Free ACH & Ventilation Calculator

Air Changes Per Hour Calculator

Calculate air changes per hour from room volume and airflow, or work backward to estimate the CFM required for a target ACH. Useful for ventilation, indoor air quality and airflow planning.

Calculate ACH

ACH, CFM & Room Volume

Calculate how often a room's air volume moves each hour.

If you need to estimate system airflow first, use our HVAC CFM calculator . For broader air-quality research, review our indoor air quality guide .

Calculate ACH or Required Airflow

Enter room dimensions, then choose whether you know CFM or your target air-change rate.

Calculation Method

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Airflow (CFM) Enter the airflow you want to evaluate.

Target Air Changes Per Hour Enter your target ACH. This tool does not prescribe the correct target for a particular building.

Calculated Air Changes Per Hour - -

ACH is a mathematical air-volume exchange rate. Actual mixing is not perfectly uniform, and total HVAC recirculation airflow is not automatically the same thing as outdoor ventilation airflow.

- Minutes Per Air Change

Floor Area -

Room Volume -

Airflow -

ACH -

ACH formula used by this calculator

ACH = CFM × 60 ÷ room volume. When solving for airflow instead, CFM = ACH × room volume ÷ 60. Room volume equals length × width × ceiling height.

Important ventilation distinction: if you are evaluating fresh-air ventilation, enter the outdoor-air airflow delivered to the space. Entering total supply-air CFM from a recirculating HVAC system calculates air movement through that system, not the amount of outdoor air entering the building.

Looking at ventilation or indoor air quality?

Request HVAC help below after reviewing the room volume, airflow and air-change calculation.