How Many Rats Are Really in the World?

Nobody has ever successfully counted the world’s rat population, and the commonly repeated claim that there is roughly one rat for every person on Earth is more folklore than science. That figure traces back to a rough estimate about England published over a century ago, and it has been recycled and scaled up without much scrutiny ever since. The honest answer is that no one knows the global total, but the evidence we do have points in a troubling direction: rat numbers in most major cities are climbing, driven by warming temperatures, expanding urbanization, and an endless supply of human garbage.

Where the “One Rat Per Person” Number Came From

The idea that there is one rat for every human traces to a 1909 book by W.R. Boelter called “The Rat Problem,” which estimated the rat population of England at about 40 million, roughly matching the country’s human population at the time. Boelter based this on surveys of rural farms, not on any kind of systematic census. Over the decades, the one-to-one ratio was repeated so often that it became treated as a rule of nature, applied to entire countries and eventually the whole planet. If you accept the ratio at face value and multiply by the current world population, you get somewhere around eight billion rats. That is the number that shows up in casual internet searches, and it is almost certainly wrong.

The problem is not just that Boelter’s estimate was imprecise. It is that the ratio between rats and people is not stable or universal. In some dense urban neighborhoods, rat populations likely exceed the human population by a wide margin. In other areas, rats barely exist at all. A single ratio applied globally smooths over these enormous differences and gives a number that sounds authoritative but rests on almost nothing.

Why a Real Count Is Practically Impossible

Rats are nocturnal, spend much of their lives underground or inside walls, and actively avoid human contact. These traits make them spectacularly difficult to count. The methods scientists use to estimate local populations are all indirect. They include mark-recapture trapping, tracking plates that record footprints, bait consumption stations, and analysis of public complaint data from city agencies. Each method has serious limitations, and none can be reliably scaled to a continent, let alone the whole world.

A 2025 study published in Science Advances used public complaint and inspection data from 16 cities around the world to estimate rat population trends. The researchers relied on things like calls to pest-control hotlines and the results of building inspections, which can tell you whether rat activity is rising or falling over time but cannot tell you how many rats are actually there.¹ In Singapore, researchers studied waste collection premises and found that active burrows were associated with higher odds of rodent sightings, but even this approach only measures relative activity, not absolute numbers.²

The fundamental challenge is that you cannot extrapolate a citywide count from a few blocks of trapping data. Rat density varies enormously even within a single neighborhood, depending on building age, sanitation, sewer access, and the proximity of food sources. Multiplying a local density estimate by a city’s area produces a number, but not a trustworthy one. And multiplying a city’s estimate across all cities on the planet produces something closer to fiction.

What We Actually Know About Trends

If the global total remains unknowable, the direction of change is clearer. The Science Advances study found that eleven of the sixteen cities analyzed showed significant increases in rat numbers, including Washington D.C., New York, and Amsterdam. Just three cities experienced declines.¹ This matches what pest-control professionals and city agencies have been reporting anecdotally for years: the problem is getting worse in most places.

Three interlocking factors appear to be driving the increase. Cities experiencing greater temperature increases over time saw larger increases in rats, consistent with the idea that milder winters extend the breeding season and improve winter survival. Cities with denser human populations and more urbanization also had bigger rat increases.¹ Warming winters may extend breeding windows and increase food availability, but these effects also interact with urban architecture, waste management systems, and overall human activity patterns.³

The Garbage Connection

If you want to predict where rats thrive, follow the trash. The single most powerful driver of urban rat populations is the availability of human food waste. Rats do not need exotic habitats or specialized diets. They need calories, water, and shelter, all of which a modern city provides in abundance through overflowing bins, restaurant dumpsters, and poorly sealed buildings.

A compelling natural experiment occurred during COVID-19 lockdowns. A study in a peri-urban environment tracked rat activity before and after the removal of human refuse and found a highly significant decrease in rat activity after waste was cleaned up. When researchers compared pre- and post-lockdown conditions, the decline in invasive rat activity was striking, while native small mammal activity stayed the same. The implication is that rats were using human food waste to sustain population levels that the natural environment could not support on its own.⁴

This has practical consequences. Cities that invest in better waste containment, more frequent garbage collection, and rat-proof bins tend to see reductions in rat complaints. Cities where sanitation infrastructure has not kept up with population growth tend to see the opposite. The rat population is, in a real sense, a reflection of how well a city manages its waste stream.

Density Extremes on Tropical Islands

Some of the highest rat densities ever recorded are not in cities at all, but on tropical islands where invasive rats arrived centuries ago and found abundant food with few predators. Pacific rats can exceed population densities of well over 100 individuals per hectare, and black rats have been recorded at 119 per hectare on tropical islands, far higher than densities on most temperate islands.⁵ To put that in perspective, 119 rats per hectare translates to roughly one rat for every 84 square meters. On a small island of a few hundred hectares, the total rat population can number in the tens of thousands.

These island populations are ecologically devastating. Rats prey on the eggs and chicks of ground-nesting seabirds, eat seeds and fruit that native plants depend on for reproduction, and compete with native wildlife for food. Because many island species evolved without mammalian predators, they have no defensive behaviors against rats, making them easy targets. Island rat eradications using aerial brodifacoum drops have been among the most dramatic conservation successes of recent decades, with seabird populations rebounding rapidly once rats are removed. But these operations work on islands specifically because they are bounded by water. Attempting the same thing on a continental scale is a different problem entirely.

