|System Reference Document v3.5|
For example, a 6th-level wizard/4th-level rogue is treated as a 7th-level caster for determining the range, duration, and other effects of her spells. Her summon monster spells last for 7 rounds, her lightning bolts inflict 7d6 damage, she rolls 1d20+7 for dispel checks, caster level checks to overcome spell resistance, and so forth. She still doesn't get 4th-level spells (as a normal 7th-level wizard would).Table: Magic Rating by Class
Optional Variant If the DM wants to discourage multiclassing berween classes with very different spell selection, he can rule that magic ratings from arcane spellcasting classes (bard, sorcerer, and wizard) don't stack with magic ratings from divine spellcasting classes (cleric, druid, paladin, and ranger). (Magic ratings from nonspellcasting classes - barbarian, figther, monk, and rogue - stack with all other magic ratings.) This system resuslts in each character effectively having two magic ratings: an arcanemagic rating and a divine magic rating. Using this variant, a druid would gain more benefit multiclassing into ranger than into bard.
Use the first coulumn if the class grants...
Use the second coulumn if the class grants...
For all other classes, use the thrid coulumn. Examples: dwarven defender, duelist.
First Coulumn: Fey, Outsider.
Second Coulumn: Abberation, Dragon, Elemental, Undead.
Thrid Coulumn: Animal, Construct, Giant, Humanoid, Magical Beast, Monstrous Humanoid, Ooze, Plant, Vermin.
Two exeptions exist to the general rule. First, if a creature has innate spellcasting ability (such as a lammasu) or at least three supernatural or spell-like abilities (such as a yuan-ti), it uses either the coulumn for its creature type or the second coulumn, whichever gives the highest result.
Second, creatures with no Intelligence score (such as vermin, oozes, some undead, and most constructs) have no magic rating. If such a creature somehow gains an Intelligence score (such as by the application of a template that doesn't otherwise change its type), use the third coulumn to determine its magic rating.
If a monster has or gains class levels, the magic rating for that class stacks with the monster's magic rating from Hit Dice (just the way it works for multiclass characters). For example, a hound archon (6 HD outsider) and a dragon turtle (12 HD dragon) both have a magic rating of 6. If either crature gained a level of sorcerer, for example, its magic rating would improve to 7 (thanks to the magic rating of 1 that a 1st-level sorcerer has), and it would cast its spells at an effective caster level of 7th.
The magic rating system has no effect on the caster level of a creature's extraordinary or supernatural abilities. For its spell-like abilities, use the creature's normal caster level as given in its monster description for the starting point, not the magic rating derived from this system.
For example, a hezrou demon's spell-like abilities have a magic rating of 13 (since it casts as a 13th-level caster), rather than a magic rating of 10 (for its 10 HD, from the first coulumn on the table). However, if the hezrou later gains class levels, the magic rating for its spell-like abilities would go up based on the levels gained.
Creatures with different caster levels for different abilities use whichever generates the most favorable result by the above rules. For example, a gynosphinx is a 14th-level caster for most of her spell-like abilities but can use any symbol spell as an 18th-level caster. She would have a magic rating of 18.