You can determine how much venting you have by simply measuring the height and width of each vent. Once you measure your vents you want to reduce the total of net free venting by 25%. You reduce it by 25% because of the restrictions your vents have. Your vents have restrictions such as lovers, filters and etc. Please check with your local building codes for each vent, we also have some listed in our installation manual. Find your model on our Documentation page and download the respective installation manual. If you have vaulted or cathedral ceilings, you will want to lean closer to 3 CFM per SQFT due to the increase of air volume in your home.