Most small bats can squeeze through a gap roughly the width of a finger, around 1 to 1.5 centimeters (about three-eighths to five-eighths of an inch). That is comparable to the thickness of a pencil or the diameter of a 10-cent coin. Larger species need a bit more room, but even they can pass through surprisingly narrow openings thanks to skeletal features that let them compress and contort their bodies. The answer matters whether you are trying to keep bats out of your attic or designing habitat for them, and the biology behind it is more interesting than you might expect.
What Determines the Minimum Gap
A bat’s ability to pass through a narrow crack comes down to one thing: the size of its skull. Bat skulls are rigid bone, so they cannot compress further. Everything else, the ribcage, the shoulders, the folded wings, is remarkably flexible. A bat’s ribs are thin and somewhat cartilaginous, letting the torso flatten substantially under gentle pressure. The shoulder joints are loosely constructed, allowing the wings to fold tightly against the body. So the skull becomes the bottleneck, literally. If the skull can fit through, the rest of the bat usually can too.
For the small Myotis species common across North America and Europe, skull width runs roughly 7 to 9 millimeters. That is how you arrive at the commonly cited figure: a gap of about 1 centimeter, or three-eighths of an inch, is enough. Bigger bats like the big brown bat (Eptesicus fuscus) have wider skulls, so they need openings closer to 1.5 centimeters (about five-eighths of an inch). Flying foxes and other large fruit bats, by contrast, have skulls several centimeters across and would not attempt to enter such tight spaces.
Skulls Built for Tight Spaces
Some bat species have skulls that seem purpose-built for crevice living. One striking example is the flat-headed myotis (Myotis planiceps), a rare species rediscovered in northeastern Mexico. Members of this species have a conspicuously flattened cranium, and researchers have speculated that this is an adaptation for squeezing into narrow rock crevices, though little is known about the species’ biology overall.1Journal of Mammalogy. Placement of the rediscovered Myotis planiceps (Chiroptera: Vespertilionidae) within the Myotis phylogeny The flat-headed myotis is not alone in this regard. Several unrelated bat lineages around the world show dorsoventrally flattened skulls or flattened body profiles, suggesting that crevice-dwelling has exerted similar selective pressure on bat anatomy multiple times independently.
This flattening does not appear to compromise brain volume much. The skull compensates by being slightly wider or longer, redistributing the cranial cavity without adding much overall height. The result is a bat that can slip into a space that looks impossibly thin for any vertebrate of its body mass.
Why Bats Prefer Tight Gaps in the First Place
You might assume bats would favor roomy entrances for easy access. In fact, the opposite is true. A study of female big brown bats roosting in rock crevices in southeastern Alberta found that the crevices they selected as roosts had openings that were smaller than those randomly available in the landscape.2Canadian Journal of Zoology. Roosting behaviour and roost selection of female big brown bats (Eptesicus fuscus) roosting in rock crevices in southeastern Alberta The bats were deliberately choosing the tightest gaps they could fit through. The same study found that selected crevices were more vertical in orientation and farther from level ground above than random crevices.
This makes sense from a predator-avoidance perspective. A crack too narrow for a raccoon’s paw or a snake’s body gives a roosting bat a measure of safety that a wider cavity would not. The tighter the fit, the fewer predators can follow. Tight crevices also offer thermal advantages: a snug space traps body heat more effectively, which matters for a small endothermic animal that often roosts in colonies where pups need to stay warm. The preference for vertical orientation may further limit access by climbing predators while also preventing rain from pooling inside the roost.
How This Plays Out for Homeowners
If you have bats getting into your attic, garage, or wall voids, the practical takeaway is clear: any gap wider than about 1 centimeter on a typical residential building is a potential entry point. Common entry locations include gaps where roof flashing meets the wall, unscreened attic vents, spaces beneath ridge caps, gaps around chimney flashing, and openings where pipes or wires penetrate the exterior. Even the gap under a poorly sealed door can be enough if the door sits on a raised threshold.
Bat-proofing a structure (typically called “exclusion” in wildlife management) involves sealing every opening larger than about 6 millimeters. That is a conservative target to account for the smallest species. Exclusion should never be done while bats are actively roosting, especially during maternity season when flightless pups are inside. The standard practice is to install one-way exclusion devices, typically small mesh tubes or netting, over the main entry points, allowing bats to leave at dusk but not re-enter. Once the colony has vacated, every gap gets permanently sealed. Timing this work for late summer or early fall, after pups can fly but before hibernation, prevents trapping juvenile bats inside, which creates both a welfare problem and a smell problem.
One mistake people make is sealing only the large, obvious openings and ignoring small cracks along soffits or rooflines. Bats will find those. Another common error is using expanding spray foam alone, which some bat species can chew or push through before it fully cures. Hardware cloth, steel wool, caulk, or commercial exclusion netting are more reliable long-term solutions for different types of gaps.
Species Size Ranges and What They Can Fit Through
The three-eighths-inch figure gets repeated so often that people sometimes treat it as universal. In reality, the minimum gap varies quite a bit across the roughly 1,400 known bat species. Here are some rough groupings:
- Small Myotis species: Little brown bats, Indiana bats, and northern long-eared bats can manage gaps of about 1 centimeter (three-eighths of an inch). These are among the species most commonly found in buildings in temperate North America.
- Mid-sized species: Big brown bats, evening bats, and similar species typically need about 1.3 to 1.5 centimeters (roughly half to five-eighths of an inch). Big brown bats are the other very common attic-dwelling species in much of the United States and Canada.
