CFM Calculator

A 3-ton system needs about 1200 CFM at the nominal 400 CFM per ton (3 x 400). Hot, humid climates run closer to 350 CFM/ton (1050 CFM) for more dehumidification; dry climates run up to 450 CFM/ton (1350 CFM) for more sensible cooling. Confirm against the OEM blower table for the actual coil.

A 2-ton system needs about 800 CFM at 400 CFM per ton. The 350-450 CFM/ton band puts it between 700 and 900 CFM depending on climate and how the system is set up for latent versus sensible load.

For the SENSIBLE portion of the load, CFM = sensible BTU / (1.08 x deltaT), where deltaT is the supply-to-room temperature difference. For 24,000 BTU sensible at a 20F deltaT that is 1111 CFM. The 1.08 constant is 60 min/h x 0.075 lb/ft3 standard-air density x 0.24 BTU/lb-F specific heat, so it assumes standard air at 70F and sea level.

1.08 is 60 x 0.075 x 0.24: 60 minutes per hour, times the density of standard air (0.075 lb/ft3 at 70F, sea level), times the specific heat of air (0.24 BTU/lb-F). It converts a sensible BTU/hr load and a temperature difference into the airflow in CFM. At altitude or high supply temperatures the air is thinner, so the real constant is lower and the required CFM is higher.

No. 400 CFM per ton is a nominal comfort starting point, not a law. ACCA gives a 350 to 450 CFM per ton band: 350 for hot, humid climates that need more moisture removal, 450 for dry climates that want more sensible cooling. The OEM blower performance table for the specific air handler is the authority; this calculator sizes the airflow, then feed it into the duct size calculator.