How to measure a door gap properly

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

1. 1Close the door and get your eye down to floor level on the inside.
2. 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.
3. 3Repeat in the middle of the door width.
4. 4Repeat at the hinge side.
5. 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.

1. 1Take five or six index cards and square them up.
2. 2With the door closed, slide the stack into the gap between the door edge and the stop.
3. 3Add or remove cards until the stack just fills the gap with light friction.
4. 4Count the cards and divide by 100. Eight cards is roughly 0.08 in, which is a shade over 1/16.
5. 5Repeat at the top corner, the middle and the bottom of the latch side, and once across the head.

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, 36 in, Fits Gaps up to 1.7 inStill the rare listing in this category that publishes a gap figure rather than only a door length — which is why it leads the selector table even though the number has changed.	A published gap figure	Up to 1.7 in (published)	$18.99 · CHECK ON AMAZON
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	$7.69 · CHECK ON AMAZON
3	Frost King A79WHA Premium Aluminum and Reinforced Rubber Door Sweep, 2 x 36 in, WhiteA 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

#ad · how we are funded · price as of October 17, 2026

1

A published gap figure

MAXTID Door Draft Stopper, 36 in, Fits Gaps up to 1.7 in

Published gap range

Up to 1.7 in (published)

$18.99 · CHECK ON AMAZON

#ad · how we are funded · price as of October 17, 2026

Still the rare listing in this category that publishes a gap figure rather than only a door length — which is why it leads the selector table even though the number has changed.

Where the number comes fromMAXTID publishes a maximum gap of 1.7 in for this stopper. That is a ceiling, not a working range: it tells you the largest gap the tube will fill, and says nothing about the smallest. Almost nothing else in this category publishes any gap figure at all, which is the whole reason it is ranked here.

Published maximum gap

1.7 in

Published length

36 in

Type

Under-door stopper

Mounting

Slide-under

Color

Gray

Mounting

Slides under the door

What it does well

• +Publishes an actual gap figure, so you can check it against your measurement
• +A 1.7 in ceiling covers badly hung doors that defeat most stoppers
• +No fixings, so nothing is done to the door

Where it fails

• -A maximum is not a range — the listing says nothing about the smallest gap it suits
• -36 in only, so a narrow bathroom or closet door will overhang
• -Fabric cover on a bathroom door will pick up damp

Who this is forSomeone who has measured a large gap and wants a figure to check it against.

• Source: Amazon listing — published 36 in length, fits door gaps up to 1.7 in

2

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

$7.69 · CHECK ON AMAZON

#ad · how we are funded · price as of October 17, 2026

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.

• Source: Amazon listing — published 5/16" W x 1/4" T x 17 ft, EPDM

3

Exterior doors, screw-mounted

Frost King A79WHA Premium Aluminum and Reinforced Rubber Door Sweep, 2 x 36 in, White

Published gap range

Roughly 1/4 in to 3/4 in

CHECK PRICE

#ad · how we are funded · price as of October 17, 2026

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
• -The matte white profile vanishes on white trim and stands out on a stained or dark door

Who this is forAn exterior door you own, with a gap you can see daylight through and a threshold that is basically flat.

• Source: Amazon listing — published material, 2 in x 36 in size and white finish
• Source: Frost King — door sweeps, bottoms and thresholds category

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.

One reading is worth checking against a published threshold rather than against a seal size. UC IPM states that mice can enter through openings as small as 1/4 inch, so an under-door gap at or above that is an entry point as well as a draft. If that describes yours, the best door sweeps for keeping mice out covers what changes about the choice.