Trafalgar Square is large enough to contain a crowd of 35,190 people. That's according to CrowdSize, a simple but time-sucking app that lets you calculate the capacity of any space on Earth. By coincidence, this is almost precisely the same as the stadium capacity at the Boleyn Ground, West Ham's recently vacated home.
The app is really designed for event organisers, but it's also a boon for anyone seeking to waste an afternoon making dumb comparisons. So, here goes...
The Queen could invite up to 49,000 people round to Buckingham Palace. That's if it's raining and everyone has to stay indoors. If she opened her back garden, this grows to 541,000. She could fit in everyone from the surrounding City of Westminster, each with two guests.
Greenwich Park might seem like one of the smaller parks, but you could ram about 1.9 million people into the space. Everyone from Birmingham, Liverpool and Leeds could be corralled within its walls, should you wish to conduct an interesting, if deeply immoral, social experiment.
Across the water, the Isle of Dogs can squeeze in 9 million people — everyone in London with room to spare. And that's before we start populating those fancy apartment blocks and commercial towers. The Square Mile would hold a similar number.
Remember how Boris Johnson wanted to close Heathrow Airport and build a new strip in the Thames Estuary? If that ever happened, it would free up enough land to hold a crowd of 30.5 million. We could fit everyone from North Korea in Heathrow Airport, with room to spare.
The app is a bit too limited to get an accurate figure for the entire city. However, we can do a back-of-the envelope calculation to work it out. We know that the Square Mile is actually 1.12 square miles. We now know it can hold about 9 million people, if everyone stands cheek-by-jowl. According to Wikipedia, Greater London is 606 square miles. If you do the maths, that means we could fit about 5 billion people into Greater London.
In other words, two-thirds of the entire world population could be squeezed into our city. And you think the underground is crowded now.
CrowdSize is available for iPhone, £2.49.