Unless you've been hiding in a cave for the past month, you know there was a big wedding in London earlier today.
Mixed in with the accounts of the wedding, you're likely to see estimates that 1/3 of the Earth's population, or 2 billion people, will have watched it on TV.
Fun numbers to throw around, but they're all just wild, unauditable guesses. No one knows how many will watch, and no one will know after the fact how many did watch worldwide. There are just too many gaps in the worldwide TV measurement system, and there's no financial incentive to bridge them (because TV audience measurement is done to price TV advertising, and no advertising is sold on a worldwide basis).
The last worldwide event I remember the worldwide audience guesses for was the 2010 World Cup Final, which was estimated to draw 700 million viewers. Nobody knows how many watched that match either.
I'm sure we'll get ratings for the US TV networks sometime on Saturday, and we'll certainly post them to the site. We're also almost certain to see media accounts of the viewership in the UK and a handful of other countries too, we'll post those as well.
Until then, enjoy the wedding you probably recorded on your DVRs!