When calculating Base Stats, how is HP determined?

Question

Is it just double the HP plus IVs, or is there more to it?

When I max out HP EVs on Chansey (for example) is there a percentage increase like good nature EV or does it just go up normal (ie 4EV per stat)?

Comments

Be careful with terminology. "Base stats" aren't the numbers you see on your Pokemon's summary screen (which I assume are what the question is actually about). Base stats are the static numbers each species has for each stat, which contribute a good chunk of a Pokemon's "real" stat points, per the formula in the answer below.

Answer 1

Well, according to this, it's not just hp and ivs. It also takes into account the level. Also, unlike others, the hp cannot be increased not deceased by any nature at all

Answer 2

The formula for HP is similar to all the other stats, except you add an extra Level + 5, and there's no possibility of getting a positive or negative effect from nature.