The Eradication Problem

New Zealand has set perhaps the most ambitious rat-control goal on record: making the entire country predator-free by 2050. The effort targets rats, stoats, and possums, all of which devastate native bird populations. Researchers have pointed out that achieving this goal is not a simple scaling-up of current island eradication methods, but requires sustained coordination of research, management, and societal elements, along with solutions to four key challenges: understanding ecological interactions better, developing new operational tools, overcoming social and ethical opposition, and building effective governance partnerships with Māori communities.⁶

Making control efforts harder is the growing problem of rodenticide resistance. In the Netherlands, a nationwide survey of over 1,400 brown rats found that about 15% carried a genetic mutation conferring resistance to anticoagulant rodenticides, the most common type of rat poison used worldwide. Among house mice, the resistance rate was even higher, at 38%.⁷ An earlier Dutch study of targeted samples found resistance-linked genotypes in over half of the rat tail samples tested.⁸ And the issue is not limited to Europe: in Australia, a genetic survey of black rats found that mutations suggesting potential anticoagulant resistance were widespread in urbanized areas.⁹

The evolutionary logic is straightforward. When you expose a rat population to a poison for years, the individuals with genetic resistance survive and reproduce more successfully. Over time, the resistant gene becomes more common. The result is that the primary chemical weapon in the global arsenal against rats is gradually losing its edge, with no clear replacement ready.

Rats and Human Disease

The reason rat numbers matter to public health goes well beyond property damage. Rats carry a long list of pathogens transmissible to humans, and leptospirosis is among the most important globally. Research in Malaysian Borneo found that the Leptospira bacteria carried by rats were more common at sites with higher human influence, particularly areas combining commercial and residential activity.¹⁰ In densely populated areas of France, a study of urban brown rats found that leptospirosis prevalence jumped sharply once rats reached a certain body weight, going from around 20 to 30% in younger rats to over 75% in heavier adults. The bacteria spread primarily through horizontal transmission between rats rather than being passed from mother to offspring.¹¹

This means that as rat populations grow denser, the proportion of infected individuals increases, and the risk to people who come into contact with contaminated water, soil, or surfaces rises accordingly. Cities in tropical and subtropical climates, where both rat populations and flooding events tend to be high, face the greatest burden. Leptospirosis is estimated to cause over a million cases and tens of thousands of deaths annually worldwide, though exact figures are uncertain because many cases go undiagnosed.

The Economic Toll

Beyond disease, invasive rodents impose massive economic costs. A global analysis of documented economic impacts found that the vast majority of costs, about 87%, were damage-related rather than management-related, meaning we spend far more dealing with the destruction rats cause than we spend trying to control them. Agriculture bore the heaviest burden, and the costs fell disproportionately on Asia, which accounted for roughly 60% of global documented costs, followed by Europe at 19% and North America at 9%.¹² The authors noted that these figures represent the tip of the iceberg, since most rat damage goes unreported and many countries lack systematic tracking.

In agricultural settings, the damage can be severe. In the Ethiopian highlands, researchers documented a clear positive correlation between rodent abundance and crop damage, with fields surrounded by dense stone walls harboring more rodents and suffering significantly greater crop losses. Damage peaked during the reproductive stage of crops and around harvest time, when fields offer the most food.¹³ In regions where subsistence farming is the norm, the difference between a normal harvest and one decimated by rats can be the difference between eating and not eating.

How Rats Conquered the Planet

The reason rats are virtually everywhere humans live is that they followed us there. The two most globally important species, the brown rat and the black rat, both originated in Asia and spread outward along human trade and migration routes. The brown rat expanded westward from central Asia into Europe, from which it rapidly colonized Africa, the Americas, and Australasia. Genomic analysis has identified at least thirteen distinct evolutionary clusters arising from five major expansion routes.¹⁴

The black rat took a different path. Ancient DNA research suggests it dispersed overland from South Asia through Iran and the Persian Gulf region to the Mediterranean and then Europe, rather than via an Indian Ocean sea route. A secondary dispersal route apparently ran through Egypt, with evidence appearing at a 6th-century site in what is now Serbia.¹⁵ Molecular clock estimates suggest the major lineages of the black rat complex diverged during the Pleistocene, roughly a million years ago, with diversification within lineages beginning around 200,000 years ago during a warm interglacial period.¹⁶

Both species owe their global success to the same set of traits: dietary flexibility, rapid reproduction, tolerance of cramped spaces, and an uncanny ability to exploit human infrastructure. Sewers, ships, grain stores, and garbage have served as stepping stones for thousands of years. Wherever humans build permanent settlements, rats eventually show up, and once established, they are extraordinarily hard to remove.

Twenty Thousand Temple Rats

Not every human community treats rats as pests. At the Karni Mata Temple in Deshnoke, Rajasthan, roughly 20,000 rats live under active protection and care. To the Charan community that administers the temple, these animals are not rats at all but kaba, the reincarnated descendants of their lineage and kin of the goddess Karni Mata. The rats are fed, sheltered, and revered by pilgrims, making the temple one of the few places on Earth where a massive rat population exists entirely by human intention.¹⁷

The temple is a useful reminder that the relationship between humans and rats is not universally adversarial. Across most of the world, rats are defined by the damage they cause and the diseases they spread. But the boundary between “pest” and “protected animal” is cultural, not biological. What makes the Karni Mata rats different from the ones in a New York subway is not the species, it is the context. The rats at Deshnoke are the same black rats that devastate island ecosystems and contaminate grain stores elsewhere. Whether a given rat population is a conservation crisis, a public health hazard, or a religious community depends entirely on where and how it lives alongside people.