Already have ductwork in place? Enter the duct size and get the estimated airflow capacity. Use this when evaluating existing systems or checking whether current ductwork can handle a new unit.
Reverse Duct Calculator
Already have ducts? Find out how much airflow they can handle.
REVERSE CALCULATOR
Capacity for 8" round
This duct can comfortably handle 140–314 CFM. Recommended: 209 CFM at 600 FPM.
This size may work for a branch run — verify if being used as a main trunk.
CFM Capacity
209
Recommended CFM for 8" round
Velocity at Common CFMs
Pressure Drop
0.08" WC
Per 100 ft of straight duct
Duct area: 50.3 sq in (0.349 sq ft)
Explain to Customer
This 8" round duct is best used for 209 CFM or less. At that airflow, air moves at about 600 feet per minute — quiet and efficient. Push it past 314 CFM and you'll start hearing it.
Airflow estimates based on standard friction rate assumptions for residential ductwork.
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