For most beds, there is no specific thickness that a mattress should be. It's purely down to what type of mattress you buy and your own personal preferences. The only bed with a requirement is a bunk bed, where Happy Beds suggest that the mattress for the top bunk is no thicker than 16cm.This is because a top bunk bed has guard rails that add safety, preventing someone from rolling off the top in their sleep. Any mattresses thicker than this can result in the guard rails not being able to do their job efficiently. For more information on this, check out Happy Beds' bunk bed guide on their website!