{"id":25095465,"date":"2026-05-31T08:31:19","date_gmt":"2026-05-31T06:31:19","guid":{"rendered":"https:\/\/pestpatrol.be\/2026\/05\/31\/souris-et-trou-de-taille-stylo-tout-savoir-pour-bloquer-les-acces\/"},"modified":"2026-05-31T08:31:19","modified_gmt":"2026-05-31T06:31:19","slug":"mouse-and-pen-hole-all-you-need-to-know-to-block-accesses","status":"publish","type":"post","link":"https:\/\/pestpatrol.be\/en\/2026\/05\/31\/souris-et-trou-de-taille-stylo-tout-savoir-pour-bloquer-les-acces\/","title":{"rendered":"Mouse and pen-cut hole: Everything you need to know to block access"},"content":{"rendered":"<h1>Can a mouse fit through a hole the size of a pen?<\/h1>\n<h3 id=\"sommaire\">Contents<\/h3>\n<ul>\n<li>\n<p><a target=\"_blank\" rel=\"noopener noreferrer nofollow\" class=\"text-primary underline toc-link\" href=\"#anatomie-d-une-intrusion-comparaison-entre-la-taille-d-un-stylo-et-le-crane-des-souris\">Anatomy of an intrusion: comparing the size of a pen with the skull of a mouse<\/a><\/p>\n<\/li>\n<li>\n<p><a target=\"_blank\" rel=\"noopener noreferrer nofollow\" class=\"text-primary underline toc-link\" href=\"#criteres-de-choix-comment-differencier-un-trou-de-souris-d-un-trou-de-rat\">Selection criteria: how can you tell a mouse hole from a rat hole?<\/a><\/p>\n<\/li>\n<li>\n<p><a target=\"_blank\" rel=\"noopener noreferrer nofollow\" class=\"text-primary underline toc-link\" href=\"#recommandations-avant-achat-les-meilleurs-materiaux-pour-bloquer-les-acces-durablement\">Pre-purchase recommendations: the best materials for long-term access control<\/a><\/p>\n<\/li>\n<li>\n<p><a target=\"_blank\" rel=\"noopener noreferrer nofollow\" class=\"text-primary underline toc-link\" href=\"#conclusion\">Conclusion<\/a><\/p>\n<\/li>\n<\/ul>\n<p>Yes, and that's probably the most frustrating answer you'll read today. An adult mouse, the one that trots around your walls at night, can squeeze through a hole the size of a ballpoint pen. We're talking 6 to 7 millimeters in diameter. Not a pipe, not a gaping crack: a Bic pen. The one you have on your desk.<\/p>\n<h3 id=\"ce-qu-il-faut-retenir\">Things to remember<\/h3>\n<ul>\n<li>\n<p>Scientifically validate the pen adage by explaining rodent cranial anatomy, while providing a professional caulking guide<\/p>\n<\/li>\n<li>\n<p>The aim is to move from \u2018myth\u2019 to concrete action with 5mm protection standards.<\/p>\n<\/li>\n<li>\n<p>Comparing the size of a pen to the skulls of mice<\/p>\n<\/li>\n<li>\n<p>Compare the different options before deciding.<\/p>\n<\/li>\n<\/ul>\n<p>It's not an urban legend or something you tell yourself to scare people. It's pure biomechanics. The rodent skeleton is designed for it, and scientific studies have been confirming this for decades. The problem is that most people underestimate this ability. They look for big holes, obvious passages. Meanwhile, mice can get in through pipe joints, electrical ducts and gaps under doors.<\/p>\n<p>This article will explain why this is physically possible, how to distinguish a mouse passage from a rat passage, and above all which materials to use to seal these accesses once and for all. No need to panic, no unnecessary jargon: just what you need to know to take action.<\/p>\n<h2 id=\"anatomie-d-une-intrusion-comparaison-entre-la-taille-d-un-stylo-et-le-crane-des-souris\">Anatomy of an intrusion: comparing the size of a pen with the skull of a mouse<\/h2>\n<p>The skull of a house mouse (<em>Mus musculus<\/em>) is between 6 and 7 mm wide at its narrowest point. This is the limiting factor. The rest of the body follows, because a rodent's skeleton doesn't work like ours.<\/p>\n<p>In humans, bones are rigid and joints fixed. In mice, it's a different story. Their rib cage is compressible. Their ribs are connected by soft cartilage, allowing the thorax to flatten considerably. A study published in the <em>Journal of Experimental Biology<\/em> has shown that some rodents can reduce their body cross-section by up to 50 % to cross narrow passages. The rodent skeleton is literally made for infiltration.