# Free cross-sectional area inside duct

Cross-sectional area of empty duct minus the cross-sectional area of all objects inside the duct.

Symbol
$A_{d,fill}$
Unit
mm$^2$
Formulae
$\frac{\pi}{4} \left({Di_d}^2-N_c {D_e}^2\right)$
Related
$D_e$
$Di_d$
Used in