PA Images

On 23 March, vast numbers of people opposed to Brexit, and in favour of returning the choice of what to do next to a second referendum, descended on London to protest for their voice to be heard.

They came from all corners of the country, some leaving in the early hours and not returning until long after the sun had set.


Organisers claimed a million people marched along the streets of London; one Guardian contributor even suggested that two million lined up to protest – a figure that was quickly shot down by others. But is there a sound way to calculate how many people actually protested on Saturday?

When it comes to counting numbers at protests and marches, “there is no perfect solution” but there are few common approaches, says Chris Green of Etherlive, a company providing event technology services, which also monitors crowds.

Read next Cummings doesn’t need a no deal Brexit to create his British Google Cummings doesn’t need a no deal Brexit to create his British Google

One way is “heat-mapping” using mobile phone or Wi-Fi signals. “This approach detects the presence and strength of the radio signals sent out from mobile devices and creates a ‘heat map’,” says Green. This can be done using software such as Ruckus SPoT – which requires access to Wi-Fi access points – or independent Wi-Fi “sniffers” able to tell the number of devices that have Wi-Fi enabled in a certain locale.

From that, one can glean a rough idea of numbers in the area. However, this system works best in controlled environments – such as arenas or stadiums – and not on the streets, where people in nearby shops could also be signed into wifi or using 4G. Another complication is that the sheer number of people present on Saturday overloaded mobile phone networks, making it impossible to use.


The most in-depth method is the RAMP analysis, which monitors ingress and egress routes to an area, the area itself, the movement of the mass of people, and the profile of the participants (for instance, their age helps establish how fast they can walk). “With this information we have a clear picture of how many people can enter the space, how long it will take to fill up the space and so on,” says Eric Kant, a crowd management specialist at Dutch company Phase01.

The total route area for the anti-Brexit march Andrew Seeds

This method also analyses images and video collected after the event. Researchers at Manchester Metropolitan University have overlaid helicopter footage of the march winding its way through the streets onto Google Maps, in order to work out the route the anti-Brexit protesters took, and the amount of space they took up – based on the width of the road along that route. The total area of the space where people were standing was around 156,250 square metres.

Read next Brexit plus coronavirus could spell disaster for Britain's universities Brexit plus coronavirus could spell disaster for Britain's universities

Crowd analysis experts estimate that the average crowd has between two and four people standing per square metre – any more than that and it becomes too uncomfortable to move. That gives a low estimate of 312,000 people and a high estimate of 624,000 people. Crowds, however, don’t congregate evenly, and the helicopter footage showed certain pockets of high density and others where there were only a smattering of people.


“Based on the visuals from the helicopter image, it’s between 312,000 and 400,000 people,” explains Manchester Metropolitan University’s Keith Still, a world leader in crowd science who has developed mandatory event-monitoring training for police. (He also debunked Donald Trump’s claims about the numbers attending his inauguration in 2017.) “Avoiding the political overtones – Trump, the Eagles, Brexit – and inevitable arguments, it’s not surprising that there are few people who would dare to quote numbers. However, the numbers don’t lie.”

A computer model of what two people per square metre looks like (top) versus four people per square metre (bottom) Keith Still

For decades there has been chronic overestimating of crowd sizes. “This is not an exact science, and people have a propensity to overestimate,” says Alan Wilson, a freelance crowd safety specialist. “Ask me how long I was on-hold on the phone the other day. I'd probably say five to ten minutes. My phone log would say one to two minutes.”

“We have an inaccurate scale for all events, so the words can be taken out of context,” Still says. As journalists have pointed out semi-jokingly, they’d often just pick a number based on their best guess and then double it, before asking the police to affirm it. And police estimates – which have not been made for the People’s Vote march – are often significantly lower than those released by organisers.

Read next The weird psychological effect of a no-deal Brexit deadline The weird psychological effect of a no-deal Brexit deadline

On the same day as hundreds of thousands of people took to the streets of London, Nigel Farage appeared at the latest stop of his pro-Brexit March to Leave in Nottinghamshire. Organisers estimated 200 people attended.

Updated June 4, 2019 09:10 BST: The headline of this article has been updated to reflect not all the marchers on the protest were counted. The statistical methods used produce estimates.

More great stories from WIRED

– The games industry should be worried about Google Stadia

– How the petition to revoke Article 50 went viral


– I tried to keep my baby secret from Facebook and Google

– Care about online privacy? Then change your phone number