Measure the clear width of your garage door opening at floor level (not the door panel), check the gap height at three points across that width, and order a kit at least 6-12 inches longer than your measurement so you have material to trim to an exact fit. Standard US single doors run 8-9 feet wide and double doors run 16-18 feet, but non-standard openings, sloped floors, and older doors all change what "the right size" actually means. This guide covers the full measuring process, including the edge cases that trip up most first-time buyers.

Why Sizing Gets Confusing

Threshold seals are sold in fixed kit lengths, not custom-cut to the inch, which means the actual buying decision isn't just "measure and match a number," it's "measure accurately, then choose the right kit length and account for trimming." Get the width measurement wrong, or skip checking the floor for slope, and you end up with a seal that's the right kit size on paper but doesn't actually seal the opening.

Step 1: Measure the Opening, Not the Door

Measure the clear width between the two vertical door tracks at floor level, wall to wall. This is different from the door panel's width, and different again from the rough framed opening, both of which can be an inch or more off from the actual clear opening the seal needs to span.

Take this measurement at floor level specifically, not partway up the tracks, since some track systems angle slightly and a measurement taken higher up won't match the floor-level width you actually need.

Standard US widths for reference:

Door type	Typical width
Single car	8 or 9 ft
Double/two-car	16 or 18 ft
Oversized/RV	10-14 ft (single), 20+ ft (double)
Commercial roll-up	Varies widely, always measure

 

Treat these as a sanity check on your own measurement, not a substitute for it. Older homes, custom garages, and any commercial building are common enough exceptions that assuming "standard" without measuring is the single most common sizing mistake.

Step 2: Check the Gap Height at Three Points, Minimum

With the door closed, measure the gap between the bottom of the door and the floor at the left edge, center, and right edge. On a wide double door, take a fourth reading at the quarter-points too, since a slab can dip in the middle without either edge showing it.

What the spread between your readings tells you:

A difference of 1/4 inch or less across all readings means your floor is close enough to level that a standard flexible PVC or vinyl seal will conform to the small variation without leaving gaps.

A difference of 3/8 to 1/2 inch means you're at the edge of what a flexible seal can reliably compensate for. It may still work, but check the specific product's stated tolerance before ordering, since not all flexible seals flex equally.

A difference beyond 1/2 inch means you're dealing with a genuinely sloped or dipped floor, and a rigid aluminum track system that can be shimmed or set to the actual floor profile is the more reliable choice over a flexible seal stretched past its comfortable range.

Step 3: Check for a Sloped Floor Across the Full Width, Not Just at the Edges

Beyond the three-point gap check, run a level or a straightedge across the full width of the opening at floor level. Many garage floors slope toward the door for drainage by design, typically a gentle grade, but older slabs can also settle unevenly over time, creating a dip that isn't part of the original design and won't be obvious from gap measurements at just three points.

If you find a slope that's consistent and gradual (built-in drainage grade), a flexible seal generally handles it fine as long as the resulting gap variation falls within the ranges above. If you find an irregular dip or high spot, that's a stronger signal to go with an aluminum track system regardless of how small the actual gap measurement is at any single point, since irregular unevenness is exactly what flexible seals struggle to compensate for consistently along the full length.

Step 4: Account for Door Type

Sectional doors, roll-up doors, and older tilt-up or one-piece doors all clear the floor slightly differently as they operate, which affects which seal profiles are compatible.

Sectional doors (the most common residential type, made of hinged horizontal panels) generally work with the widest range of threshold seal profiles, since the bottom panel's clearance and angle of approach is fairly standardized across manufacturers.

Roll-up doors (common in commercial and some residential garages, coiling up rather than lifting as panels) sometimes need a specific low-profile seal design to avoid interference with the coiling mechanism at the base.

Older tilt-up or one-piece doors swing out and up as a single panel, which changes the angle the door's bottom edge approaches the floor at compared to a sectional door, and can affect whether a given seal profile makes clean contact across its full range of motion.

Confirm your door type against the specific seal's stated compatibility before ordering. This is a more common cause of a seal not sealing properly than an actual sizing error.

Step 5: Buy Longer Than You Measure, Then Trim

Kits come in fixed lengths, commonly around 9 ft, 16 ft, and 18 ft, corresponding to standard door widths plus trimming margin. If your measured opening falls close to a standard length, round up to the next kit size rather than down. It's a straightforward cut to shorten a seal to an exact fit; there's no reliable way to extend one that's too short.

For openings that fall well between standard kit lengths, for a custom or oversized door for instance, check whether the manufacturer offers a longer commercial-length kit or multiple kits joined at a seam, since some product lines only publish their standard residential lengths prominently even when longer options exist for non-standard openings.

Measuring for Commercial and Multi-Bay Installations

Commercial applications add two considerations beyond the residential steps above.

Measure each bay separately, even on a building with multiple identical-looking overhead doors. Manufacturing and installation tolerances mean openings that look the same can vary by an inch or more, and ordering one kit length for all bays based on a single measurement is a common and avoidable mistake.

Factor in traffic type when choosing not just length but profile and weight rating. A loading dock crossed by forklifts needs a low-profile, heavy-rated seal that won't be damaged or displaced by repeated heavy point loads, which affects the product choice independent of the length calculation. See GaraDry's commercial threshold seal buyer's guide for the full material and weight-rating comparison.

A Quick Pre-Order Checklist

Measured the clear opening width at floor level, not the door panel or rough opening.

Checked gap height at a minimum of three points across the width, four or more on a double door.

Checked for a sloped or dipped floor with a level or straightedge across the full width, not just at the measurement points.

Confirmed door type (sectional, roll-up, tilt-up) against the specific seal's compatibility.

Rounded up to the next standard kit length rather than ordering short.

For commercial jobs, measured each bay individually and confirmed the weight rating matches actual traffic type.

FAQ

What size threshold seal do I need for a standard single garage door?

Most single garage doors are 8 or 9 feet wide. Measure your specific opening at floor level to confirm, since older homes and custom garages don't always match the standard, then order a 9 ft kit (or the next size up from your measurement) to allow for trimming.

What if my garage floor isn't level?

A gap variation of 1/4 inch or less across the width of the door is well within what a flexible PVC or vinyl seal can handle. Beyond about 1/2 inch of variation, or if you find an irregular dip rather than a consistent drainage slope, an aluminum track system that can be shimmed to the actual floor profile is the more reliable choice.

Can I join two threshold seal kits together for a wider opening?

Some product lines support this for oversized or custom openings; check with the specific manufacturer before assuming it's possible, since not every seal profile is designed to be seamed cleanly. It's generally better to check whether a longer single kit is available for your exact width first.

Do I measure the door panel or the door opening?

The opening, not the panel. Measure the clear width between the two vertical tracks at floor level, wall to wall. The door panel itself can be a different width than the opening due to how it sits within the tracks.

How much extra length should I buy beyond my measurement?

6 to 12 inches beyond your measured opening width is a safe margin for trimming to an exact fit. Since kits come in fixed standard lengths, in practice this usually means rounding up to the next available kit size rather than calculating an exact custom length.