Are you never more than 6ft away from a rat?

The old adage has it that we're never more than six feet away from a rat - but how was this worked out, and is it true?

It's a saying that seems almost deliberately contrived to get a reaction, but isn't exactly clear where the adage comes from.

It may derive from the former Ministry of Agriculture, suggests rodent expert Prof Rob Smith from the University of Huddersfield. They circulated many public health announcements to promote hygiene in homes.

It is also associated with another commonly quoted statistic, that the population of rats outnumbers the population of humans.

"This statistic is ritually used in news stories about rats and has been for more than 100 years," says Robert Sullivan author of Rats: A Year with New York's Most Unwanted Inhabitants.

The one rat per person rule seems to come from a 1909 book entitled The Rat Problem by WR Boelter. In it he conducted a survey where he asked people in the English countryside whether it was reasonable to say that there was one rat per acre of land.

"In the end he made an educated guess," says Sullivan. "There were 40 million cultivated acres of land in England at the time so he concluded there were 40 million rats. It just happened that the population of the UK in 1909 was also about 40 million."

So just how many rats are there in the UK?

Dr Dave Cowan, leader of the wildlife programme at the Food and Environment Research Agency, has analysed previous studies to try to estimate a total population.

"Rats are almost completely commensal in Britain, [meaning] they're associated with human activities. That gives us a start because we only really need to think about how many rats there might be living in close proximity to ourselves."

"It's pretty rare that rats are inside our dwellings. Less than half a per cent of dwellings have rats," he says.

In those cases, "it would be just a couple of rats".

A better figure to take is the number of dwellings that have rats outside, in the garden or driveway or compost heap and so on, Cowan suggests. "Around 3% of our dwellings have rats present outside. We can come up with a figure of 1.5 million rats in total in Britain in or around our homes."

The next place they are likely to live is in commercial premises like warehouses, factories and shops.

"There are about 1.8 million commercial premises in Britain. A survey suggested around 5% of those commercial premises had rats present. Again, each presence was a relatively small number of animals. The total estimate for the number of rats in and around commercial premises is around 200,000."

Then we have the place most of us associate with rats - the sewers.

"This is a bit tricky to estimate," says Cowan. "There is an estimate of 16,000 square kilometres of sewers in Britain. And around 5% of the sewers have rats present.

"In the 1950s people were allowed to go down and count rats in the sewers. From that work, we had an estimate of around 2,000 rats per square kilometre of sewer".

If you multiply those two figures you get a figure of around 1.6 million rats in sewers, and a total of 3.3 million rats in urban environments.

"That's the streetwise, urban rats - between 3 and 3.5 million in total," says Cowan.

But then there's also rats in rural habitats. "That is mainly reflecting rats in and around agricultural buildings, on farms, where they have got lots of food and shelter," says Cowan.

"There are around 200,000 agricultural premises in Britain. Various surveys suggest that around 40% of those premises will have rats present. We actually have pretty good census data for the average numbers of rats on each of those farms where rats are present."

The figure works out to be about 90 rats per farm. Do the multiplication and you get an estimate of about seven million rural rats. This would give a total estimate of 10.5 million rats in the UK.

Given the human population is around 60 million, we actually outnumber rats by six to one.

The maths for working out the average distance to a rat is a bit rough and ready because rodents are not evenly spread.

It makes sense to discount all the rats on farms, because the phrase is most often used about cities. That leaves us with about 3.1 million urban rats.

Urban areas in the UK cover around 16,000 square kilometres. If we distribute the rats evenly across the urban areas, which is clearly unlikely but necessary for the calculation, each rat has a rather spacious 5,000 square metres to roam around in.

Assuming you're standing at a given spot in an urban area you would be at most 164ft (50m) away.

Saying you're always 164ft away from a rat doesn't have quite as much of a fear factor as 6ft away, but it's much more of a realistic estimate.

You can follow the Magazine on Twitter and on Facebook