Grille & Diffuser Calculator — Sizing and Selection

For comfort cooling and heating, CIBSE Guide B recommends neck velocities of 2.0-3.0 m/s for ceiling diffusers in occupied spaces. Lower velocities (1.5-2.0 m/s) are needed in noise-sensitive areas like bedrooms and recording studios. Higher velocities (3-5 m/s) may be acceptable in industrial or retail applications where noise is less critical.

Free area ratio is the proportion of the grille face that is open to airflow, typically 45-80% depending on the grille type. External louvres have lower free area ratios (45%) due to rain defence blades, while ceiling diffusers can achieve 75-80%. The gross grille area must be larger than the required free area by this ratio.

Throw distance is how far the supply air jet travels before slowing to its terminal velocity, conventionally taken as 0.25 m/s (CIBSE notes 0.25–0.75 m/s for the edge of the occupied zone). It depends on the diffuser type, neck velocity and discharge pattern, and is published per product by the manufacturer (throw ∝ √(Q/v) from the diffuser’s Ak/K-factor) — there is no universal rule. Square ceiling diffusers with 4-way discharge typically throw about 3–5 metres. As a layout guide the throw should reach roughly 75% of the distance to the nearest wall or opposing diffuser. The calculator’s throw figure is an order-of-magnitude screen only; confirm it from the maker’s throw tables.

Noise Rating (NR) curves define the maximum acceptable background noise level for a space. CIBSE Guide A (2015) Table 1.5 gives typical comfort criteria: bedrooms NR 25, living rooms NR 30, offices NR 35, classrooms NR 30, restaurants NR 40, and industrial spaces up to NR 50; detailed services-noise design sits in CIBSE Guide B4. Diffuser noise rises with neck velocity, so keeping velocity below about 3 m/s generally avoids problems — but the actual sound level must be read from the manufacturer’s published Lw/NC data for the chosen diffuser at the design flow, not from velocity alone.

The number of diffusers depends on the total flow rate, maximum flow per diffuser (to avoid noise), and the throw distance relative to room dimensions. As a rule of thumb, space diffusers so their throw distance covers the room without dead spots. For offices, one 600×600mm diffuser per 10-15 m² of floor area is typical.