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, and after the duct friction loss calculator has checked the run pressure drop. If catalog data gives a grille pressure drop, add it to the HVAC static pressure calculator with the duct, filter and coil losses. For return-air search intent, use the return air grille size calculator.
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 and the duct friction loss calculator for total equivalent length pressure drop.
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.