Minecraft wiki entry for Blast_Protection, associated text: The formula for damage reduction by explosions is (8 × level)%, meaning that for every additional level another 8% of the damage is absorbed. The highest level (Blast Protection IV) absorbs 32% (4 x 8%) of the damage inflicted by any explosion. Theoretically at level 13 no damage from explosions is possible (because it should absorb 100%), but the damage reduction is capped at 80%.