Free Duct Size & Airflow Calculator
Calculate a preliminary round metal duct size from CFM, available pressure and effective length. Free Heco duct airflow worksheet with visible assumptions.
How to use this estimate
This worksheet evaluates one constant-flow round metal duct section. An installed duct network changes airflow at takeoffs and needs equipment, fittings, leakage and balancing checks.
See the method, assumptions and source references
Full calculation assumptions appear with your result.
Local weather references: CEC 2025 Joint Appendix JA2, Table 2-3. Cooling uses 1% dry-bulb; heating uses 0.6% dry-bulb. Nearby references are identified explicitly. These are planning weather values, not a property-specific climate-zone determination.
Final professional work: ACCA Manual J loads, equipment selection and Manual D duct design. This tool is not ACCA-approved software or a permit compliance calculation.
How do I calculate duct size from airflow?
Duct sizing starts with required airflow and the pressure available to move that air through the duct path. This worksheet searches whole-inch round metal sizes and checks estimated friction loss and air speed against the limits entered. It does not diagnose a weak room from floor area alone.
Use the design CFM for the section being evaluated. Enter the pressure remaining for ducts after equipment and component losses, then include fittings in the total effective path length. The model evaluates one constant-flow section under standard air conditions; it does not create a complete duct layout.
Can square footage tell me the correct duct diameter?
Not by itself. Room load determines the airflow target, and airflow, pressure, duct material, fittings and equipment performance determine suitable dimensions. A weak register may also involve a restricted return, filter, coil, damper or blower setup.
What is available pressure versus total external static pressure?
Available duct pressure is the portion left for the duct path after applicable component losses have been accounted for. It is not interchangeable with a measured total external static pressure. A qualified technician should establish the appropriate values from the system and manufacturer information.
Will the round metal result work for flex duct?
Do not treat it as a flex-duct recommendation. Flex friction depends strongly on extension, compression, bends and installation. The worksheet models galvanized round metal; actual materials and fittings need the appropriate design data.
Should I replace a duct if the calculator gives a larger diameter?
No. First verify delivered airflow, pressure, blower capability, return capacity, leakage and fitting losses. Final residential duct design requires a full Manual D assessment. Request a Heco airflow assessment if the necessary measurements are unavailable.
Published by Heco Heating and Air Conditioning • California C-20 contractor license #1050351. Tools are for preliminary planning; contact Heco to verify property conditions and service availability.