Resistance in classic WoW is the average share of spell damage a target shrugs off; it is 0.75 times resistance over five times the attacker level.
average damage resisted = 0.75 × resistance / (5 × attacker level) never more than 75%
a non-binary spell is resisted in steps of 25% (0, 25, 50, 75, 100%) with this average
a binary spell (most spells with a direct effect, such as a stun or a slow) either lands in full or is resisted in full
Resistance does not make a spell miss more often; it takes an average share off the damage of each spell that lands, in quarters. Against a level-60 caster 315 resistance is the cap at 75%, which is why resistance sets stopped around 315 against level-63 bosses, whose own level sets the divisor.
| 150 Fire resistance vs a level-60 caster | 0.75 × 150 / 300 = 37.5% resisted on average |
| 150 Fire resistance vs a level-63 boss | 0.75 × 150 / 315 = 35.7% |
| 315 resistance vs a level-63 boss | 0.75 × 315 / 315 = 75%, the cap |
Last checked: unverified against Forever. These are the long-established values of the classic ruleset (the 1.x game of 2005-2006); World of Warcraft Forever is in beta and may differ. When you test this one in Forever, post your numbers in Theorycraft.