The only time a large gap may be needed is for return air in a forced air system.
Other than that, I usually aim for about 1/8 to 1/4 inch depending on how level and flat the floor is.

Thanks Brickeyee, what do you mean with forced air system? Is that like normal central ac/heat? There are only 2 air returns in the house. If so what gap is needed?

If you are planning on having a rug or carpet right near the door, you'll need to allow for it, too.

Brickeyee or anybody else? Any input on gap with a central ac and not a return in every room?

my 1921 apt. has a threshold at every interior door.

I've decided that its purpose is to close the gap left under the door, because the door is sized so that there's about 1 inch between door and floor when it's open.

The "why" of this is now clear to me--it's so that the door will have enough room to swing OVER the area rug that's required in each room.

There is no similar threshold at either of the two front doors, and I cannot put a normal rug in the front hall--there's not enough clearance for the door to swing over it.

I'll have to buy some sort of ultrathin rug to go there, which sort of limits my choices in design, etc.

So if you decide you need the door to be shorter in order to clear the rug, you can still close that gap w/ a threshold at the door.

lyfia, typical central air systems have an outlet in every room where the air blows out, and a return where the air gets suck in. The air circulates from the outlet, through the space, and into the return. The gap allows this to happen when the door is closed. I'm not aware of a code requirement, but I've hear 3/4" referred-to as a rule of thumb. Something like 1/8" seems very small, but I think the gap could be closed down to 1/2" without problems. If you don't have the blowing outlets then this is all a non-issue for you. You can always leave the gap small and cut the bottom of the door if there's a problem with a rug or something - the reverse doesn't work.

Before changing out the door close it and see how much air blows when the system fan is turned on.
If you can feel it moving you probably should at least duplicate the existing gap.
Matching the cross section of the ducts supplying the room is another way to get an estimate.
Just remember it is a lot easier to remove more if the flow is low then add it back.

There needs to be a minimum of 2 inch gab between the base of the interior doors and the top of the flooring to allow proper HVAC system circulation. Provided that there is not a return or separate HVAC system installed in each single room. This gap is necessary to allow the forced air a place to leave the room. Otherwise, when the doors are nearly shut the room can pressurize and force the doors to close/slam shut. Once closed the interior temperature of the closed room will be different than the remaining interior rooms as there is poor conditioned air circulation occurring in that room. Another problem with poor gap beneath the interior doors is that the pressurized conditioned air can impregnate dust/dirt into the carpet fibers leaving a stain line beneath the area where the closed door would rest. Just wish I saw this years ago when originally posted.

Almost a 7 year old post, hope they have it worked out by now!!!

Our home was built in 2002, and has central air and heating. The space between the floor and interior doors is 3/4". I just measured. Our existing floors are mainly tile, but the doors swing easily over the bedroom carpet. We are moving soon, and are also replacing doors in our new remodel.

I freaked when I read your topic because I missed the "interior" part, and thought "Oh no!" Snakes can get in! I'm a little paranoid because our new remodel is on a rocky hillside, and I've seen snakes squeeze into the teensiest cracks in rocks!

Thanks for the post! The answers are all good! We do have a contractor who should know codes for hanging doors, but I'll be right there with my yard stick!

Suzi