TradesHow this works
A grille is sized by how fast the air moves through its face. Divide the airflow by that face velocity and you have the free area the grille needs, then gross it up for the louvers that block part of the opening. Returns and filters run slow to stay quiet, supplies run faster to throw the air.
The tool sizes the grille at three velocities, from quiet and big to compact and noisy, and picks a standard size for each. The face velocity is the decision: a bigger grille is quieter and easier on the blower, a smaller one saves space and, on a supply, reaches farther into the room. It stops at planning grade, so confirm the final pick against the manufacturer's free-area data.
Common questions
How do you size a return air grille?
Divide the airflow in CFM by a target face velocity to get the free area you need, then divide by the grille free-area fraction to get the gross size to buy. Returns are sized slow, around 300 to 500 feet per minute, to stay quiet and keep static pressure low. A 1000 CFM return at 350 feet per minute needs roughly 400 square inches of free area, which is a large grille once you account for the louvers.
What face velocity should a grille run at?
It depends on the grille. Return and filter grilles run slow, roughly 300 to 500 and 250 to 300 feet per minute, so they stay quiet. Supply registers run faster, roughly 500 to 700, because the velocity is what throws the air across the room. Slower is bigger and quieter, faster is smaller but noisier, so the target velocity is really a size-versus-noise decision.
What is free area on a grille?
A grille is not an open hole. The bars and louvers block part of the face, so only a fraction of the gross size actually passes air. That fraction is the free area, typically around 75 percent for a common stamped or louvered grille. The tool sizes the free area from the airflow and then grosses it up so the grille you order actually delivers the flow.
Sources: Face velocity guidelines: return ~300-500, supply ~500-700, filter ~250-300 fpm (ACCA Manual T style) · Free area = CFM / velocity; net free area ~75% of gross face (typical). Engine version 1.0.0. Risk tier 2.