Size a supply or extract grille / diffuser by velocity. Returns both the
face (approach) and core (free-area) velocity and grades the core against your target.
Indicative selection only. Confirm against the manufacturer's catalogue and CIBSE Guide B2.
For design guidance only. Confirm the face vs free-area basis of your target and verify with a qualified engineer.
About this grille / diffuser sizing calculator
This free grille and diffuser sizing calculator helps ventilation engineers select a supply or extract air terminal by velocity. Enter the airflow, the nominal face width and height, the free-area ratio from the manufacturer's data and your target core velocity, and it returns the gross face area, the effective free (core) area, both the face and core velocities, and the face area you would need to hit the target. It then grades the core velocity against that target with a green, amber or red verdict. Everything runs in your browser. No data is uploaded. So it is ideal for a quick tender-stage or commissioning check before you confirm against a catalogue.
How grille and diffuser sizing is calculated
The gross face area is simply width × height (converted from mm to m), and the effective free area is that gross area multiplied by the free-area ratio, typically 70–90% for a louvred grille. Velocity is volumetric flow divided by area: the face velocity is Q ÷ gross face area (the bulk approach velocity), while the core velocity is Q ÷ free area, the higher inter-vane figure that drives generated noise and throw, with Q in m³/s. The verdict bands the core velocity: green when it is at or below the target, amber up to 1.25 × the target, and red beyond. The default 3 m/s sits inside the CIBSE Guide B2 2–4 m/s air-distribution range for occupied spaces, with quiet rooms at the low end. Catalogue selection convention varies by maker, so confirm whether your quoted target is a face or a free-area velocity. Both are output so a face-velocity figure can be compared directly.
Frequently asked questions
What is the difference between face and core velocity?
Face (or approach) velocity is the airflow spread over the whole gross face area. Core (or free-area) velocity is the higher velocity through the actual gaps between the vanes or louvres. Because the core velocity governs noise and throw, manufacturer catalogues commonly select on it. So it is the figure this calculator grades.
What free-area ratio should I use?
Use the figure from the manufacturer's data sheet for your specific terminal, usually 70–90% of the gross face for grilles and diffusers. If you do not have it to hand, 80% is a reasonable starting assumption, but always confirm before finalising a selection.
What is a sensible target velocity?
CIBSE Guide B2 places occupied-space air distribution around 2–4 m/s, with quiet rooms such as bedrooms and meeting rooms at the lower end to limit noise. Because the basis can differ between makers, check whether your catalogue target is a face or a free-area velocity before reading the grade.