ACH to CFM Calculator

This calculator turns a target number of air changes per hour (ACH) into the airflow a fan or air handler must move. Enter the room volume, the ACH you want and an allowance for losses in ducts, grilles and filters. It returns the theoretical airflow, the fan size to look for, and the same figure in m³/h and L/s.

ACH targets are used for workshops, clinics, animal rooms, garages, warehouses, enclosures and anywhere a guideline or code sets ventilation by air changes rather than per person. For occupied offices, classrooms and shops, outdoor air is normally sized per person and per square foot with the fresh air CFM per person calculator instead.

The result assumes the air mixes well. A fan blowing straight from inlet to outlet can leave stagnant corners even at a high ACH, so place supply and exhaust on opposite sides of the space. For total room airflow from other inputs, see the general CFM calculator.

ACH is defined in the ASHRAE Handbook—Fundamentals as the volume of air supplied or exhausted per hour divided by the space volume. The same relationship run backwards checks an existing room: ACH = CFM × 60 ÷ volume. In metric units no factor of 60 is needed: airflow in m³/h ÷ volume in m³ = ACH.

The percentage allowance is a rough stand-in. The reliable way to size a fan is to read its rated CFM at your system's static pressure from the manufacturer's fan curve; a fan rated 300 CFM at free air may move far less through long ducts and filters.

Required air change rates come from different standards depending on the building, and local codes can override them. Use the source that applies to your project rather than a generic figure.

To check whether a room meets a target, measure or look up the actual airflow — from a flow hood, the fan's rating at its operating pressure, or the balancing report — and run the formula backwards: ACH = CFM × 60 ÷ volume. The 343 CFM fan in the first example gives 343 × 60 ÷ 2,240 ≈ 9.2 ACH if it delivers its full rating.

For small enclosures such as a 3D printer cabinet, the volume is tiny, so even a small fan gives a high ACH. A 3 × 2 × 2 ft enclosure is 12 cu ft; at 60 ACH it needs only 12 CFM. What matters more there is keeping the enclosure under slight negative pressure and filtering or venting the exhaust outdoors.

• Using floor area instead of volume. A 300 sq ft room with a 12 ft ceiling has 50% more air than the same room at 8 ft.
• Ignoring make-up air. Exhaust fans cannot move air that has no way in. Large exhaust systems need dedicated make-up air.
• Trusting free-air ratings. Long or narrow ducts can cut a fan's output sharply; check duct friction loss on long runs.
• Counting recirculated air as fresh air. An air purifier adds clean-air changes but not outdoor air. Some standards count both; others count only outdoor air.