From Pokemon World Online Wiki
Stealth Rock lays a trap of stones around the foe, which hurts Pokemon switching in to the opposing field. The rocks inflicts variable damage - a fraction of the Pokemon's maximum HP, based on its susceptibility to the Rock type.
|Pokémon that learn it via level-up:|
|0.25x effective||1/32 max hp|
|0.50x effective||1/16th max hp|
|1x effective||1/8th max hp|
|2x effective||1/4th max hp|
|4x effective||1/2 max hp|