Fan airflow is expressed in cubic metres per hour. To find the airflow you need, multiply the room volume by the number of times the air should be replaced each hour.
Guide values: 4 air changes per hour for storage, 6 for a construction site, 8 for crowded spaces and 10 or more for drying out.
Example: 200 m² at 3 m high is 600 m³. At 6 air changes per hour you need 3,600 m³/h.
Long ducting and bends reduce the real airflow. Add 10 to 20 percent margin in that case.