<\/p>\n<p>A standard ballpoint pen has a diameter of around 7 mm. When we say that a mouse can pass through a hole the size of a pen, we're not talking about an exceptional feat: it's their daily routine. They do it naturally, without any apparent effort, sometimes dozens of times a day to get food and return to their nest.<\/p>\n<p><img decoding=\"async\" class=\"max-w-full h-auto rounded-md\" src=\"https:\/\/nghaeknymynesecnqcmd.supabase.co\/storage\/v1\/object\/public\/article-images\/article-1780208122181-souris-et-trou-de-taille-stylo-tout-savoir-pour-bloquer-les-acces.png\" alt=\"Mouse and pen-cut hole: Everything you need to know to block access\"><\/p>\n<p>How big is the mouse hole? If you can slip your little finger in, a mouse can get through. This is the rule of thumb used by professional pest control operators in the field. A mouse's passage diameter starts at 6 mm for juveniles and stays under 10 mm for the strongest adults.<\/p>\n<p>What fools many people is the mouse's appearance. It looks round, fluffy, much bigger than it really is. Remove the fur, and you'll see a wiry animal, all length, with a narrow skull and a body that deforms at will. The slow-motion videos are quite impressive: the mouse approaches the hole, tests with its whiskers (which serve to measure the opening), then engages the head. If the head fits, the body follows. And always.<\/p>\n<p>That's why professionals reason in terms of cranial diameter, not body size. A mouse hole in a house isn't necessarily a round, visible hole. Sometimes it's a 6 mm crack under a skirting board, a loose seal around a heating pipe, or a gap between a door frame and the wall. Places we never look.<\/p>\n<p>The standard that pest control companies apply is simple: any passageway of 5 mm or more must be plugged. Not 10 mm, not 8 mm. Five millimeters. This is the safety margin that takes into account juvenile mice, which are even smaller than adults, and capable of squeezing into spaces you'd think impossible.<\/p>\n<h2 id=\"criteres-de-choix-comment-differencier-un-trou-de-souris-d-un-trou-de-rat\">Selection criteria: how can you tell a mouse hole from a rat hole?<\/h2>\n<p>Before you get out the putty and steel wool, you need to know what you're dealing with. Because a mouse hole and a rat hole aren't the same thing, and the solutions aren't the same either.<\/p>\n<p>Rodent identification starts with the size of the hole. A mouse needs 6 to 7 mm, as we've seen. A brown rat (<em>Rattus norvegicus<\/em>), that's another category altogether. The size of a rat hole is about 20 to 25 mm in diameter. The trick de-raters use: if a 2 euro coin (25.75 mm in diameter) fits through the hole, so can a rat. It's a simple, reliable visual marker, and you probably have one in your pocket.<\/p>\n<p>Beyond the size, look at the marks around the hole. Rats leave greasy marks (the sebum from their fur) on the edges of the passages they regularly use. These marks are brownish, a little shiny. Mice leave them too, but they're much less visible to the naked eye because the animal is lighter and smaller.<\/p>\n<p>Droppings are another reliable indicator of rodent identification. Mouse droppings are 3 to 6 mm long, shaped like grains of rice and pointed at the ends. Brown rat droppings are much larger, 15 to 20 mm, capsule-shaped with rounded ends. If you find droppings near a hole, the size will immediately tell you what you're dealing with.<\/p>\n<p>What about a 5 cm hole in your garden? It's a question that comes up a lot. A hole of this diameter in the ground, especially if surrounded by loose soil and droppings, is most likely a rat burrow. Brown rats dig gallery systems with several entrances and exits. Mice, on the other hand, prefer to exploit existing structures: they settle into walls, false ceilings and insulation. They rarely dig into the ground outside.<\/p>\n<p>An important point that many people forget: you can have both. Mice and rats don't usually cohabit (rats eat mice, which settles the issue), but in a block of flats or a house with a garden, it's quite possible to have rats in the basement or drains and mice in the attic. Two distinct problems, two different approaches.