Free Online Diffuser & Grille Sizing Calculator

Air terminals must move the design airflow without excessive velocity, which causes noise and draughts. This calculator returns the required free area and an equivalent square or round neck for a supply diffuser or return grille from your airflow and target face velocity, and flags the likely noise level.

Face velocity is the key trade-off: too low and the terminal is oversized and costly; too high and it whistles. Typical targets are 2-4 m/s for supply diffusers and 1.5-3 m/s for return grilles. The tool divides airflow by velocity to get free area, then applies the free-area ratio (usually 80-90%) to give the physical neck size.

Example: A supply diffuser handles 200 L/s (0.2 m³/s) at a target face velocity of 2.5 m/s. Free area A = 0.2 / 2.5 = 0.08 m². With a 90% free-area ratio the neck area is 0.089 m², giving a 298 mm square or 336 mm round neck. At 2.5 m/s the terminal sits in the quiet-to-moderate NC band, suitable for offices and classrooms.