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Hi PrincessandthPea,

There really isn’t a minimum thickness that is necessary for a mattress (outside of for esthetic reasons) because it would depend on the type of material, the design of the mattress, and the body type and preferences of the person. If a mattress provides the PPP that you need (pressure relief, Posture and alignment,and Personal preferences) … then it’s thick enough for that person. The material inside the mattress would also make a big difference. For example 6" of a very high quality foam (such as latex or HR polyfoam) may be absolutely fine for many people but in a lower quality material that doesn’t get firm as quickly with deeper compression you may need something thicker to prevent heavier people from bottoming out. Many designs also use multiple layers of foam to achieve their target performance and “feel” so they need more thickness to accommodate the design goals of the mattress. There is more about the effect of layer thickness and the potential benefits of a thicker mattress in some cases in post #14 here.

As you mention shape (and body type in general) plays an important role the ideal design for any person and has a lot to do with how weight is distributed on a mattress. Shape also plays a larger role in the more “curvy” sleeping positions (such as on the side). There are some guidelines that includes weight and shape in the “your statistics” article here and in “tips and tricks” article here as well. There is also a lot more discussion about the effects of body shape in various posts around the forum. A forum search on “curvy” for example (you can just click this) will bring up about 150 posts :).

The “best” way to address this IMO would be the use of an adjustable bed rather than in the design of the mattress or the use of a topper, both of which aren’t designed for a position other than lying down and don’t offer the even support that upright positions require for the longer term. For a lower cost alternative yet than a “basic” adjustable bed, something like this would also be a much better solution.

You’ve probably read this already but the first place I would start is post #1 here and the information it links to. The next step would be some local testing. Your budget is very low so I would make sure you included any factory direct manufacturers in your area who may have (or could build) a mattress in your budget range that uses better quality materials than you would normally find at this price. I would also consider a single layer of HR foam covered with a reasonable quality cover that is generally available at most foam shops. This can produce a very comfortable mattress if the ILD and thickness is “right” for you and it is also bendable and can be used in any position.

Hope this helps.

Phoenix