A futon mattress is different than a regular bed mattress because it can be folded. In fact, it is important not to use a bed mattress on a futon because it doesn't fold, it is too heavy and its construction is not designed for a futon frame. A futon mattress must correspond with the type of futon frame used. E.g. a bi-fold frame requires a bi-fold mattress
As a rule of thumb the more you use the mattress the thicker the mattress should be. Daily use requires a futon mattress 9 inches thick or greater, semi-regular use requires a 7 or 8 inch thick futon mattress and occasional use only requires a 6 inch thick futon mattress.
Futon mattresses come in a wide variety of materials: 100% cotton, 100% foam, cotton and foam blend, cotton and polyester blend, 100% polyester and also innerspring. Each futon mattress type offers unique levels in weight, firmness, flexibility and rigidity. When shopping for a futon mattress don't be afraid to lie down on the store model and try it out. It's the only way to tell if the futon mattress provides you with optimal comfort and support.