Free area is the whole idea
A register or grille has two areas. The gross face is what you measure with a tape: a 12 by 6 register is 72 square inches. The free area is what air can pass through once you subtract the bars, the frame, the louvre blades and the damper behind them. On a typical stamped supply register that free area is somewhere around three quarters of the gross face, so those 72 square inches become roughly 54 square inches of actual opening.
Velocity is airflow divided by area, and the area that matters is the free one. Push 100 CFM through 54 square inches and the air is moving at about 267 feet per minute. Push the same 100 CFM through a 4 by 10 register instead and the free area drops to 30 square inches, giving 480 FPM. Same room, same fan, and the second one is the one people hear.
This is why sizing on outside dimensions goes wrong. The dimension printed on the box and the dimension the air experiences are different numbers, and the ratio between them is a property of the product, not a constant.
Typical free area by type
| Type | Free area, roughly | Why |
|---|---|---|
| Stamped supply register | 70–80% | Curved blades with a damper plate behind the face |
| Ceiling diffuser | 80–90% | Open cone or stepped face, little metal in the path |
| Return grille, fixed bar | 65–75% | Heavier bar stock, no damper, but more of it |
| Filter grille | 50–60% before the filter | Deeper frame to hold media, and the media adds its own drop |
| Linear slot or bar grille | 35–55% | Narrow slots between substantial bars |
Treat those as orientation for a first pass. Every manufacturer publishes the real figure for each model and size, usually alongside a throw and a sound rating, and the published figure is worth looking up before you buy twenty of anything. The default this calculator uses for the type you pick is a middle-of-the-road number, and you can overwrite it in the free area field.
Face velocity and neck velocity are two different problems
Face velocity governs noise in the room and how far the air throws before it mixes. Neck velocity governs the branch duct behind the register and feeds back into the pressure the blower sees. They are usually set independently: a common approach is to pick a face velocity low enough to be quiet, then size the neck for whatever the branch design uses, and accept that the boot behind the register is a transition between the two.
The neck is also where a balancing damper belongs. A damper at the register face throttles air right where it enters the room, which converts pressure into noise at exactly the point you can hear it. The same damper at the takeoff off the trunk does the same balancing work with the noise buried in the duct. If a room is loud only when the damper is partly closed, that is the mechanism, and moving the restriction upstream fixes it without changing any flow.
Throw, spread and where the air ends up
The calculator here is about area and velocity. It does not model throw, which is how far a jet of supply air travels before it slows to a crawl, or spread, which is how wide it fans out. Those depend on the blade pattern of the specific register, the direction it is aimed, and the geometry of the room, and manufacturers publish them as tables because they cannot be derived from area alone.
What is worth knowing without the tables: a register aimed at an exterior wall or a window generally outperforms one aimed at the middle of a room, because the cold surface is where the load is. A ceiling diffuser sized for a low face velocity has a short throw and can dump conditioned air straight down onto whoever is under it. And a supply register blowing directly at a return grille four feet away short-circuits, so the room gets the air change on paper and not in fact.
Where this sits relative to the rest of the system
Register sizing is the last link in a chain. The room needs a flow rate, which comes from a load calculation or from an air change target — the CFM and air changes calculator handles the air change side and the heat loss calculator the load side. That flow then needs a branch to carry it, which is the duct size calculator. Once several branches share a trunk, the flow each one actually gets depends on the others, which is the duct branch balancing calculator. And every one of those pieces contributes a pressure drop that the blower has to overcome, added up in the duct static pressure calculator.
A register cannot fix any problem upstream of it. If a room is short on air, a larger register lowers the noise and changes nothing about the quantity, because the restriction that is limiting the flow is somewhere else in the path. Size the register for comfort and noise, and go looking elsewhere for the missing CFM.
Questions people ask
What size register do I need for 100 CFM?
It depends entirely on how quiet you want it and what the free area of the product is, which is why this page asks for both rather than giving you one answer. Working an example through: at a 500 FPM target through the free area, 100 CFM needs 100 divided by 500, which is 0.2 square feet, or 28.8 square inches of actual opening. On a stamped register with 75 percent free area that is 38.4 square inches of gross face, so a 4 by 10 (40 square inches) just covers it and a 6 by 10 is comfortable. Drop the target to 300 FPM because it is a bedroom and the same 100 CFM wants 64 square inches of face, which is a 6 by 12 or larger. Same airflow, two very different registers, and the difference is a decision about noise rather than a calculation.
Why is my supply register whistling?
Almost always velocity through a restriction, and there are only a few candidates. The register face may just be too small for the flow, which this calculator will tell you. More often the damper behind the face is partly closed, which turns the register into a much smaller opening and puts the resulting jet right at the room. Occasionally the boot behind the register turns the air ninety degrees an inch before it arrives, so the flow hits one side of the face and screams through part of it while the rest does nothing. Open the register damper fully and see whether the noise goes; if it does, move the balancing restriction back to the takeoff at the trunk where the noise stays in the duct.
Should return grilles be bigger than supply registers?
In practice they usually are, for two reasons that compound. Most systems have far fewer returns than supplies, so each return carries much more air than any single supply. And returns have no damper to trim them, so whatever the grille does is what you get. A house with twelve supply registers and two returns is asking each return to handle six times the flow of a supply, and if the grilles are the same physical size the return side is running at six times the face velocity. That is the single most common source of a system that roars when it starts. Size returns on the flow they actually carry, using the same free area arithmetic, and give a filter grille extra allowance for the filter behind it.
Does a register damper change how much air the room gets?
Yes, and that is exactly the problem with using it. Closing a register damper raises the resistance of that branch, so less air goes down it and more goes to every other branch that shares the trunk. It works, but it does the throttling at the noisiest possible location and it changes the balance of every other room at the same time. A balancing damper at the takeoff does the identical thing with the noise inside the duct rather than in the room. If you are balancing a system by walking around adjusting register faces, expect to go round several times, because every adjustment moves all the others.
What free area figure should I use if I do not have the data sheet?
Use the type default this page offers as a placeholder and treat the result as provisional, then look the real number up before committing to a size. The reason to bother is that free area varies more between products than most other inputs vary between installations: two return grilles of the same nominal size can differ by twenty percentage points of open area, which moves the velocity by a third. Manufacturers publish free area, and often a sound rating and a throw table, for every model and size. If the product you are buying does not publish it, that is itself information about how much engineering went into it.