Work out the airflow three different ways

Multiply length by width by height to get the volume in cubic feet, multiply by the air changes per hour you are targeting, then divide by 60 to get cubic feet per minute. A 20 by 14 room with an 8 ft ceiling is 2,240 cubic feet, so at 6 air changes an hour it needs 224 CFM.

The airflow a cooling system should move for each ton of capacity, and 400 CFM per ton is the standard design point. Dry climates often run closer to 450 to move more air, and humid climates closer to 350 so the coil has longer to pull moisture out of the air. One ton is 12,000 BTU/h.

Duct area in square feet multiplied by air velocity in feet per minute. An 8 inch round duct is 50.3 square inches, which is 0.349 square feet, so at 700 FPM it carries about 244 CFM. Divide the area in square inches by 144 to get square feet — forgetting that step is the usual mistake.

How many times the entire volume of air in a space is replaced each hour. Living spaces are typically designed around 4 to 6, kitchens nearer 8, bathrooms 10, and workshops or paint booths 15 to 20. It is a ventilation rule of thumb rather than a load calculation.

For residential work, trunk lines are usually designed around 700 to 900 FPM and branches around 600 to 700. Going faster moves more air through the same duct but noise rises sharply with velocity, so a system pushed too hard tends to become audible before it becomes efficient.

No — they answer different questions and depend on each other. Sizing tells you the capacity in BTU or tons the space needs, and airflow is what that capacity requires to actually deliver it. Size first from the load, then work out the CFM. Our HVAC load calculator handles the capacity side.