The wiki says it's **some **amount based on max and current health, and will be lower if you're currently taking damage. But there's no exact number for it.
edit: the italics don't work?
EDIT2: to mystic and lemur: http://leagueoflegends.wikia.com/wiki/Health
Leveling up will restore some maximum health, and some current health. Actual health regained is lower depending on how wounded the champion is upon leveling up.
50000000000000003 voteshttp://leagueoflegends.wikia.com/wiki/List_of_champions'_health Look under "Growth (per level coefficient)"
AdeBug1 votesThank you very much. And is there some additional page for a formula for leveling while taking dmg?
50000000000000001 votesNo difference. It would look like you're gaining less health if you're taking damage, since you're both gaining and losing HP. Other than that, I don't think you gain any more or less HP when you level.
Reaper Soraka1 votesI think it's just the amount that your max health increases. idk if taking damage beforehand changes it or not.
LegendaryLemur1 votesWhich wiki page are you referring to? _And italics work fine i think. _
AdeBug1 votesI edited to have the wiki page and quote. As for italics, it was showing up with underscores after posting. random bug?
LegendaryLemur1 votesThe way it reads makes me think it's proportional. Like if you are at 58% health and then level up you'll still be at 58% of max, but i have no evidence of that.
Mysticman891 votesI don't think it cares if you're currently taking damage.
Everyone champion has a different health growth statistic that determines how much health they get when they level up, which goes into a formula which isn't linear so it's not as simple as champion X gets Y hp every level.
AdeBug1 votesI edited to have the wiki page and quote.
Moooose31 votesIt varies depending on the champion which is why it doesn't give an exact value.