Spell penetration Spell penetration reduces the target ' s resistance to your spells ; it is a different mechanic from spell hit rating . Spell penetration only works against targets with resistance s , and will not reduce resistances below zero . Due to its implementation , it is primarily a PvP - oriented stat . Spell penetration typically is intended to help casters against players ( PvP ) who wear resistance gear or are buffed with extra resistance , such as Mark of the Wild . In PvE , targets of a higher level than the caster have a base resistance ( 2 % of mitigation per level difference ) which cannot be overcome by spell penetration nor by any other stat . Otherwise , very few mobs and raid - level bosses have a substantial amount of resistance to overcome . In Mists of Pandaria . Spell penetration is being removed as a stat and mechanic in Mists of Pandaria according to Ghostcrawler ( Greg Street ) because it is only necessary if the resistance mechanic exists , but resistance is going away also . Formulas . The formula for spell penetration is quite simple . Each point of spell penetration negates one point of a target ' s resistance . Spell penetration cannot reduce a target ' s resistance below zero . The following equation gives effect of spell penetration on resistance , where P is the caster ' s spell penetration , and Rb is the target ' s base resistance . Here , the min ( ) function means the minimum of two values - in this case , it shows that spell penetration greater than a target ' s base resistance has no effect . For example , a level 70 player with 30 spell penetration casts a fire spell at another level 70 player buffed with 40 fire resistance . The spell resist check will be done as if the target only had 10 fire resistance . A level 70 player with 100 spell penetration casting at the same target will have his fire spell ' s resist check done as if the target had zero fire resistance ( not - 60 ) . For additional resistance due to level difference , spell penetration has no effect . In this equation , Lt is the target ' s level and Lc is the caster ' s level . For more details on this equation , see the resistance article . For example , A level 70 player casts a fire spell at a level 73 raid boss with no innate resistance . The level difference is 3 , so the raid boss has effectively 15 resistance to all schools against the level 70 caster . No matter how much spell penetration the caster has , his spell ' s resist check will always be done as if the target had 15 resistance . Increasing Spell Penetration . The following increase spell penetration : Talents such as the mage ' s Elemental Precision , Arcane Focus , the priest ' s Shadow Focus , and the warlock ' s Suppression do not affect spell penetration . Those talents affect spell hit rating . Using Spell Penetration . Especially in PvP , it is difficult to find a well - balanced amount of spell penetration . Against targets with no resistance , it has no effect ( and as such is wasted ) . However , against targets with resistances , spell penetration is a powerful and , in terms of item values , cheap stat . Ideally , a caster should have exactly as much spell penetration as necessary to lower the target ' s resistance to zero . In PvP , good spell penetration values are 97 ( to counter Mark of the Wild and / or Blessing of Kings ) , and 142 ( to counter various only self - cast resistance buffs , such as the Mage ' s Mage Armor ) ; these are usually enough . 97 will often do for ' occasional ' PvP . In PvE , where only few mobs have resistances , especially with Curse of Elements active , spell penetration ' s value is situational at best . The Hungarfen in Heroic Underbog , for example , have some Nature Resistance that Spell Penetration is useful against , and the Greater Lava Spiders in Searing Gorge have some Fire Resistance . It has been estimated that Rage Winterchill has approximately 200 Frost Resistance and that Supremus has approximately 200 Fire Resistance . Since neither of these fights have a challenging time requirement , stacking enough spell penetration to compliment the curses certainly isn ' t mandatory but will noticeably improve individual performance . Paladins actually need no spell penetration since the Holy school of magic has no resistance at all , in PvP or otherwise . For Discipline or Holy priest s , spell penetration is not nearly as useful as it is for other casters , but since some critical PvP spells belong to the Shadow school ( e . g . , Psychic Scream ) it isn ' t completely useless .