft ceiling has a volume of 12 x 10 x 8 = 960 ft3. For 6 air changes per hour: 960 x 6 / 60 = 96 CFM. Run it the other way: a 120 CFM fan in the same room gives 120 x 60 / 960 = 7.5 ACH.

Formula, assumptions and sources

Step 1: volume (ft3) = length x width x height. Step 2: CFM = volume x ACH / 60, because the air-change rate is per hour and CFM is per minute. CCOHS gives the underlying relation as air flow rate = air exchanges x room volume, and defines an air change as the replacement of the entire volume of air in the space. The cubic-metre figure uses 1 ft = 0.3048 m. Real rooms mix air imperfectly and ducts, grilles and filters cut a fan's delivered airflow, so treat the result as a minimum starting point. The tool does not choose an ACH for you: required rates come from building codes, standards and the room's use.

Source: Canadian Centre for Occupational Health and Safety, OSH Answers: Industrial Ventilation - 2. Units and Measures (opened 2026-10-08)

Safety and limits

This is a planning aid, not professional advice. Electrical and HVAC work can cause fire, shock and carbon monoxide hazards; have a licensed professional size, install and inspect equipment.

FAQ

How do you calculate CFM from air changes per hour?

Multiply the room volume in cubic feet by the air changes per hour and divide by 60. A 25 x 20 ft room with an 8 ft ceiling is 4,000 ft3, and 4 ACH needs 266.7 CFM.

What does one air change mean?

CCOHS defines an air change as the replacement of the entire volume of air in the space. One ACH means the fan moves the room's volume of air in an hour.

Which ACH should I choose?

This tool does not recommend one. Codes and standards set required rates by room type and jurisdiction; check yours or ask a ventilation professional.