The formula for damage reduction is
(4 × level)%, meaning for every additional level another 4% of damage is absorbed. The highest level (Protection IV) will absorb 16% (
4 × 4%) of damage. Theoretically at level 25 no damage is possible (because it would transcend 100%), but because the damage to be absorbed is capped at 80%, the maximum efficient level would be 20.
Protection, Blast Protection, Fire Protection, and Projectile Protection are mutually exclusive.[until 1.14] However, if commands are used to have two or more of these enchantments on the same item, the effects will stack.