What is Stealth Rock's effectiveness based on?

Question

How does the game calculate Stealth Rock's effectiveness?
Like how most Pokemon only lose a few percentage in health, while some can lose up to half!

Most Fying types lose 50% of their health from Stealth Rock, but is it because of their Rock weakness?
What about other types that have Rock as a weakness?

Answer 1

Stealth rock does 1/8 to someone that its neutral effective to lets say someone has a 4x weakness to rock then it does 4/8 its the pokemons weakness/resistance to rock over 8

Comments

Ahh, so it IS because of its weakness... Thanks! ~

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Yeah and if a pokemon is resistant lets say rock does 1/2 damage then stealth rock only does 1/16 (1/2 over 8)

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Oh dang 8 minutes off

Answer 2

Http://www.serebii.net/attackdex-bw/stealthrock.shtml
Pokemon with 4X weakness will receive 50% of their health gone
Pokemon with a 2X weakness will receive 25% of their health gone
Pokemon with normal damage will receive 12.5% of their health gone
Pokemon with a 2X resistance will receive 6.25% of their health gone
Pokemon with a 4X resistance will receive 3.125% of their health gone