For a round duct, airflow is the duct cross-sectional area multiplied by air velocity. In imperial units, the duct area is converted to square feet and multiplied by feet per minute to give cubic feet per minute.
NFPA wood-processing committee material describes 20 m/s — about 4,000 FPM — as a generally accepted minimum conveying velocity for wood particulate. The calculator uses 4,000 FPM as a visible planning baseline, but leaves it editable because the correct design velocity depends on the material, process and system design.
A 6-inch round duct has an area of about 0.1963 ft². At a selected conveying velocity of 4,000 FPM, the calculated airflow is about 785 CFM. The same condition is about 20.3 m/s and 1,334 m³/h.
- CAPTUREConveying velocity is not the same thing as hood capture velocity. A duct can move air fast enough to carry material while the hood still fails to capture dust at the machine.
- STATICDuct length, elbows, flex hose, branches, transitions, hoods, filters and separators create static-pressure losses. Those losses need to be checked against the fan curve rather than represented by an arbitrary percentage penalty.
- WOODThe 4,000 FPM default is a wood-particulate planning reference. Different dusts and processes can require different conveying velocities.
- MULTIDo not simply add every machine's airflow unless those branches are genuinely open and operating together. Simultaneous-use assumptions belong in the system design.
- DUSTCombustible wood dust can introduce fire and explosion hazards that airflow alone does not address. System layout, collection equipment, explosion protection, housekeeping and applicable requirements need separate review.
Some simple web calculators apply a fixed percentage loss for every elbow or length of duct. That is not how a real fan-and-duct system is evaluated. Fittings and duct runs create pressure losses, which combine into total system resistance and are then compared with the collector's fan performance curve.
This calculator therefore stays narrow: it calculates the relationship between duct diameter, airflow and conveying velocity. Static-pressure design is a separate calculation.