Do runes round up?

Question

I'm looking into my rune pages now that I have hit level 30, and there are a lot of decimal values for the runes, I want to know if those values round up or down while in game.

For example, for my AP page, I currently have 9 Greater Marks of Magic Penetration, giving me +0.87 Magic Pen each. This totals 7.8 Magic Pen (as shown to the right of the rune page)

In game, does the 7.8 round to 8? Does it round to 7? Or is there some wacky calculation for 0.8 Magic Pen?

If it doesn't round it seems like it would make sense to remove one of the runes, have 7 Magic Pen and have a single mark giving me something else, yet all the recommended builds I've found online always show having 9 of the same rune category.

Answer 1

All calculation use decimal points when calculated, even if not displayed in game

So you actually have 7.8 MrPen when you apply magic damage: not 7, not 8, not 7.5

This is confirmed in a red post here: http://euw.leagueoflegends.com/board/showthread.php?t=687992

Answer 2

"There is no rounding" said a Riot Software Software Engineer.

Answer 3

No, they do not round.

The numbers are pretty wacky and don't make sense for almost all of the runes. They seem to be trying to fix this though as they just adjusted cool-down Glyphs recently (patch 3.04) to give a full 10% instead of 8.8%.

Answer 4

It does not round, and will treat the opponent as if he had 7.8 fewer magic resist (min 0).

This is because the magic resistance calculation is as follows %Damage Resisted= MR/(MR+100), which yields decimal answers regardless. Most abilities have ratios that are not multiples of whole numbers as well.

As far as I know, the only pure integer stats in the game are health, and I'm not certain how the rounding for health is done. (e.g. if your attack were to do 131.6 damage after armour/mr)