How to use this calculator
- Enter airflow. Use the airflow through the grille, register or diffuser, not total system airflow unless it is a single terminal.
- Set target face velocity. Choose a face velocity that fits the room noise and throw target.
- Enter free-area ratio. Use catalog free area when available; otherwise use a conservative estimate.
- Check installed size. Enter a candidate grille width and height to see actual face velocity.
How it works
Required free area comes from airflow divided by target face velocity:
A_free = Q / v_f
The nominal grille face area is larger because only part of the grille is open:
A_nom = A_free / FAR
where FAR is the free-area ratio.
The rectangular suggestion uses the entered aspect ratio:
H = √(A_nom/(W/H)) and W = (W/H)·H. The installed
face-velocity check uses the entered grille width and height:
v_installed = Q / (W_i · H_i · FAR)
Use this after the duct size calculator has set the branch airflow and duct velocity. If catalog data gives a grille pressure drop, add it to the HVAC static pressure calculator with the duct, filter and coil losses.
Worked example
Verified against the live calculator
For about 250 CFM at a target face velocity near 500 fpm, the required free area is about 0.52 ft². With a 0.75 free-area ratio, nominal grille area is about 0.69 ft². At a 2:1 aspect ratio, that is roughly 14 × 7 in.. A smaller 12 × 6 in. candidate would run closer to 700 fpm through the free area.
Frequently asked questions
How do you size an HVAC grille from CFM?
First calculate required free area: free area = airflow / target face velocity. Then divide by the grille free-area ratio to estimate the nominal face area. The calculator converts that nominal area into a rectangular width and height.
What is free-area ratio?
Free-area ratio is the open area available for air divided by the nominal face area of the grille. Louvers, bars, dampers and screens reduce free area, so catalog free-area data is better than a guess.
What face velocity should I use?
Comfort terminals are often screened around 300-600 fpm, with lower values for quieter spaces and higher values only when catalog noise, pressure drop and throw are acceptable.
Is grille size the same as duct size?
No. A duct is sized for duct velocity and pressure loss. A grille or diffuser is selected for face velocity, pressure drop, throw, spread, noise and appearance. Use the duct size calculator for the duct run.
Method & assumptions
- Uses free-area and face-velocity sizing only; it does not predict throw, spread, NC/noise or pressure drop.
- Free-area ratio is bounded internally between 0.05 and 1.0 to avoid impossible geometry.
- Final grille/register/diffuser selection should use manufacturer catalog data at the design airflow, room layout, pressure-drop target and noise limit.