<\/p>\n<p>To sum up identification: take a pen and a 2 euro coin. Pen goes through the hole? Smile. Coin goes through? Rat. Neither goes through? It's probably not a rodent hole, or it's too recent to have been enlarged. But keep an eye on it, because rodents enlarge existing holes by gnawing the edges, including on wood, plaster and some plastics.<\/p>\n<h2 id=\"recommandations-avant-achat-les-meilleurs-materiaux-pour-bloquer-les-acces-durablement\">Pre-purchase recommendations: the best materials for long-term access control<\/h2>\n<p>Now that we know what we're looking for, let's talk about what really works to seal off mouse access. Because not everything is equal, far from it. And some \u00abhome remedies\u00bb are a waste of time, pure and simple.<\/p>\n<p>Expanding foam alone? Forget it. A mouse will get through it in a matter of hours. They'll gnaw through polyurethane like you'd bite through butter. I've seen cases where the foam was laid the night before and punctured the next morning. It's not a protective material against rodents in the home, it's a thermal insulator. Point.<\/p>\n<p>The material of choice for professionals is steel wool. Anti-mouse steel wool works because the metal strands injure the rodent's gums when it tries to gnaw. They give up. Stainless steel wool is preferred, because standard steel wool will rust in a few months in damp environments (under sinks, around drains, outdoors). The Xcluder brand, for example, offers a specially designed, corrosion-resistant, laboratory-tested copper mesh.<\/p>\n<p>The pro technique is to combine steel wool with a filler or sealant. You pack the steel wool into the opening, then cover with a resistant filler. The steel wool prevents rodents from breaking through, while the filler prevents the wool from dislodging and ensures a watertight seal. For larger holes, add a metal plate or rodent-proof mesh underneath.<\/p>\n<p>Let's talk about rodent-proof mesh. This is the solution for air vents, window wells and service duct entrances. Use mesh with a maximum mesh size of 5 mm (not 10 mm, not 6 mm: 5 mm). Above 5 mm, a young mouse will get through. The ideal material is galvanized steel or stainless steel. Aluminum is too soft for a rat to penetrate. Fix the mesh with screws and dowels, not with glue or silicone. Rodents will peel off silicone in a matter of days.<\/p>\n<p>Wire-bristle door sweepers are ideal for sealing mouse entry points under doors. Standard nylon brushes don't last long. There are models specifically designed to protect against rodents, with stainless steel filaments integrated into an aluminum profile. They cost between \u20ac20 and \u20ac40 per door, but are long-lasting.<\/p>\n<p>Where should you focus your attention? Here are the most frequent points of entry into a dwelling:<\/p>\n<ul>\n<li>\n<p>Passage of pipes (water, gas, heating) through walls and floors<\/p>\n<\/li>\n<li>\n<p>Electrical and telecommunications ducts<\/p>\n<\/li>\n<li>\n<p>Joints between exterior door frames and masonry<\/p>\n<\/li>\n<li>\n<p>Window wells and air vents in crawl spaces or cellars<\/p>\n<\/li>\n<li>\n<p>Gaps around drainpipes under sinks and washbasins<\/p>\n<\/li>\n<li>\n<p>Cracks in foundations, no matter how small<\/p>\n<\/li>\n<li>\n<p>Roof\/wall junctions (for climbing mice, and they all climb)<\/p>\n<\/li>\n<\/ul>\n<p>Walk around your dwelling with a flashlight, preferably in the evening when the outside light allows you to spot daylight passages through the walls. If you see light filtering through, a mouse may be passing through. Methodically inspect every room, from floor to ceiling. Mice climb on rough vertical surfaces (plaster, brick, wood) without any difficulty.<\/p>\n<p>A final word of advice: don't caulk holes if you suspect mice are already inside. You'll trap them in your walls, and they'll dig new passages (or die inside, which poses other problems you don't want to know about). Eliminate the population first with mechanical traps, then seal all accesses. It's in that order, never the other way around.<\/p>\n<h2 id=\"conclusion\">Conclusion<\/h2>\n<p>A mouse that fits through a hole the size of a pen is no myth. It's a documented anatomical fact, and it's the reason why so many people can't get rid of it: they look for the big holes and ignore the small ones. The professional standard is clear: any passage of 5 mm or more must be filled.