- Large insectivorous bats: Species like the greater mouse-eared bat (Myotis myotis) in Europe or the pallid bat in western North America have larger skulls and generally need gaps upward of 2 centimeters.
- Fruit bats and flying foxes: These are not crevice dwellers. They roost hanging from branches in the open and would not attempt to squeeze through tight gaps. Their skulls can be 4 or 5 centimeters wide.
Within a single species, juveniles and small females can fit through slightly narrower gaps than large adult males. But the difference is not dramatic, maybe a millimeter or two, because skull width does not vary as much within a species as you might expect.
Bats That Skip Crevices Entirely
Not all bats are crevice specialists. Some have evolved completely different roosting strategies that rely on adhesion rather than squeezing. Spix’s disk-winged bat (Thyroptera tricolor) in the Neotropics has suction cups on its wrists and ankles that let it cling to the smooth inner surfaces of unfurling leaves, such as rolled Heliconia or banana leaves.3Canadian Journal of Zoology. Sticking ability in Spix’s disk-winged bat, Thyroptera tricolor (Microchiroptera: Thyropteridae) Testing showed that these disks work primarily through suction and sometimes through wet adhesion. The bat does not need a crack at all; it glues itself to a temporary shelter that will unfurl in a day or two, then finds a new one.
Madagascar’s endemic sucker-footed bat (Myzopoda aurita) has a similar setup: specialized pads on its wrists and ankles that let it cling head-up to smooth leaves, reversing the head-down posture typical of most bats.4Biological Journal of the Linnean Society. How do sucker-footed bats hold on, and why do they roost head-up? These species are not closely related, which means adhesive roosting evolved independently at least twice. For these bats, the question of how small a crack they can fit through is beside the point; they have sidestepped the crevice-roosting lifestyle altogether.
Building Habitat Instead of Blocking It
The flip side of exclusion is conservation. As natural roosting habitat disappears through deforestation, building renovation, and changes in land use, wildlife managers increasingly turn to artificial roosts, commonly called bat boxes or bat houses, to provide alternatives. A review of artificial roost designs found that the most effective structures mimic the features of natural roosts, with an emphasis on the requirements of target species.5Wildlife Society Bulletin. Thinking outside the box: A review of artificial roosts for bats That includes getting the entrance gap right. For a bat box targeting small Myotis species, the crevice slot is typically 1.5 to 2 centimeters wide: snug enough to provide the tight-space security bats prefer, but not so tight that the bats struggle to enter. Wider slots tend to let in more airflow, cooler temperatures, and potential predators, all of which make the box less attractive.
Placement matters as well. Mounting a bat box high on a building or pole, in a spot that gets several hours of direct sun, mimics the thermal environment that maternity colonies seek in natural rock crevices. A box mounted in deep shade stays too cool for pup development in most temperate climates. Color also plays a role: darker-painted boxes absorb more solar heat, which benefits colonies at higher latitudes, while lighter boxes may be preferable in hotter southern climates to avoid overheating.
Getting these details right is not trivial. Many commercially available bat boxes are poorly designed, with chambers too shallow, entrances too wide, or landing areas too smooth for bats to grip. If you have just excluded bats from your building and want to offer them a nearby alternative, choosing or building a box with the right internal dimensions and slot width substantially increases the odds that bats will actually use it. A box that matches the tight-crevice preferences bats evolved for is far more likely to be occupied than a box designed for human convenience.
When Tight Spaces Become Traps
The same ability to squeeze into narrow gaps occasionally works against bats. Construction materials like corrugated metal roofing, PVC pipes, and chain-link fence posts can create spaces that a bat enters but cannot back out of. A bat crawling into a vertical pipe, for instance, may be unable to spread its wings to fly out and lack the traction to climb back up smooth walls. Wildlife rehabilitators regularly see bats that have become stuck in downspout pipes, wall cavities opened during remodeling, or gaps between window frames and screens.
Open-top pipes and vents are a recognized hazard. Capping exposed pipe ends with hardware cloth or commercial wildlife-exclusion caps is a simple fix. During building construction or renovation, temporarily covering openings at the end of each workday prevents bats (and birds) from entering unfinished wall cavities overnight. These are small measures, but they prevent situations where a bat’s extraordinary ability to fit through narrow spaces leads it into a space it cannot escape.
How Bats Find the Gap in the First Place
A gap barely wider than a centimeter is not easy to spot from the air, even for an animal with echolocation. Bats do not typically find new roost entries by sonar-scanning a building from a distance. Instead, they rely heavily on scent cues. A crevice that has been used by roosting bats accumulates oils from their fur, guano, and urine, creating a chemical signature that passing bats can detect. This is why a previously colonized building often gets recolonized within a season if the entry points are not sealed: the smell acts as a beacon.
Social information also plays a role. Bats returning to a roost at dawn produce audible social calls near the entrance, and other bats in the area can follow these calls to the site. Some species have been documented circling a building and testing multiple cracks with their noses before committing to an entry. The behavior is exploratory and deliberate, not a matter of accidentally stumbling into a hole. A bat investigating a gap will often land next to it, fold its wings, and push its head in first, testing whether its skull fits before committing the rest of its body. If the skull passes, the bat proceeds. If not, it moves on to the next candidate.
For homeowners, this means that simply patching one or two visible holes while leaving others open is unlikely to solve a bat entry problem. The bats know the building well and will relocate to the next available gap. A thorough inspection, ideally conducted at dusk while watching where bats emerge, is the most reliable way to identify all active entry points before sealing them.