Hi Zing,
Mattresses as a whole don’t have an ILD rating and ILD information only applies to individual layers in a mattress. I would also keep in mind ILD/IFD by itself is only one of several factors that can affect the softness/firmness of a material or a mattress as a whole and there are several specs besides just ILD/IFD that can affect how soft or firm a material or a mattress feels. In addition to that an ILD that would be considered to be medium in a comfort layer that is designed to relieve pressure can be relatively soft when it is used as the support core of a mattress. In many cases using ILD/IFD by itself as the only point of reference for firmness/softness without regard to the function of a specific layer or the other specs that can contribute to firmness/softness can be very misleading (see post #4 here and post #2 here).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
There are also no “standardized” definitions or consensus of opinions for overall firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that is firm for one can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. This is all relative and very subjective and is as much an art as a science. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here).
The bottom line with all of this is that regardless of how a mattress is “rated” the two most important functions of a mattress and the most important part of any mattress purchase is that the specific design of the mattress keeps you in good alignment and relieves pressure points in all your sleeping positions regardless of its firmness or softness rating or the ILD of any individual layers in the mattress.
Phoenix