<\/p>\n<p>Grab a pen and a 2 euro coin, and take a tour of your home this weekend. Every crack, every joint, every pipe run. Stainless steel wire mesh in holes, 5 mm mesh over vents, wire brushes under doors. It's neither complicated nor expensive, but you have to be methodical and leave no stone unturned.<\/p>\n<p>If the scale of the problem overwhelms you, or if you notice signs of active infestation (fresh droppings, night-time noises, traces of grease), call in a professional. Caulking alone isn't enough when mice are already established in your home.<\/p>\n<h2 id=\"questions-frequentes\">Frequently asked questions<\/h2>\n<details class=\"faq-item\">\n<summary>Can a mouse really fit through a hole the size of a pen?<\/summary>\n<p>Yes, an adult mouse can squeeze through a hole 6 to 7 millimeters in diameter - exactly the size of a standard ballpoint pen. This is made possible by their very narrow skulls and highly compressible ribcages. If the animal's head manages to clear the obstacle, the rest of its body will systematically follow.<\/p>\n<\/details>\n<details class=\"faq-item\">\n<summary>What's the minimum hole size for a mouse?<\/summary>\n<p>Pest control professionals apply a strict safety standard: any gap of 5 mm or more must be filled. This minimum size takes into account juvenile mice, which are even smaller and more agile than adults. For quick detection in the field, remember that if you can slip your little finger through, a mouse can.<\/p>\n<\/details>\n<details class=\"faq-item\">\n<summary>How can you tell a mouse hole from a rat hole?<\/summary>\n<p>The major difference lies in the diameter of the opening: a mouse hole measures less than 10 mm, while a brown rat hole reaches 20 to 25 mm. A simple trick is to use a 2 euro coin; if it passes through the hole, you've got a rat problem. What's more, rats leave strong brownish grease marks (sebum) and much larger droppings nearby.<\/p>\n<\/details>\n<details class=\"faq-item\">\n<summary>Why is expanding foam ineffective against mice?<\/summary>\n<p>Expanding polyurethane foam is not at all a rodent-proof barrier, as its texture is too soft. A mouse or rat can gnaw through it in just a few hours to reopen its passageway. It should only be used as a complement, to cover a protective and abrasive material previously installed.<\/p>\n<\/details>\n<details class=\"faq-item\">\n<summary>What are the best materials for permanently plugging a mouse hole?<\/summary>\n<p>The material of choice is steel wool (or stainless steel wool), as its metal strands injure the gums of rodents that try to gnaw on it. For air vents and window wells, we recommend galvanized steel mesh with a maximum mesh size of 5 mm. Finally, door bottoms should be fitted with special brushes with stainless steel filaments.<\/p>\n<\/details>","protected":false},"excerpt":{"rendered":"<p>Can a mouse fit through a hole the size of a pen? ContentsAnatomy of an intrusion: comparing the size of a pen with the size of a mouse's skullCriteria for choice: how to differentiate...<\/p>","protected":false},"author":2,"featured_media":25095464,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_et_pb_use_builder":"","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"categories":[1],"tags":[],"dipi_cpt_category":[],"class_list":["post-25095465","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-non-classe"],"_links":{"self":[{"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/posts\/25095465","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/comments?post=25095465"}],"version-history":[{"count":0,"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/posts\/25095465\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/media\/25095464"}],"wp:attachment":[{"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/media?parent=25095465"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/categories?post=25095465"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/tags?post=25095465"},{"taxonomy":"dipi_cpt_category","embeddable":true,"href":"https:\/\/pestpatrol.be\/en\/wp-json\/wp\/v2\/dipi_cpt_category?post=25095465"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}