TOOL GUIDE
Air grille sizing
Estimate outlet face sizes from equally divided airflow, free-area velocity and free-area ratio.
Purpose & scope
Estimate outlet face sizes from equally divided airflow, free-area velocity and free-area ratio.
Face-area sizing, not neck selection. Throw, NC, pressure drop and distribution require catalogue verification.
Inputs
Use drawings, design conditions and manufacturer data. Units and example values follow the calculator; example values are not universal design requirements.
| Parameter | Value / interpretation |
|---|---|
| Diffuser / grille type | Linear — elongated face / Rectangular grille / Circular diffuser / Square diffuser |
| Total airflow · m³/h | Volumetric flow at the calculation location; check the selected unit. |
| Number of grilles | Positive integer count of outlets sharing airflow equally. |
| Free-area velocity · m/s | Velocity through grille free area, distinct from face velocity. |
| Free area / face area ratio · % | Free-area to face-area ratio from catalogue data. |
| Quick reference face size | Reference face sizes. Check neck dimensions and free area against the catalogue. |
| Face width / length / diameter · mm | Grille face width/length or circular face diameter, not neck size. |
| Grille face height · mm | Rectangular grille face height; square diffusers use the width for both sides. |
Steps
- Select outlet type, total airflow and count.
- Enter catalogue free-area ratio and design velocity.
- Review candidates or check an existing face size; save the outlet group in the project.
Worked example
This reproducible training example uses the values below. Select the appropriate mode and overwrite restored project values. Example rates, K values and equipment losses are assumptions, not mandatory design criteria.
Examples use a decimal point and no thousands separator.
All example inputs
| Parameter | Value / interpretation |
|---|---|
| Diffuser / grille type | Rectangular grille |
| Total airflow | 2400 m³/h |
| Number of grilles | 4 |
| Free-area velocity | 2.5 m/s |
| Free area / face area ratio | 60 % |
| Quick reference face size | Custom dimensions |
| Face width / length / diameter | 400 mm |
| Grille face height | 300 mm |
Expected results (rounded)
| Parameter | Value / interpretation |
|---|---|
| Flow per outlet | 600 m³/h |
| Required face area per outlet | 0.1111 m² |
| First candidate | 450 × 250 mm |
Small differences in displayed rounding are normal. A result that passes the entered limits does not establish suitability for the complete system.
Method & limitations
Face-area sizing, not neck selection. Throw, NC, pressure drop and distribution require catalogue verification.
Free area = flow per grille / velocity. Face area = free area / free-area ratio. Flow is distributed equally among grilles.
60% is an example; replace it with catalogue data. Dimensions are nominal face sizes, not neck sizes or actual open area. Quick sizes are references and are not available from every manufacturer.
This is preliminary area sizing, not product selection. Check noise, throw, drop, pressure loss and air distribution in the manufacturer catalogue.
Square diffusers use equal sides; circular uses πD²/4; rectangular and linear use length × width. Linear sizing uses face area and the entered free-area ratio; it does not select slot count or slot width. Changing type does not change the free-area ratio.
Up to 10 smallest qualifying options are selected before column sorting. Square/circular: 50 mm increments, 100–2,000 mm; rectangular: 50 mm increments and aspect ratio ≤4. Linear: length 300–2,000 mm, width 50/75/100/150/200/250/300 mm, length/width ≥3. Options are geometric preliminary checks.
Common mistakes
Confusing face and free area; entering neck instead of face dimensions; failing to divide total airflow by outlet count.
Projects & reports
- Choose or create an engineering project and enter your actual design inputs.
- Project inputs/results are autosaved in this browser. Where a saved-list panel is available, name and add the selected section or alternative to compare multiple options.
- Review the report before exporting PDF. In the print dialog select Save as PDF. Export JSON for backup and use Open file to restore it; PDF cannot restore project data.
Saved results do not automatically recalculate when another tool changes. Review any dependency warning, recalculate and update the affected results.
References & related tools
ASHRAE — Room Air Distribution
The tool implements the methods and limits stated above; linking a standard does not establish complete compliance with that standard.