01 / DOORS
How to measure a door gap properly
The measurement is the whole purchase. This is the page we link to from everywhere else, because everything on this site starts here.
Measure the gap under the door at the latch side, the middle and the hinge side, and use the largest reading. For the frame, fold an index card until it just fills the gap and count the folds — each card is about 0.01 inch. Buy a seal whose published thickness is slightly larger than what you measured, never smaller.
Almost nothing in the weatherstripping aisle publishes a working gap range. Sellers publish a section — 5/16 by 1/4 inch, 1-3/4 by 36 inches — and leave you to work out whether that reaches your floor. Which means the number you take with a tape measure is doing the work that the packaging should have done.
The under-door gap: three readings, take the largest
- 1Close the door and get your eye down to floor level on the inside.
- 2Push the end of a tape measure into the gap at the latch side until it stops against the floor and the door. Read it.
- 3Repeat in the middle of the door width.
- 4Repeat at the hinge side.
- 5Write all three down. The largest is your buying number. The spread between them tells you whether the floor is level.
The frame gap: the index-card method
The gap between the door and the stop is usually a fraction of an inch and a tape measure is useless on it. Index cards are not: a standard card is close to 0.01 inch thick, so a stack you can just slide into the gap gives you a number to two decimal places for free.
- 1Take five or six index cards and square them up.
- 2With the door closed, slide the stack into the gap between the door edge and the stop.
- 3Add or remove cards until the stack just fills the gap with light friction.
- 4Count the cards and divide by 100. Eight cards is roughly 0.08 in, which is a shade over 1/16.
- 5Repeat at the top corner, the middle and the bottom of the latch side, and once across the head.
| You measured | Seal thickness to buy | Type |
|---|---|---|
| Under 0.05 in (5 cards) | V-strip | Tension seal, springs back |
| 0.05 to 0.12 in | 1/8 in bulb or V-strip | Either works; V-strip is less visible |
| 0.12 to 0.25 in | 1/4 in D-section or silicone bulb | The most common answer |
| 0.25 to 0.4 in | 1/2 in open-cell foam | Indoors only — foam sets over a season |
| Over 0.4 in | Rope caulk, or fix the door | A gap that big usually means a hinge or frame problem |
The dollar-bill test: where, not how much
Close the door on a dollar bill and pull. If it slides out freely, that point is not sealing. Do it at eight or ten points around the frame and you get a map of where the leaks are — which is a different and equally useful piece of information from how big they are. Use the bill to find them, then the cards to size them.
One rule for reading the answer
A compressible seal works from about half its published thickness up to its full thickness. So a 1/4 inch D-section covers a gap of roughly 1/8 to 1/4 inch. Under half, it is being crushed and will take a permanent set; over its thickness, it is not touching at all. When your measurement sits on a boundary, go up a size — a seal slightly too thick still seals, and one slightly too thin does nothing.
Quick picks
Ranked for fit, not for hype. Tap a row to jump to the full write-up, or go straight to the listing.
| # | Product | Best for | Gap range | Price |
|---|---|---|---|---|
| 1 | MAXTID Door Draft Stopper for 30 in Interior DoorsThe rare listing in this category that publishes a numeric gap range — 0.5 in to 1.2 in — which is why it leads on the selector table. | A published gap range | 0.5 in to 1.2 in (published) | CHECK PRICE |
| 2 | Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16 in x 1/4 in x 17 ftEPDM in a hollow D — the profile that keeps sealing at minus ten where foam tape has already gone hard. | Medium frame gaps | Published 1/4 in thick; compresses to seal gaps of roughly 1/8 in to 1/4 in | CHECK PRICE |
| 3 | Frost King A79/36H Premium Aluminum and Reinforced Rubber Door SweepA screw-on aluminum carrier with a reinforced rubber blade — the version of this part that survives a winter of a storm door slamming into it. | Exterior doors, screw-mounted | Roughly 1/4 in to 3/4 in | CHECK PRICE |
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A published gap range
MAXTID Door Draft Stopper for 30 in Interior Doors
Published gap range
0.5 in to 1.2 in (published)
#ad · how we are funded · no live price right now
The rare listing in this category that publishes a numeric gap range — 0.5 in to 1.2 in — which is why it leads on the selector table.
Where the number comes fromMAXTID states this fits door gaps of 0.5" to 1.2" in the listing title. That is a genuine published working range, not our estimate, and it is the exception in a category that mostly publishes nothing.
- Published gap range
- 0.5" to 1.2"
- Door width
- 30 in
- Type
- Under-door stopper
- Mounting
- Slide-under
- Color
- Black
- Mounting
- Slides under the door
What it does well
- Publishes an actual gap range, so you can check it against your measurement
- Sized for narrower bedroom and bathroom doors that most stoppers overhang
- No fixings
Where it fails
- 30 in sizing is a real constraint, not a suggestion
- A 1.2 in ceiling rules out badly hung doors
- Fabric cover on a bathroom door will pick up damp
Who this is forAnyone who measured their gap first and wants a part whose published range actually covers it.
