I always have the hardest time finding things to for our bed correctly. I buy regular king, they may end up being too small depending on brand. If I buy deep pocket, sometimes they are too big depending on brand. I had similar concerns when it came to this mattress cover. Our mattress is 11.5 inches and the mattress cover fits. Barely. It was definitely a chore to get it to zip, but it did.
I have been looking for a mattress cover though because I have severe issues with allergies. I am very excited for allergy season to kick into gear again so I can tell just how much a relief the mattress cover can provide. I also enjoy the waterproof aspect. One of my dogs just loves to lick the sheets; she is a weird dog, lol. Every night, she walks around the bed just licking, especially the first evening after washing them. Oftentimes, when I take the sheets off each week, I find a stain on the mattress to be washed off while the sheets are washing. I have not pulled the mattress cover off yet to see whether she has managed to lick through that too but I am praying the waterproof feature is prevented those horrid splotches from forming.
Otherwise the mattress cover is 100% comfortable and you have no clue it is under your sheets. There is not any irritating bunching under the sheets. As of the last time I washed sheets over the weekend, it still looks great itself!