The formula for damage reduction by projectiles is
(8 x level)%, meaning for every additional level another 8% of the damage is absorbed. The highest level (Projectile Protection IV) will absorb 32% (
4 x 8%) of the damage inflicted by any projectiles. Theoretically at level 13 and above no damage by projectiles are possible (because it would transcend 100%), but absorbed damage is capped at 80%.
Protection, Blast Protection, Fire Protection, and Projectile Protection are mutually exclusive.[Bedrock and Legacy Console editions only] However, if commands are used to have two or more of these enchantments on the same item, the effects will stack.