Armour in classic WoW is the share of physical damage a target does not take; it is armour over armour plus a constant set by attacker level, up to 75%.
reduction = armour / (armour + 400 + 85 × attacker level) (attacker level up to 59)
reduction = armour / (armour + 400 + 85 × (attacker level + 4.5 × (attacker level − 59))) (attacker level 60 and up)
never more than 75%
Physical damage is reduced by a share that grows with armour but never reaches 75%; the same armour is worth less against a higher-level attacker. At attacker level 60 the constant is 5,500; a level-63 boss raises it to 7,285.
| 5,000 armour vs a level-60 attacker | 5,000 / (5,000 + 5,500) = 47.6% |
| 5,000 armour vs a level-63 boss | 5,000 / (5,000 + 7,285) = 40.7% |
| 10,000 armour vs a level-63 boss | 10,000 / 17,285 = 57.9% |
| A 3,500-armour mob hit by a level-60 player | 3,500 / 9,000 = 38.9%; 1,000 armour less (Sunder, Faerie Fire) brings it to 31.3% |
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.