How do I calculate CFM from a grille velocity reading?

Multiply the measured face velocity in feet per minute by the net free area of the grille in square feet: CFM equals velocity times area. The net free area is the grille's open portion, found by multiplying gross neck area by a free area factor of about 0.7 to 0.8.

What is the net free area factor?

Grille bars and frames block part of the opening, so air only passes through the free area. The factor is the open fraction of the gross neck area, typically 0.7 to 0.8 for standard bar grilles. Diffusers and the exact model can vary, so use the manufacturer figure when available.

Why does face velocity cause noise?

Air moving fast across grille bars generates turbulence and whistling. ASHRAE publishes recommended maximum face velocities by space type, from about 500 FPM for quiet libraries to 1000 FPM for noisy stores, to keep that noise acceptable.

Should I use gross or net area for velocity?

Use net free area. Face velocity is the speed of air through the open portion of the grille, so dividing CFM by net area, not gross neck area, gives the velocity that determines noise and throw.

What CFM should each register deliver?

That comes from the room's design airflow, which depends on the cooling or heating load. Once you know the target CFM per register, this tool checks whether the chosen grille size keeps velocity within the noise limit.

Supply Grille/Diffuser Velocity & CFM Calculator

When balancing an air distribution system you constantly move between airflow, velocity, and grille size. This tool applies the airflow equation Q = V × A in both directions and adds a noise check against ASHRAE face-velocity limits so a grille is not only sized for flow but also for quiet.

How it works

Airflow, velocity, and net free area are related by a single equation:

gross area (ft²) = width × height ÷ 144
net free area    = gross area × free area factor
CFM              = velocity (FPM) × net free area (ft²)
velocity (FPM)   = CFM ÷ net free area (ft²)

The free area factor accounts for the grille bars and frame that block part of the opening. Velocity is always computed on net free area, because that is the speed of the air actually leaving the grille, which drives both throw and noise.

Example and tips

A 12 by 6 inch neck with a 0.75 free area factor has 0.375 ft² net free area. At 700 FPM that delivers about 263 CFM, and 700 FPM is within the 750 FPM office limit but above the 600 FPM residential limit. If the tool flags excess velocity, step up to a larger grille rather than accept whistling. Always take the free area factor from the grille’s catalog data when you can, since it varies by core style.