An exhaust fan with a 300 airflow has an opening size of approximately 300mm.An exhaust fan of this size provides air flow between approximately 200-400 cubic meters per hour. A 280mm x 280mm aperture size has an airflow of up to 240 cubic meters per hour, which is equivalent to up to 40 cubic meters per minute. This airflow range applies to 4-6 times the volume of the room space.