Air velocity in a duct is nothing more exotic than flow divided by the area it flows through. $v = \dfrac{4\dot{V}}{\pi d^{2}}$ — read aloud v equals four V-dot over pi d squared. $v$ is the average velocity in metres per second, $\dot{V}$ is the airflow in cubic metres per second, and $d$ is the duct's inside diameter in metres. The 4 and the $\pi$ are just the area of a circle, $A = \pi d^{2}/4$, turned inside out.

Two unit traps live in that one line, and both are quiet. Duct is drawn in millimetres and air is scheduled in litres per second — neither is what the formula wants. Convert both, or work with the combined form $v = \dfrac{4000\dot{V}}{\pi d^{2}}$ with $\dot{V}$ in L/s and $d$ in mm, which is the version worth memorising. And square the diameter: an area is built from two lengths, so a duct one size up does far more for velocity than it looks like it should.

Rectangular duct fits in a ceiling where round does not, and every friction chart in the office is plotted for round. Huebscher's equation bridges them: $D_e = 1.30 \dfrac{(ab)^{0.625}}{(a+b)^{0.25}}$, where $a$ and $b$ are the two sides of the rectangle in the same length unit and $D_e$ is the equivalent round diameter that comes out in that unit. Equivalent means same friction loss at the same airflow — not same area, and emphatically not the average of the sides. A 600 × 300 duct is not a 450 round; it is a 457, and it does not have the same face area as either. Flatten a duct and you buy headroom with pressure drop; Huebscher is the receipt.