This mattress protector is 100% waterproof.About a year ago I purchased a different mattress cover that claimed to be waterproof, put it on my kid's bed, and it's been fine. But after seeing comments that it wasn't really waterproof, I tested. Yes, you can spill a cup of water and it won't go through, but if you press/squeeze into it for a few seconds (or imagine an adult sleeping atop for several hours) it definitely goes through.Tested this mattress cover and it absolutely won't let liquid pasSee full review