From the Leaky Cauldron interview:

Well, Hogwarts. All right. Here is the thing with Hogwarts. Way before I finished “Philosopher's Stone,” when I was just amassing stuff for seven years, between having the idea and publishing the book, I sat down and I created 40 kids who enter Harry's year. I'm delighted I did it, [because] it was so useful. I got 40 pretty fleshed out characters. I never have to stop and invent someone. I know who’s in the year, I know who's in which house, I know what their parentage is, and I have a few personal details on all of them. So there were 40. I never consciously thought, “That's it, that' s all the people in his year,” but that's kind of how it's worked out. Then I've been asked a few times how many people and because numbers are not my strong point, one part of my brain knew 40, and another part of my brain said, “Oh, about 600 sounds right.” Then people started working it out and saying, "Where are the other kids sleeping?" [Laughter.] We have a little bit of a dilemma there. I mean, obviously magic is very rare. I wouldn't want to say a precise ratio. But if you assume that all of the wizarding children are being sent to Hogwarts, then that's very few wizard-to-Muggle population, isn’t it? There will be the odd kid whose parents don't want them to go to Hogwarts, but 600 out of the whole of Britain is tiny.

Let's say three thousand [in Britain], actually, thinking about it, and then think of all the magical creatures, some of which appear human. So then you've got things like hags, trolls, ogres and so on, so that's really bumping up your numbers. And then you've got the world of sad people like Filch and Figg who are kind of part of the world but are hangers on. That's going to bump you up a bit as well, so it's a more sizable, total magical community that needs hiding, concealing, but don't hold me to these figures, because that's not how I think.

But those numbers don't make any sense (I think that JKR was making them up as she was talking). If you have around 600 kids in school at any one time, but only have 3000 active wizards in all of Britain, that means that there are only 5 times as many wizards as there are Hogwarts students! There were more than that just at the Quidditch World Cup championship, and I'm sure that only a small fraction of the British wizards attended.

There were more than that just at the Quidditch World Cup championship, and I'm sure that only a small fraction of the British wizards attended.

She's self-admittedly not a good source. How many people do you figure were at the World Cup? How many of them do you think were international?

There are 1,100 people on the two floors of the building I work in. Three times that is one hell of a tiny community.

I honestly have no idea. Most of my knowledge on older Jewish customs comes from Fiddler on the Roof.

Yeah, mine too.

And sort of on topic, my wife sent me a link to the Myers-Briggs test here: [link] which eventually led me to this site: [link] which suggests a relationship between personality types and the Houses of Hogwarts. And apparently, I'm a Ravenclaw.

Plus, 40 kids per year times 7 years = 280 kids not 600.

Well, if there were 40 in Harry's year, and about the same in the rest of the years, that makes 280 kids. Which makes, let's assume, 560 parents. Muggle-borns are very rare so lets say that out of the 560 parents, only 50 are Muggle couples. But pure blood wizards are getting rare, so let's say that out of the remaining 510 parents, 300 of them are Wizards/Witches.

THAT'S TEENY TINY!

And I have waaaaaaaaaay too much time on my hands.

My impression was she fleshed out 40 kids from Harry's year, not that there are only 40 kids in each year. That would make 10 kids per house per year, and that number just seems small to me.

Aaaaaaaaaaaaaah!

Plus, 40 kids per year times 7 years = 280 kids not 600.

She cops to that error in the quote. Also it wouldn't multiply (probably) by 7 -- there'll be some attrition after fifth form, and not really many other places to get students.

So I'm thinking her 3000 population is from the 600 school roster.

That would make 10 kids per house per year, and that number just seems small to me.

But it can't be that much bigger than that -- it seems like a lot of their classes are all the kids in a certain year from two houses together, and these seem to be close to regular classroom sizes (I'd say no more than 40 or so kids, total, so 20 per house per year) rather than lecture halls.