Calculation Results
About This Calculator
Calculate room supply airflow (CFM) based on cooling square footage, air changes per hour (ACH), round flexible/rigid duct collar diameter, and supply register velocity.
Frequently Asked Questions
How many CFM are required per square foot in residential HVAC?
A standard residential rule of thumb is 1.0 CFM per square foot for average rooms with 8-foot ceilings, 1.25 CFM/sq ft for sunny master bedrooms or rooms with large west-facing windows, and 1.5 CFM/sq ft for high-ceiling rooms or hot attics.
How many CFM does a 1-ton air conditioner provide?
In standard air conditioning systems, 1 ton of cooling (12,000 BTU/hr) circulates 400 CFM of conditioned airflow across the evaporator coil.
What happens if a duct is sized too small for the required CFM?
An undersized duct creates high static pressure and forces air velocity above 800-1,000 feet per minute (FPM), causing loud whistling, rushing air noise at registers, and reduced system efficiency.
What velocity is recommended for residential branch ducts?
Supply branch ducts are typically designed for 600 to 700 FPM, and return grilles for 400 to 500 FPM to ensure near-silent HVAC operation.