Originally posted by markreed:https://en.wikipedia.org/wiki/Twickenham_Stadium
and
https://en.wikipedia.org/wiki/Wembley_Stadium
Though by the looks of it, figures vary.
There will be a lot of spare space at the back of GA's : the stadium has allocated a safe 'square metre' per standing person and therefore, if it's 1sqm per person, the stadium needs to have 40,000 square metre size for 40,000 people - even if everyone squeezes in the front.