The run is not the perimeter
A door is stripped on three sides and swept on the fourth, in two different materials. A 36 by 80 door takes 2 x 80 + 36 = 196 inches, or 16.3 feet, of jamb and head material, plus a 36 inch sweep. Nobody sells those two things together in the right lengths, which is why the take-off has to keep them apart.
Windows split the same way. On a double-hung sash the two joints that actually pass air are the meeting rail where the two sashes cross and the bottom rail where the lower sash sits on the sill — the sides are usually already in a channel. Stripping only those two is 2 x the width. Stripping the full sash perimeter is 2 x width + 2 x height, which on a 32 by 54 window is 14.3 feet instead of 5.3. That is nearly three times the material for a job that may not be the one you needed. The selector on this page exists because the honest answer depends on which window you are looking at.
A gap you can barely see, over a length you never think about
Take the five-line default list: three doors and six windows, with the windows stripped on the two rails only. At a one-eighth inch gap the geometric open area comes to 140 square inches, which is a square 11.8 inches on a side. The front door alone is 2 x 80 + 2 x 36 = 232 inches of perimeter, so at one eighth it is 29 square inches — a hole 5.4 inches square, in one door.
| Gap all round a 36 x 80 door | Open area | Same as a square |
|---|---|---|
| 1/32 in | 7.3 sq in | 2.7 in a side |
| 1/16 in | 14.5 sq in | 3.8 in a side |
| 1/8 in | 29 sq in | 5.4 in a side |
| 1/4 in | 58 sq in | 7.6 in a side |
Area is linear in the gap, so halving the gap halves the area. That is the whole reason weatherstrip works: it does not have to close the joint perfectly, it only has to make the number smaller. A rough geometric area is also not the same as flow — the stop, the latch, the hinge side and the roughness of the joint all interrupt it — so treat the table as scale rather than as a leakage measurement. If you want a leakage figure, that comes from a fan test, and the leak area calculator turns one of those readings into an area you can compare this against.
Compression, and the door that will not shut
Every seal has a working range: a thickness at which it touches and a thickness at which it is squashed so hard the latch will not reach the strike. That range is a product number and it is on the packet. It matters more than the material, because a bulb seal set a sixteenth too proud turns into a door that gets slammed, and a door that gets slammed tears the seal off within a season. If a new seal stops a door latching, the answer is a thinner seal or a moved strike, not more force.
The other honest limit: weatherstrip only ever addresses the joints that move. Everything else — the top plates, the wiring holes, the chases, the rim joist, the hatch — is a different material and a different job. Foam coverage covers the static gaps, the caulk calculator covers the tubes for the joints that get sealed shut, and the air sealing payback calculator is where a measured before-and-after turns into money.
Related pages
For the deliberate gap under an interior door that a return-air path depends on, the door undercut calculator works out the free area that gap represents. For a new opening rather than an existing one, flashing and sealant around the rough opening is the take-off. For glass rather than gaps, the window U-factor payback calculator compares storms, inserts and replacement. The home insulation guide puts the sealing work in order against everything else.
Questions people ask
How many feet of weatherstripping does one door take?
Two jambs and a head, which is twice the height plus the width. A 36 by 80 door is 196 inches, or 16.3 feet, and the bottom is a separate sweep in a different material rather than more of the same roll. A 32 by 80 door is 192 inches. If you are buying a packaged door kit, check what is in it before comparing prices — some are sold as three cut sticks that suit a standard door, others by the foot on a roll, and the two are not interchangeable across a whole house.
Should I strip the whole window or just the rails?
It depends which joints on that window actually pass air. On a typical double-hung the sides of each sash already run in a channel, and the two joints that leak are the meeting rail where the sashes cross and the bottom rail where the lower sash sits down. Sealing those two is twice the width. Sealing the full perimeter is twice the width plus twice the height, which on a 32 by 54 window is 14.3 feet against 5.3. Look at the window on a windy day with a smouldering incense stick before deciding which number to buy.
What gap should I enter for the open area section?
Whatever you actually measure. Slide a folded strip of paper or a feeler gauge into the closed joint at a few points and take a rough average, because the gap is rarely uniform — hinge sides are usually tighter than latch sides and the top is usually tighter than the bottom. A tight modern door reads around 1/32, a lot of ordinary doors read about 1/8, and a door you can see daylight past is a quarter inch or more. The area figure is geometry, not measured leakage.
Does closing these gaps save a specific amount of money?
Not one this page can tell you. Sealing gaps changes how much outside air the house exchanges, and turning that into fuel needs a before-and-after measurement, your own degree days and your own fuel price. That is what the air sealing payback calculator on this site does, and it needs a fan test to be worth anything. Any figure quoted as a percentage saving for weatherstripping in general is an average across a housing stock, not a prediction about your house.
Is there any risk in sealing a house up tightly?
Yes, and it is worth stating plainly. Air that used to arrive through gaps also fed anything in the house that burns fuel, and it carried moisture out. Tightening the shell can starve or reverse a furnace, boiler, water heater or fireplace, and the result of a reversed flue is carbon monoxide in the house. A tighter house also needs its ventilation thought about rather than assumed. Neither of those is something to work out from a calculator: if there is a combustion appliance inside the conditioned space, get somebody in who can test it under worst-case conditions with an instrument.