Miss chance in classic WoW is the share of melee attacks that fail; it is 5% plus 0.1% per point of target defence over weapon skill, with a jump past 10.
difference = target defence − your weapon skill (a mob's defence = 5 × its level)
difference ≤ 10: miss = 5% + 0.1% × difference
difference > 10: miss = 7% + 0.4% × (difference − 10)
dual wielding: white swings miss 19% more; instant attacks are not affected
+hit% from gear and talents is taken off the miss chance (never below 0)
A level-63 raid boss has 315 defence. With 300 weapon skill your instant attacks miss 9% of the time and need +9% hit to never miss; with 305 skill the step at a difference of 10 is avoided and the cap is 6%. White swings while dual wielding add 19% on top.
| Against a level-63 boss (315 defence) | 300 skill | 305 skill | 310 skill |
|---|---|---|---|
| Instant attacks and two-hand white swings miss | 9.0% | 6.0% | 5.5% |
| White swings, dual wielding, miss | 28.0% | 25.0% | 24.5% |
| Against a level-60 mob (300 defence) | 5.0% | 4.5% | 4.0% |
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.