Grille sizing is about free area, not face size
A grille's visible opening, its face size, is not the area air actually passes through. The vanes or bars that give a grille its look and its ability to direct or filter airflow also block a meaningful chunk of that opening. What's left, the actual open area, is called free area (sometimes labeled Ak on a spec sheet), and it's always smaller than face area, commonly 60 to 80 percent of it for ordinary bar-type supply and return grilles. A 12x6 grille has 72 square inches of face area, but if its free area is only 70 percent of that, it behaves, for sizing purposes, like a 50.4 square inch opening. Sizing off face size alone overstates how much air a grille can comfortably pass.
Where free area actually comes from
Free area is manufacturer-measured, published per model in that manufacturer's catalog or cut sheet, it isn't a physical constant this calculator can derive or reasonably guess for a specific product. That's exactly why this page doesn't ship a lookup table of grille models and their free areas: doing so would mean reproducing licensed manufacturer data, and getting it wrong (or out of date) for a specific SKU is worse than not offering it at all. Instead, free-area ratio is an input you provide, read directly off your grille's spec sheet (often given as a percentage, or as Ak in square inches you can convert to a ratio by dividing by face area). A default of 0.70 is offered as a stated, generic starting point for common bar grilles, not a substitute for the real number.
Face velocity and why it matters for comfort
Face velocity is how fast air moves through the grille's free area, and it's the main driver of how a supply register or return grille sounds and feels in a room. Too slow and a register can feel like it's barely moving air; too fast and the same CFM through a smaller opening produces a whistle, a hiss, or a noticeable draft right at the grille. A common convention for supply registers is roughly 500 to 750 feet per minute, chosen mainly for occupant comfort in typical rooms; return grilles, which don't need to throw air across a room the way supply registers do, commonly run lower, roughly 400 to 500 fpm. These are guidance ranges to size toward, not hard computed limits, a bathroom exhaust-adjacent register or a grille in a mechanical space can reasonably run hotter than a bedroom register would.
Two ways to use this calculator
"Size a grille" starts from what you know before buying: the CFM the run needs to deliver, a target face velocity you're comfortable with, and your grille's free-area ratio. It returns the face area (and free area) that combination requires, so you know what size grille to look for. "Check a grille" runs the same math the other direction: given a specific grille's face width and height, its free-area ratio, and the CFM it will carry, it reports the actual face velocity that grille produces, so you can tell whether a grille you already have (or are considering) will feel right in that CFM.
0.70 is a midpoint of the range commonly cited for ordinary bar-type grilles, useful for a rough first pass when you don't yet have a spec sheet in hand. It is not a measured figure for any specific product, and real grilles vary meaningfully around it: a plain fixed-bar grille might sit closer to 75 to 80 percent, while a grille with dense, angled vanes for better air direction, or one built for a filter to sit behind it, can run closer to 55 to 65 percent. Once you have an actual grille in mind, replace the default with its real free-area ratio (or compute one from a published Ak figure divided by face area) before finalizing a size.
How these numbers are derived
Every figure here comes from the same public face-velocity relationship used throughout HVAC sizing: required free area equals CFM divided by target face velocity; required face area equals that free area divided by the free-area ratio; and, in reverse, the effective free area of a known face size (width times height, converted to square feet, times the free-area ratio) divides into CFM to give actual face velocity. None of it depends on a specific manufacturer's catalog, the free-area ratio is the one input that does, and it's always yours to supply, never something this page assumes on your behalf.
Frequently asked questions
What size return grille do I need for 400 CFM?
At a typical return face velocity of 450 fpm and a 0.70 free-area ratio, 400 CFM needs about 1.27 sq ft of face area, roughly 183 sq in (the calculator above computes the exact figure live). A 20x10 grille (200 sq in) comfortably clears that. Your grille's actual free-area ratio, from its spec sheet, changes the exact number.
What is free area on a grille?
Free area (sometimes called Ak) is the actual open area air passes through, the gaps between the vanes or bars, not the full visible face opening. It is always smaller than face area, typically 60-80% of it for common bar grilles, and it is measured and published per model by the manufacturer, not something this calculator can look up for you.
How do you size a supply register?
Divide the CFM by a target face velocity (commonly 500-750 fpm for supply, chosen for comfort and noise) to get required free area, then divide that by the grille's free-area ratio to get required face area. Both numbers come from the same public formula; only the free-area ratio depends on the specific grille.
What face velocity is too high for a grille?
There is no single hard cutoff, but supply face velocities meaningfully above about 750 fpm commonly produce noticeable whistling or a draft at the register, and return velocities above roughly 500 fpm start to feel and sound like suction rather than gentle airflow. Treat these as comfort guidance, not a computed limit.
Why isn't there a lookup table for my exact grille model?
Free area is manufacturer-measured, per-model data published in that manufacturer's catalog or spec sheet, it is not public information this calculator can legally or accurately reproduce. The formula that turns free area into a sizing answer is public and is exactly what this page computes; the free-area number itself has to come from your grille's spec sheet.