What neck velocity should I use for a supply diffuser?

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.

What is free area ratio?

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.

How do I determine the throw distance?

Throw distance is how far the supply air jet travels before slowing to 0.25 m/s (the occupied zone velocity limit per CIBSE). It depends on the diffuser type, neck velocity, and discharge pattern. Square ceiling diffusers with 4-way discharge typically throw 3-5 metres. The throw should reach 75% of the distance to the nearest wall or opposing diffuser.

What are NR noise rating criteria?

Noise Rating (NR) curves define the maximum acceptable noise level for a space. CIBSE Guide B specifies: bedrooms NR 25, living rooms NR 30, offices NR 35, restaurants NR 40, and industrial spaces NR 50. Diffuser noise increases with neck velocity — keeping velocity below 3 m/s generally avoids noise problems in most applications.

How many diffusers do I need for a room?

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.

TradeCalculator

Grille & Diffuser Calculator — Sizing and Selection

Size supply diffusers and extract grilles for correct neck velocity, free area, throw distance, and noise rating. Based on CIBSE Guide B recommendations.

Grille / Diffuser Type

Determines free area ratio

Total Flow Rate

Target Neck Velocity

CIBSE: 2-3 m/s for occupied spaces

Number of Diffusers

Room Type (NR Criteria)

Noise rating limit for the space

Neck Diameter (if known)

Leave blank to calculate from flow rate

How We Calculate This

This calculator sizes grilles and diffusers by determining the required free area for a target neck velocity, accounting for the free area ratio of the selected grille type.

Key calculations

Free area: Flow rate (m³/s) / target velocity (m/s)
Gross area: Free area / free area ratio
Neck velocity: Flow rate / neck area
Pressure drop: 0.5 × rho × v² × loss coefficient

Recommended velocities (CIBSE Guide B)

Supply diffusers: 2.0-3.0 m/s
Extract grilles: 2.0-3.5 m/s
Transfer grilles: 1.0-1.5 m/s
External louvres: 2.5-4.0 m/s

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.