The gap under the door is the return path, whether anyone designed it or not
Most houses with central air have a small number of return grilles, often just one in a hallway, and a supply register in every room. Close a bedroom door and that room now has air being pushed in with no direct way out. The pressure rises until enough air is forced through whatever gaps exist — under the door, around the trim, through the ceiling — to balance the supply. In a reasonably tight room, the undercut is doing nearly all of that work.
The arithmetic is the same as any duct opening. Free area in square inches equals airflow in cubic feet per minute times 144, divided by the velocity in feet per minute you are willing to run. Then the undercut is that area divided by the door width, because the gap is a long thin rectangle the full width of the slab.
| Door width | Undercut | Free area | CFM at 400 FPM |
|---|---|---|---|
| 30" | 1/2" | 15 sq in | 42 |
| 30" | 3/4" | 22.5 sq in | 63 |
| 32" | 3/4" | 24 sq in | 67 |
| 32" | 1" | 32 sq in | 89 |
| 36" | 3/4" | 27 sq in | 75 |
| 36" | 1-1/2" | 54 sq in | 150 |
The 400 FPM column is a design choice, not a requirement — it is a velocity commonly used as a quiet target for a low-pressure path. Run the same gap at 700 FPM and it passes nearly twice the air and you can hear it. That is the whole reason the velocity is a field on this page rather than a constant.
Flooring is where existing houses lose the gap
Doors are usually hung before the finish floor goes down, with the undercut set for whatever floor was planned. Then somebody puts three quarter inch engineered plank plus an eighth of underlayment over the original vinyl, and 7/8 of an inch of gap disappears. A door that had a three quarter inch undercut now has none, and either it drags or it was cut down on site.
The airflow consequence is invisible and nobody connects it to the flooring job. The room that used to balance now runs positive, the door thumps when the system starts, and there is a whistle at the latch side because all the air is being forced through the strike-side gap instead. Enter the flooring build-up in the field above and the calculator subtracts it, which is the version of the question worth asking before the flooring is ordered rather than after.
What cutting a slab costs you
Taking material off the bottom of a door is normal work, and it has a limit that depends on how the door is built. A solid slab has material all the way down. A hollow core door has a bottom rail, a strip of solid wood glued between the two skins, and above it there is a cardboard grid holding nothing. Cut past the rail and the skins have no edge support: the bottom flexes, the veneer chips, and there is nothing to hold a sweep or a threshold seal. Door manufacturers publish how much can come off their product, and that number is the one to use.
Two things not to do here on the strength of a calculator. A fire-rated door is a tested assembly, and cutting it means it is no longer the thing that was listed — the manufacturer is the only authority on any modification, and for practical purposes the answer for a job-site cut is no. And an exterior door with a bottom sweep and a threshold is a water and air detail, not a ventilation one; undercutting it to move return air creates a path for water and insects.
The transfer grille alternative and what it costs
When the gap cannot get any bigger, the standard alternative is to move the air path somewhere else: a grille through the door, a pair of grilles high and low through the wall, or a jumper duct through the ceiling connecting the room back to the hallway. All of them provide free area without shortening the slab, and all of them let sound and light through in the same proportion. A bedroom with a large transfer grille to the hallway is acoustically much closer to a room with the door open, which is exactly the thing the door was closed to avoid.
A jumper duct — a short run of flex up into the ceiling and back down in the hallway — is the version that keeps the sound path long and crooked, at the cost of being real ductwork with real static pressure. The duct static pressure calculator and the flex duct penalty calculator cover what that run costs the system, and register and grille sizing handles the face size once you know the free area you need.
Where the numbers should come from
Two inputs on this page carry all the weight and neither should be guessed. The airflow into the room comes from the duct design — the load calculation says how much that room needs, the design says what the branch delivers, and a balancing hood at the register says what it actually delivers, which is often a different number again. The duct branch balancing calculator and air changes calculator deal with that side. The velocity is a comfort and noise choice.
Whether any of this is required where you live is a separate question with a local answer. Mechanical codes address return air paths for closed rooms, but which code, which edition, and how the building department applies it varies, and so do the exemptions. Ask them before you cut a hole in a wall or a door on the strength of an internet calculator. The rest of the door dimension work lives on the jamb width calculator and the floor transition calculator, which is where the flooring build-up number you entered above comes from.
Questions people ask
How much undercut does a bedroom door need for return air?
It depends entirely on how much air is being supplied to that room and what velocity you are willing to run through the gap, which is why both are input fields. As an illustration, 100 CFM at a quiet target of 400 feet per minute needs 36 square inches of free area, which on a 32 inch door is an undercut of about 1-1/8 inches. The same room at 600 FPM would balance through a 3/4 inch gap but you would hear it. Whether an undercut is required at all, and at what size, comes from the mechanical code adopted where you are.
Is a 3/4 inch undercut enough?
On a 32 inch door a 3/4 inch undercut is 24 square inches of free area, which passes about 67 CFM at 400 feet per minute or about 100 CFM at 600 FPM with the noise that implies. So the honest answer is that it is enough for a room getting up to roughly 70 CFM if you want it quiet, and short for a larger bedroom or a room with two supply registers. Put the actual supply airflow for that room into the calculator rather than working from the door size alone.
The new floor closed the gap under my doors. What are my options?
Three, roughly. Take the slabs off and cut them down, within whatever the door manufacturer says can come off that product — a hollow core door has a limited bottom rail and cutting past it ruins the slab. Add a transfer path elsewhere, through the door, the wall or the ceiling, which buys free area but also passes sound. Or accept the pressure imbalance, which is what most houses quietly do, at the cost of doors that thump, whistling at the latch side and rooms that never quite reach temperature with the door shut. A fire-rated door is not in the first category at all and must not be cut.
Does the gap around the sides of the door count?
A little, and this calculator ignores it deliberately. The hanging gap at the hinge and strike sides is typically an eighth of an inch, and while it does pass air, most of it is interrupted by the stop, the latch, the strike plate and the weatherstrip if there is any. Counting it would flatter the result. The undercut is the path that is genuinely open along its whole length, so it is the one worth sizing. If you want the extra credit, measure the side gaps that are actually clear of the stop and add the area yourself.
Why does velocity matter if the air gets through either way?
Because velocity is what you hear and what costs pressure. Air squeezing through a small gap at high speed makes broadband noise right at head height when you are lying in bed, and it also means the room is running at a higher pressure difference, which pushes conditioned air into wall cavities and out of the building instead of back to the return. Doubling the free area halves the velocity for the same airflow, and noise falls much faster than that. The velocity field is where you decide how much of that trade you want.