CFM, velocity and duct area: how the sizing works
How airflow and air velocity set the cross-section of a duct, why the equivalent round diameter matters, and how to pick a stocked size.
The airflow equation behind the tool
Duct sizing starts from one relationship: airflow equals velocity multiplied by cross-sectional area. Rearranged, area equals airflow divided by velocity, which is exactly what this calculator does with CFM and feet per minute. Because CFM is cubic feet per minute and velocity is feet per minute, the units cancel to leave square feet of area. That area is the amount of open space the moving air needs, and everything else in the result is just a unit conversion of it.
From square feet to a round diameter
A cross-section in square feet is hard to shop for, so the tool converts it to square inches by multiplying by 144, then finds the round duct that has that area. The circle area formula, area equals pi times radius squared, is inverted to solve for diameter. That is why 82.29 square inches becomes a 10.24 inch round duct and 160 square inches becomes 14.27 inches. Round duct is quoted by diameter, so this figure maps straight to a catalog size.
Choosing a velocity that fits the room
Velocity is a design choice, not a fixed constant, and it controls both duct size and comfort. Faster air packs the same CFM into a smaller duct, which saves space and material, but it also increases friction loss and the whooshing sound at registers. Residential supply trunks often target 700 to 900 feet per minute, branch runs a little less, and return air is kept slower still to stay quiet. Entering different velocities in the tool shows the size and noise trade-off instantly.
Rounding up to a stocked size
Ducts are sold in whole-inch steps such as 6, 8, 10, 12 and 14 inches, so the exact diameter almost never matches stock. Always round up to the next available size, because a smaller duct forces the air faster than you designed for and raises both static pressure and noise. If the next size up feels oversized, nudge the velocity higher in the calculator and re-check, but keep the whole system in mind rather than one branch in isolation.