Medium frame gaps
Frost King EPDM Rubber Self-Stick Weatherseal Tape, D-Section, 5/16 in x 1/4 in x 17 ft
Published gap range
Published 1/4 in thick; compresses to seal gaps of roughly 1/8 in to 1/4 in
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EPDM in a hollow D — the profile that keeps sealing at minus ten where foam tape has already gone hard.
Where the number comes fromFrost King publishes the section dimensions exactly: 5/16" wide by 1/4" thick, 17 ft long. A hollow D seals across roughly half to full its stated thickness, which is where the 1/8-1/4 in working figure comes from.
- Profile
- Hollow D
- Published width
- 5/16 in
- Published thickness
- 1/4 in
- Published length
- 17 ft
- Material
- EPDM rubber
- Mounting
- Self-adhesive onto the door stop
What it does well
- EPDM stays flexible in cold, which is the entire point of a winter seal
- Published dimensions to three fractions — you can actually match it to a measurement
- Compresses without taking a permanent set the way open-cell foam does
Where it fails
- 17 ft is one door frame; a house needs several rolls
- Adhesive still needs a clean, smooth surface
- Thicker than a V-strip, so it can stop a tight door from latching
Who this is forA door stop or window sash with a visible, consistent gap you have measured.
Exterior doors, screw-mounted
Frost King A79/36H Premium Aluminum and Reinforced Rubber Door Sweep
Published gap range
Roughly 1/4 in to 3/4 in
#ad · how we are funded · no live price right now
A screw-on aluminum carrier with a reinforced rubber blade — the version of this part that survives a winter of a storm door slamming into it.
Where the number comes fromFrost King does not publish a numeric working range for this sweep. The 2-inch face and the rubber blade's projection put it in the quarter-inch-to-three-quarter-inch band once mounted; the blade must touch the threshold under light compression, not bend flat. Measure before you buy.
- Carrier
- Aluminum
- Blade
- Reinforced rubber
- Published size
- 2" x 36"
- Mounting
- Screws (supplied)
- Maker's claim
- Lifetime guarantee on weathersealing ability
- Mounting
- Screws through the door face, above the threshold
What it does well
- Screws, not adhesive — nothing to peel off in July
- Cuts down with a hacksaw for a narrower door
- The rubber blade is replaceable in practice; the carrier is what lasts
Where it fails
- You are drilling into the door face — renters should look at an adhesive or slide-on option
- No published gap range, so measuring first is not optional
- Aluminum shows a hard line on a painted door; the finish choice matters
Who this is forAn exterior door you own, with a gap you can see daylight through and a threshold that is basically flat.
How we picked
- Every product on this site is placed by the published working range of its own manufacturer, matched against the measurement this page teaches you to take.
- Where no range is published, we say so and explain how to decide from the dimensions that are.
We have not tested these products and we do not claim to have. The full method, including what we refuse to claim, is on how we pick.
The same measurement logic scales to a garage door, where the numbers are bigger and the floor is usually worse: see garage door seal for an uneven floor.
Questions people actually ask
- How do I measure the gap under my door?
- Close the door and take a tape-measure reading at the latch side, the middle and the hinge side. Use the largest of the three as your buying number, and note the spread — a spread over about a quarter inch means the floor is uneven and you need a brush sweep.
- How do I measure a gap too small for a tape measure?
- Stack index cards until they just fill the gap and count them. A standard index card is about 0.01 inch, so ten cards is a tenth of an inch. It is more precise than any tape measure at this scale.
- What size weather stripping do I need?
- One whose published thickness is between your measured gap and twice it. A compressible seal works from roughly half its thickness up to its full thickness, so a 1/4 inch section covers a 1/8 to 1/4 inch gap.
- My door gap is different at each end. What now?
- That is an uneven floor or a dropped hinge. If the door itself has sagged, tightening or shimming the top hinge may square it up. If the floor is uneven, buy a brush sweep, which bends to the changing height rather than fighting it.
Sources
- MAXTID — an example of a listing that publishes an actual gap range (0.5 in to 1.2 in)
- Frost King — published D-section dimensions used in the sizing table above
Every specification on this page traces to a published manufacturer or retailer listing. Where a maker publishes no working gap range, we say so rather than estimating one.
Read next
The best door sweeps, sorted by the gap they actually close
Six sweeps placed by the gap they close, from a slim vinyl blade to a brush for uneven floors.
The best weather stripping for doors, by published thickness
V-strip, D-section, silicone and foam placed against the gap each published thickness can actually seal.
Sealing a garage door on an uneven floor
Measure the dip first. Past about a quarter inch, only a floor-bonded threshold can close it.